Iiw Diploma - Wpe1 Course Notes

EWF/IIW Diploma – Welding Processes and Equipment (Foundation) WPE1

Training and Examination Services Granta Park, Great Abington Cambridge CB21 6AL United Kingdom Copyright © TWI Ltd

EWF/IIW DiplomaWelding Processes and Equipment (Foundation) Contents Section

Subject Pre training briefing Objectives

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11

General Introduction to Welding

2 2.1 2.2 2.3 2.4

Fabrication Standards

3 3.1 3.2 3.3 3.4 3.5

Weld Symbols

4 4.1 4.2 4.3

Introduction to Fusion Welding

Introduction Joining methods Welding processes Joint configuration Types of weld Features of the completed weld Weld preparation Types of preparation Size of butt welds Size of fillet welds Welding position, slope, rotation and weaving IWS revision questions on general introduction Application standards and codes Approval of welding procedures and welders Process terminology Revision questions on standards Standards Basic representation Edge preparation symbols Weld sizing Revision questions on weld symbols Creation and protection of weld pool Direction of welding Bead shape IWS questions on fusion welding Introduction and safety

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5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9

Arc Welding Safety

6 6.1 6.2 6.3 6.4

Gas Welding

7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11

Electricity as Applicable to Welding

8 8.1 8.2 8.3 8.4 8.5 8.6

Power Sources

9 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9

TIG Welding

Introduction Electric shock Heat Light Fumes and gases Noise Gas handling and storage Working at height and in restricted access areas Mechanical hazards Oxyacetylene welding Equipment Operating characteristics Equipment safety checks IWS questions on gas welding Introduction Ions and electrons Electricity generation Current, voltage, watts and resistance Direct and alternating current Transforming electricity Rectification Series and parallel Inductance Transistors and thyristors Inverters Revision questions on electricity Types of power source Power source characteristics Pulsed power Slope control and gas purging Duty cycle Bibliography Revision questions on power sources Process characteristics Arc Initiation Current and polarity Preparing the tungsten electrode Shielding gas Filler wires Potential defects Advantages of the TIG process Disadvantages of the TIG process Revision questions on TIG

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10 10.1 10.2 10.3 10.4 10.5 10.6 10.7

MIG/MAG Welding

11 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10

Manual Metal Arc (MMA) Welding

12 12.1 12.2 12.3

Welding Consumables

13 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8

Submerged Arc Welding

14 14.1 14.2 14.3 14.4 14.5 14.6

Electroslag Welding

Process characteristics Transfer modes Welding parameters Contact tip and nozzle set-up Shielding gases Solid wire consumables Flux-cored arc welding Revision questions on MIG/MAG History Process characteristics MMA basic equipment requirements Electrode types Setting up for welding Welding parameters Practical aspects of MMA Storage and handling Baking electrodes Electrode classification Revision questions Consumables for MMA welding AWS A 5.1- and AWS 5.5Inspection points for MMA consumables History Process characteristics Power source Equipment Consumables Welding parameters Potential defects Classification of consumables Revision questions

History Process characteristics ESW materials other than steel Stainless steel and nickel alloys Current status Benefits and disadvantages

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15 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 15.10 15.11

Thermal Cutting and Gouging

16 16.1 16.2 16.3 16.4 16.5

Surfacing and Spraying

Introduction General safety Oxy-fuel cutting Powder cutting Oxy-fuel gouging MMA gouging Air carbon arc gouging Plasma arc cutting Plasma arc gouging Laser cutting IWS Revision questions Background Friction surfacing Surfacing by arc welding Thermal spraying IWS Revision questions.

Appendix 1 Practice Questions

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Objectives

Objectives What the Welding Processes and Equipment Module is About Welcome to the International Institute of Welding (IIW) and European Welding Federation (EWF) approved Diploma course offered by TWI Training and Examination Services. Successful completion of your course leads to qualification recognised in more than 40 countries. TWI-TES also offers tuition to those who do not meet the IIW/EWF access criteria. The syllabus and expected learning outcomes are given in an IIW publication, IAB-252r8-07, of which a short version may be downloaded from either the IIW website: www.iiw-iis.org or from the EWF website: www.ewf.be. This course is designed to cover the syllabus but we emphasise that self-study should account for at least as much time as the lectures. Larry Jeffus (Welding Principles and Applications) is an excellent source for basic information, with coloured easy to follow diagrams. There are good books covering the topics in greater depth: AC Davies - The Science and Practice of Welding is a classic, but now rather dated, reference. Jeffries (Welding Principles and Application) and Althouse, Turnqist, Bowditch, Bowditch, Bowditch (Modern Welding) are newer titles with good explanations. The internet is, of course, a prime source of reference, though care must be taken as anyone can set up a website and post information, not all of which is accurate. We strongly suggest that you use the technical information available from TWI’s website http://www.twi.co.uk/content/tec_index.html Others that you may find helpful are: www.gowelding.com www.welding-technology-machines.info www.electronics-tutorials.com With the changing face of the internet we cannot say that these sites will remain in place and as useful as they seemed when we looked at them. We recommend that you use a search engine to explore what is available for any topic that you to learn more about. We hope that you enjoy this learning experience. Good luck in the exams! What does this module cover? We will take you from the absolute basics - defining a weld, for instance through to quite detailed understanding of the make-up and characteristics of arcs and plasmas. You will learn the basic electricity functions applicable to welding and the relationship between such fundamentals as transformation, rectification, inductance, etc and the behaviour of a welding process.

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We cover all of the commonly used processes and many of those considered advanced or specialised. The basic principles behind each process are described together with the equipment and materials necessary for a quality joint. Standards applicable to welding and symbols used on drawings to indicate specific joints are covered and safety aspects are emphasised throughout. Much of the module concerns fusion welding but solid state processes, brazing, soldering, surfacing and cutting are also dealt with. What is the final outcome that I can expect? We emphasise that we work to an international syllabus, at one of three levels, in order to prepare you for examinations that will qualify you to the same level as welding co-ordinators trained in any of the countries complying with the International Accreditation Board’s requirements. Your qualification will be recognised in more than 40 countries around the world. This module prepares you for specific exams on welding processes and equipment, one of four modules that you need to achieve the end qualification. Even if you choose not to be tested in this way, your involvement in the course will have given you a much greater understanding of the most influential parameters in welding and how to exert control over them in order to achieve quality welds. What sort of material and learning methods are used? The rest of this volume contains notes and slides that show you the depth to which we take each topic. We lecture and expect active participation. This involvement increases as you progress through the levels - we expect those at the Engineer Level to be making significant personal input into the learning process. We must point out that simply learning the notes is not enough. We make frequent reference to private study and expect you to use all facilities - library, reference books and the internet, especially the TWI website with its Job Knowledge series of articles - to give you a fuller understanding of the subject. Our lecturers and course manager are always keen to hear from you. If you have input to give, ideas for improvement, or you just have a concern over the learning or examination, please speak to us. Why is this module important to me? All welding engineers, technologists and specialists are expected to know the fundamentals of the welding processes. There is no-one in the company with better knowledge, so if the welding operation does not go smoothly everyone will turn to the specialist, ie you, for advice. A key decision the welding specialist must make is to determine the best process for the company to use for any application. This will require an understanding, not only of the pros and cons of each process, but also any attendant requirements necessary to make the process work efficiently. This module will give you an understanding of how each process works and the differences between them; the equipment, control and operator skill required for each and the economic factors associated with choosing a welding process.

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My company has fixed ideas, who am I to change them? We’re not saying change is necessary, nor always desirable, but WL Bateman famously said: If you keep on doing what you've always done, you'll keep on getting what you've always got. Maybe your company has got it right and wants to continue getting what it always got, but we doubt it. Everyone wants to remain competitive and seeks to improve productivity. If not, we would still see rows of scribes with quill pens rather than computers in offices. Welding is a traditional process, but the equipment and control available today make even the set-up of ten years ago obsolete. This course will place recently developed processes and newer equipment types and controls in context with traditional units. It will teach you how to judge true advances and their benefit to your company. My company just wants me to be IIW/EWF qualified so that I can sign the paperwork, do I really need this knowledge? Companies do have short-term goals and getting someone qualified as a welding co-ordinator is an admirable one, but this shows that it is working on contracts that demand that welding is taken seriously as a special process. Having succeeded with the first of such contracts, your company will surely look to take on more. A welding co-ordinator does far more than sign the paperwork and will play a big part in determining the success of future contracts of ever increasing technological and quality demands. This module will give you the confidence to speak with authority on fabrication techniques to be used and the cost-effectiveness of welding processes at your disposal. What will I be able to do at the end of this course that I can’t do now? This is a tricky one, as everyone has different skills coming into the course and different requirements that they wish to gain from it. However, even if you are on top of the game with regard to the applications you see every day in your job, exposure to the requirements and decisions from other quarters can only be of benefit. Who knows, maybe laser cutting or friction stir welding is the next logical step for your company with regard to cost and quality improvement. This module will give you details of a wide range of processes available for many different types of material. So, in a nutshell, what’s in it for me? The acquisition of knowledge about your speciality is never wasted. Even if you don’t use all that you learn on this course immediately, your awareness will be raised so that you will remember where to look for information when circumstances demand it.

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If your company develops opportunities in applications and materials currently unfamiliar, you will be in a position to come to terms rapidly with any new approaches necessary. Whilst we recognise that you are likely to be sponsored by your company against a company objective, we should also point out that your personal development and the gaining of professional qualifications is of great benefit to you, the individual, as you follow your career path.

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Section 1 General Introduction to Welding

1

General Introduction to Welding

1.1

Introduction Welding and joining, like any other technologies, have their own terminology and are liberally endowed with abbreviations and acronyms, but these soon become familiar. In this section we give the definitions of basic terms.

1.2

Joining methods Joining is the most general term used to refer to any process or procedure by which two or more separate pieces of material are physically attached to each other so as to create a single larger piece.

1.2.1

Welding Welding is defined as an operation in which two or more parts are united by means of heat or pressure or both, in such a way that there is continuity in the nature of the metal between these parts. Many materials such as metals, plastics and ceramics may be welded though some require the use of specific processes and techniques and a number are considered unweldable, a term not usually found in dictionaries but useful and descriptive in engineering.

The parts that are joined are termed parent material and any material added to help form the join is called filler or consumable. The form of these materials may see them referred to as parent plate or pipe, filler wire, consumable electrode (for arc welding), etc. Consumables are usually chosen to be similar in composition to the parent material thus forming a homogenous weld but there are occasions, such as when welding brittle cast irons, when a filler with very different composition and therefore properties is used, such welds are called heterogeneous. The completed welded joint may be referred to as a weldment. 1.2.2

Brazing A process of joining generally applied to metals in which, during or after heating, molten filler metal is drawn into or retained in the space between closely adjacent surfaces of the parts to be joined by capillary attraction. In general, the melting point of the filler metal is above 450oC but always below the melting temperature of the parent material.

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The composition of the filler for brazing is often very different from parent material; for instance, steel may be brazed with a copper alloy filler. 1.2.3

Soldering A similar process to brazing, relying on capillary attraction to draw molten filler into a gap between parts that remain solid throughout. Solders melt at low temperatures – less than 450ºC. For steel and copper, solders are usually alloys of tin.

1.3

Welding processes Welding processes fall into two groups – those in which fusion takes place and those that achieve solid state bonding. Fusion welding includes oxy-fuel gas welding (OFW); manual metal(lic) arc (MMA); metal inert/active gas (MIG/MAG); flux-cored arc welding (FCAW); tungsten inert gas (TIG); submerged arc welding (SAW); electron beam welding (EBW); laser welding (laser is an acronym: light amplification by stimulated emission of radiation) and others. United States codes and standards use different terminology and abbreviations for these processes: MMA MIG/MAG TIG Laser

– – – –

shielded metal arc welding (SMAW) gas metal arc welding (GMAW) gas tungsten arc welding (GTAW) laser beam welding (LBW)

Solid state processes do not involve melting because some materials can be permanently welded together by pressure if in a suitably malleable state. This may require the application of some heat, eg forge welding as carried out by blacksmiths and friction welding in its many forms. Explosive welding; cold pressure welding and ultrasonic welding are examples of welding processes in which heat is not deliberately generated. The most common of the above mentioned welding processes are described in these notes and some further ones are given in the Advanced Welding Processes notes, but neither attempts to give an exhaustive listing of all of the welding processes that have been demonstrated.

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1.4

Joint configuration The table below defines some of the more common configurations: Type of Sketch Definition joint Butt joint

A connection between the ends or edges of two parts making an angle to one another of 135-180° inclusive in the region of the joint.

T joint

A connection between the end or edge of one part and the face of the other part, the parts making an angle to one another of more than 5 up to and including 90° in the region of the joint.

Corner joint

A connection between the ends or edges of two parts making an angle to one another of more than 30 but less than 135° in the region of the joint. A connection between the edges of two parts making an angle to one another of 0 to 30° inclusive in the region of the joint.

Edge joint

A connection in which two flat plates or two bars are welded to another flat plate at right angles and on the same axis. Cruciform joint

A connection between two overlapping parts making an angle to one another of 0-5° inclusive in the region of the weld or welds.

Lap joint

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1.5

Types of weld

1.5.1

Based on configuration

Butt weld

Fillet weld

Slot weld Joint between two overlapping components made by depositing a fillet weld round the periphery of a hole in one component so as to join it to the surface of the other component exposed through the hole.

Plug weld Weld made by filling a hole in one component of a workpiece with filler metal so as to join it to the surface of an overlapping component exposed through the hole (the hole can be circular or oval).

Based on penetration Full penetration weld Welded joint where the weld metal fully penetrates the joint with complete root fusion. In US the preferred term is complete joint penetration weld (CJP, see AWS D1.1).

Partial penetration weld Weld in which the fusion penetration is intentionally less than full penetration. In the US the preferred term is partial joint penetration weld (PJP).

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1.5.2

Based on accessibility

Single side weld

Double side weld

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1.6

Features of the completed weld

Butt weld

Fillet weld Parent metal Metal to be joined or surfaced by welding, braze welding or brazing. Filler metal Metal added during welding; braze welding, brazing or surfacing.

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Weld metal All metal melted during the making of a weld and retained in the weld. Heat affected zone (HAZ) The part of the parent metal metallurgically affected by the heat of welding or thermal cutting, but not melted. Fusion line Boundary between the weld metal and the HAZ in a fusion weld. This is a nonstandard term for weld junction. Weld zone Zone containing the weld metal and the HAZ. Weld face The surface of a fusion weld exposed on the side from which the weld has been made. Weld root Zone on the side of the first run furthest from the welder. Weld toe Boundary between a weld face and the parent metal or between runs. This is a very important feature of a weld since toes are points of high stress concentration and often they are initiation points for different types of cracks (eg fatigue cracks, cold cracks). In order to reduce the stress concentration, toes must blend smoothly into the parent metal surface. Excess weld metal Weld metal lying outside the plane joining the toes. Other non-standard terms for this feature: reinforcement, overfill. Note: the term reinforcement, although commonly used, is inappropriate because any excess weld metal over and above the surface of the parent metal does not make the joint stronger. In fact, the thickness considered when designing a welded component is the design throat thickness, which does not include the excess weld metal. Run (pass) The metal melted or deposited during one passage of an electrode, torch or blowpipe.

Single run weld

Multi run weld

Layer 1.7

Stratum of weld metal consisting of one or more runs. Weld preparation Preparation for making a connection where the individual components, suitably prepared and assembled, are joined by welding or brazing.

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1.7.1

Features of weld preparation Angle of bevel Angle at which the edge of a component is prepared for making a weld. For an MMA weld on carbon steel plates, the angle is typically: 25-30° for a V preparation. 8-12° for a U preparation. 40-50° for a single bevel preparation. 10-20° for a J preparation Included angle Angle between the planes of the fusion faces of parts to be welded. In the case of single V or U and double V or U this angle is twice the bevel angle. In the case of single or double bevel, single or double J bevel, the included angle is equal to the bevel angle. Root face The portion of a fusion face at the root that is not bevelled or grooved. Its value depends on the welding process used, parent material to be welded and application; for a full penetration weld on carbon steel plates, it typically is around 1-2mm (for the common welding processes). Gap Minimum distance at any cross-section between edges ends or surfaces to be joined. Its value depends on the welding process used and application; for a full penetration weld on carbon steel plates, it is usually 1-4mm. Root radius The radius of the curved portion of the fusion face in a component prepared for a single J or U, double J or U weld. In case of MMA, MIG/MAG and oxy-fuel gas welding on carbon steel plates, typical root radii are 6mm for single and double U preparations and 8mm for single and double J preparations. Land The straight portion of a fusion face between the root face and the curved part of a J or U preparation. It is not essential to have a land but it is usually present in weld preparations for MIG welding of aluminium alloys.

1.8

Types of preparation

1.8.1

Open square butt preparation

This preparation is used for welding thin components, either from one or both sides. If the root gap is zero (ie if components are in contact), this preparation becomes a closed square butt preparation (not recommended due to the lack of penetration problems)! The exception to this is submerged arc welding, where this preparation is used.

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1.8.2

Single V preparation

The V preparation is one of the most common preparations used in welding; it can be produced using flame or plasma cutting (cheap and fast). 1.8.3

Double V preparation

For thicker plates a double V preparation is preferred since it requires less filler material to complete the joint and the residual stresses can be balanced on both sides of the joint resulting in lower angular distortion. The depth of preparation can be the same on both sides (symmetric double V preparation) or can be deeper on one side compared with the opposite side (asymmetric double V preparation). Usually, in this situation the depth of preparation is distributed as 2/3 of the thickness of the plate on the first side with the remaining 1/3 on the backside. This asymmetric preparation allows for a balanced welding sequence with root back gouging, giving lower angular distortions. Whilst single V preparation allows welding from one side, double V preparation requires access to both sides (the same applies for all double side preparations).

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1.8.4

Single U preparation

U preparation can be produced only by machining (slow and expensive). However, tighter tolerances obtained in this case provide for a better fit-up than in the case of V preparations. Usually it is applied to thicker plates as it requires less filler material to complete the joint compared with single V preparation and this leads to lower residual stresses and distortions. Double U preparation

As with V preparation, in the case of very thick sections a double U preparation can be used. Usually this type of preparation does not require a land, except for aluminium alloys. Single V preparation with backing strip

Backing strips allow the production of full penetration welds with increased current and hence increased deposition rates/productivity without the danger of burn-through. Backing strips can be permanent or temporary. Permanent types are made of the same material as being joined and are tack welded in place. The main problems related to this type of weld are poor fatigue resistance and the probability of crevice corrosion between the parent metal and the backing strip. It is also difficult to examine by NDT due to the built-in crevice at the root of the joint. Temporary types include copper strips, ceramic tiles and fluxes.

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For further details regarding weld preparations, refer to Standard BS EN ISO 9692. 1.9

Size of butt welds Full penetration butt weld

Partial penetration butt weld

As a general rule: Actual throat thickness = design throat thickness + excess weld metal. Full penetration butt weld ground flush Actual throat thickness = design throat thickness Butt weld between two plates of different thickness

1.10

Size of fillet welds

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1.11

Welding position, slope, rotation and weaving Welding position The orientation of a weld expressed in terms of working position, weld slope and weld rotation (for further details, see BS EN ISO 6947). Welding position

Definition and symbol according to BS EN ISO 6947

Sketch

Flat

A welding position in which the welding is horizontal, with the centreline of the weld vertical. PA

Horizontalvertical

A welding position in which the welding is horizontal (applicable in case of fillet welds). PB

Horizontal

A welding position in which the welding is horizontal, with the centreline of the weld horizontal. PC

Vertical-up

A welding position in which the welding is upwards. PF

Vertical-down

A welding position in which the welding is downwards. PG

Horizontaloverhead

A welding position in which the welding is horizontal and overhead (applicable in case of fillet welds). PD

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Weld slope The angle between root line and the positive X axis of the horizontal reference plane, measured in mathematically positive direction (ie counter-clockwise). Weld rotation

The angle between the centreline of the weld and the positive Z axis or a line parallel to the Y axis, measured in the mathematically positive direction (ie counter-clockwise) in the plane of the transverse cross-section of the weld in question. Weaving Weave Transverse oscillation of an electrode or blowpipe nozzle during the deposition of weld metal, generally used in vertical-up welds.

Stringer bead A run of weld metal made with little or no weaving motion.

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IWS questions on general introduction 1

Sketch a double bevel T butt weld with full penetration and superimposed mitre fillet welds.

2

Sketch a single V butt weld and indicate the features.

3

Sketch a double J butt weld.

4

Indicate the typical excess weld metal dimension on a butt weld in 6mm thick material.

5

The abbreviation MMA stands for?

6 Sketch actual throat thickness compared with design throat thickness.

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Section 2 Fabrication Standards

2

Fabrication Standards Application standards and codes of practice ensure that a structure or component will have an acceptable level of quality and be fit for the intended purpose. The requirements for standards on welding procedure and welder approval are explained below. It should be noted that the term approval is used in European standards in the context of both testing and documentation. The equivalent term in the ASME standard is qualification. A standard has also been constructed that gives a unique number to a welding process. This is also described below.

2.1

Application standards and codes There are essentially three types of standards that are referenced in fabrication:   

Application and design. Specification and approval of welding procedures. Approval of welders.

There are also specific standards covering material specifications, consumables, welding equipment and health and safety. British Standards are used to specify the requirements, for example, in approving a welding procedure, they are not a legal requirement but may be cited by the Regulatory Authority as a means of satisfying the law. Health and Safety guidance documents and codes of practice may also recommend standards. Codes of practice differ from standards in that they are intended to give recommendations and guidance, for example, on the validation of power sources for welding. It is not intended that they should be used as a mandatory or contractual documents. Most fabricators will be working to one of the following:     

Company or industry specific standards. National British Standard (BS). European British Standard European Standard (BS EN). US American Welding Society (AWS) and American Society of Mechanical Engineers (ASME). International Standards Organisation (ISO).

In European countries, national standards are being replaced by EN and ISO standards. However, when there is no equivalent EN standard, the National standard can be used. For example, the BS EN 287 and BS EN ISO 9606 series replaced BS 4871, but BS 4871 Part 3 and 4872 Part 1 remain as a valid standard.

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Examples of application codes and standards and related welding procedure and welder approval standards are listed below: Application Pressure vessels Process pipework

Structural fabrication

Storage tanks

2.2

Application code/standard BS 5276 BS PD 5500 ASME Section VIII BS 2633 BS 2971 BS 4677 ASME B31.1/B31.3 BS EN 1090 BS 8118 AWS D1.1/ D1.2/ D1.6

Welding procedure approval BS EN ISO 15614 ASME Section IX

BS EN 12285 BS EN 14015 API 620/650

BS EN ISO 15614 ASME IX

BS EN ISO 15614 ASME IX

BS EN ISO 15614 AWS D1.1/ D1.2/ D1.6

Welder approval BS EN 287 BS EN ISO 9606 ASME Section IX BS EN 287 BS 4872 BS EN ISO 9606 ASME IX BS EN 287 BS 4872 BS EN ISO 9606 AWS D1.1/ D1.2/ D1.6 BS EN 287 BS EN ISO 9606 ASME IX

Approval of welding procedures and welders An application standard or code of practice will include requirements or guidelines on material, design of joint, welding process, welding procedure, welder qualification and inspection or may invoke other standards, for example for welding procedure and welder approval tests. The requirements for approvals are determined by the relevant application standard or as a condition of contract. The manufacturer will normally be required to approve the welding procedure and welder qualification, or to have it witnessed by an independent inspection authority. Welding procedure approval test Carried out by a competent welder and the quality of the weld is assessed using non-destructive and mechanical testing techniques. The intention is to demonstrate that the proposed welding procedure will produce a welded joint that will satisfy the specified requirements of weld quality and mechanical properties. As shown in the table above, welding procedure approval is carried out according to BS EN 15614 series (different parts correspond to different fusion welding processes), Section IX of the ASME Boiler and Pressure Vessel Code, and other codes such as AWS D1.1 for structural welding. DNV-OS-F101 (offshore structures) includes requirements for welding procedure qualification.

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Welder approval test Examines a welder's skill and ability at producing a satisfactory test weld. The test may be performed with or without a qualified welding procedure. (Note, without an approved welding procedure the welding parameters must be recorded.) BS EN 287, BS ISO EN 9606 and ASME Section IX would be appropriate for welders on high quality work such as pressure vessels, pressure vessel piping and off-shore structures. They are also used for other products where the consequences of failure, stress levels or complexity mean that a high level of welded joint integrity is essential. In less demanding situations, such as small to medium building frames and general light structural and non-structural work, an approved welding procedure may not be necessary. However, to ensure an adequate level of skill, welders are often approved to a less stringent standard, eg BS 4872. Coded welder is an expression often used to denote an approved welder but the term is not recognised in any of the standards. However, it is used in the workplace to describe those welders whose skill and technical competence have been approved to the requirements of an appropriate standard. 2.3

Process terminology The European standard, BS EN ISO 4063:2000 Welding and allied processes - Nomenclature of processes and reference numbers, assigns a unique number to the main welding processes. These are grouped as follows:      

Arc welding. Resistance welding. Gas welding. Forge welding. Other welding processes. Brazing, soldering and braze welding.

Each process is identified within the group by a numerical index or reference number. For example, the MIG welding process has a reference number of 131 which is derived as follows: 1 2 3

Arc welding. Gas-shielded metal arc welding. Metal arc inert gas welding.

The main arc welding process reference numbers are: 111 114 121 125 131 135 136 141 15

Manual metal arc welding. Self-shielded tubular-cored arc welding. Submerged arc welding with one wire electrode. Submerged arc welding with tubular cored electrode. Metal inert gas welding (MIG welding). Metal active gas welding (MAG welding). Tubular cored metal arc welding with active gas shield. Tungsten inert gas arc welding (TIG welding). Plasma arc welding.

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Revision questions on standards 1

What is the purpose of a welding procedure approval test?

2

What is the purpose of a welder approval test?

3

What is the difference between a Standard and a Code of Practice?

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Section 3 Welding Symbols

3

Welding Symbols Weld symbols are a simple way of communicating design office details to the variety of shop floor personnel eg welders, supervisors and inspectors, in a consistent manner. Non-company staff such as sub-contractors and insurers may also need to interpret the engineering drawings. It is essential therefore that everyone should have a full understanding of the system of weld symbols in use to ensure that the design requirement is met.

3.1

Standards The most common international standards for weld symbols are the ISO 2553/European EN 22553, published in the UK as BS EN 22553 and the American AWS/ANSI A2.4. Most of the details are the same, but it is essential that everyone concerned knows the standard to be used. The UK traditionally used BS 499-2 to define weld symbols which was superseded by BS EN 22553. Confusingly, the BSI still publishes BS 499-1 containing weld symbols as well as other terminology for welding and a chart, BS 499-2C that shows the symbols pictorially.

3.2

Basic representation All the standards use a reference line plus an arrow line and head placed at an angle to the reference line, viz:

The V-shaped tail is optional as it is used to show the welding process, in Europe with the reference numbers defined in BS EN ISO 4093. If only one process is to be used throughout the construction, this can be shown once on the drawing rather than repeated for each weld. The reference line has a parallel dotted line to show the other side. This is a refinement introduced in the European standard that is not present in the American one. In AWS A2.4, the top of the line is always the near side and information attached to the underside represents the far side. On these two lines (or two sides if a single line is used) symbols are placed representing the weld preparation on the near and, if appropriate, far side of the joint line. The arrow line can be at any angle (except 180o) and can point up or down. The arrow head must touch the drawn surfaces of the components to be joined at the location of the weld.

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3.3

Edge preparation symbols To the basic set-up of the arrow and reference line, the design draughtsperson can apply the appropriate symbol, or symbols for more complex situations. The symbols, in particular for arc and gas welding, are shown as simplified cross sectional representations of either a joint design or a completed weld, as shown below:

Supplementary symbols are added to the edge preparation to show the shape of the finished bead profile:

Aspects of welding not immediately apparent from the basic symbols can be added as complementary symbols:

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3.4

Weld sizing So that the correct size of weld can be applied, it is common to find numbers to either the left or to the right of the symbol. For fillet welds, numbers to the left of the symbol indicate the design throat thickness, leg length, or both design throat thickness and leg length requirements. Numbers to the right of the symbol show the length of the weld and where the welding is intermittent, the number of welds to be made in the location:

As per ISO 2553/EN 22553: a = Design throat thickness. z = Leg length. s = Penetration throat thickness.

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The large Z through the reference line shows that intermittent weld beads are placed in a staggered arrangement on either side of the component. When there are no specific dimensional requirements specified on the weld symbol, it would normally be assumed that the requirement is for a full penetration, full length weld. Summary of information on symbols.

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IWS questions on weld symbols 1

What is the symbol for: Weld all round.  Single bevel butt weld.  Site weld. 

2

Draw an indication for a fillet on the near side.

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Section 4 Introduction to Fusion Welding

4

Introduction to Fusion Welding

4.1

Creation and protection of weld pool Fusion welding requires a source of heat sufficient to melt both parent plate and filler and a means of protecting the molten material from unwanted chemical reactions with the atmosphere. Heat may be provided by a flame, electric arc or resistance or a power beam. Protection from reactions with oxygen and nitrogen in air may be achieved by placing the pieces in a vacuum or controlled atmosphere or more usually by providing local cover from a shielding gas or flux. In some processes, such as flux-cored wire welding a combination of gas and flux may be used.

TIG welding.

MMA welding; Welding flux operates in two ways to protect weld metal. It forms a gas around the arc that keeps air away from the pool and creates a slag that freezes (usually at a similar temperature to the metal) and protects the solidified, but still hot and reactive, metal from oxidation.

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Gas shielding is usually with an inert gas, argon or helium, protecting the pool and adjacent hot metal from oxidation, but there is no protection for the still hot solid metal beyond the range of the gas flowing from the nozzle. A thin layer of oxide therefore often tarnishes MIG and TIG welds. Some metals, notably titanium, cannot accept exposure to air whilst hot, even if solidified, so require extra, trailing shields to provide gas coverage until the metal has dropped temperature considerably. Carbon and C-Mn steels do not oxidise rapidly so the protective gas can be active rather than inert, usually carbon dioxide or an Ar-CO2 mixture and the process is then often referred to metal active gas (MAG). 4.2

Direction of welding When welding with a manual technique, the torch is very rarely held upright over the weld pool. It is usually inclined in the line of the welding direction, with the tip either pointing away from the previously deposited weld metal or towards it. For a right handed person, the usual method is to move the torch or electrode from right to left, with the torch/electrode pointing in the direction of travel. This is often referred to as the pushing technique and results in a fairly smooth weld profile. There are occasions where it is advantageous to weld in the opposite direction using a dragging technique and this gives deeper penetration but at the expense of a more convex weld profile. When using the oxy-acetylene process the movement is usually similar and is referred to as the leftward technique. However for oxy-acetylene pipe welding a technique known as all positional rightward may sometimes be used, where the filler wire is fed into the weld behind the weld pool. This allows greater deposition (compared with leftward) but is again at the expense of weld appearance, which will be coarser than a leftward weld.

4.3

Bead shape If welding progresses directly in a straight line with no sideways movement, a stringer bead is laid.

The weld bead is the same width as the molten weld pool. If travel speed increases, the weld pool will become elongated in the direction of travel and narrower in width. The resultant stringer bead will also be narrower. If the current is insufficient for the travel speed adopted, there may be only limited melting of the parent plate resulting in a bulbous cross-section bead and, in the extreme, lack of fusion.

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Conversely, excessive current will lead to the pool being pushed into the surface of the plate and on freezing grooves will be left at either side of the bead, termed undercut.

The welder can deliberately move the torch from side-to-side during the laying of a bead, called weaving.

This has the advantage of dwelling at the edges of the bead giving more time to melt the parent plate. It can achieve a better blend of the bead shape to the parent plate surface and can be used by a skilled welder to bridge larger than expected root gaps. It is particularly used for vertical up welding but care must be taken to keep the depth of bead to only a few millimetres. It is possible to use a wide, triangular weave technique when working in the vertical position, often known as blocking out. This should be exercised with caution as the very high heat input associated with it can cause deterioration of the mechanical properties of the parent material. It is often thought that blocking out is faster than using a stringer bead technique, but this is an incorrect. The deposition rate is controlled by the welding current or wire feed speed, not the movement of the torch. It is important to attempt to achieve a smooth profile change from the weld bead to the surface of the parent plate as sharp discontinuities create stress raisers from which defects such as hydrogen or fatigue cracks may initiate.

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IWS questions on fusion welding Introduction and safety 1

What are the essential requirements to achieve a successful weld?

2

Describe stringer beads, weaving and blocking.

3

What is the effect of excess current?

4

List the general safety aspects required for welding.

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Section 5 Arc Welding Safety

5

Arc Welding Safety Introduction Working in a safe manner, whether in the workshop or on site, is an important consideration in any welding operation. The responsibility for safety is on all individuals but especially for welders, not only their own safety but also to avoid endangering other people. The welding co-ordinator has an important function in ensuring that safe working legislation is in place and safe working practices are implemented. The co-ordinator should ensure compliance with all appropriate documents, for example:      

Government legislation – The Health & Safety at Work Act. Health & Safety Executive – COSHH Regulations, statutory instruments. British Standards – OHSAS 18001. Company health and safety management systems. Work instructions – permits to work, risk assessment documents, etc. Local authority requirements.

There are many aspects of arc welding safety that the co-ordinator needs to consider:       

Electric shock. Heat and light. Fumes and gases. Noise. Gas cylinder handling and storage. Working at height or in restricted access. Mechanical hazards: trips, falls, cuts, impact from heavy objects.

To find out if welders and other operatives are at risk the co-ordinator needs to consider the working conditions. The Management of Health and Safety at Work Regulations 1999 require employers assess the risks to health of employees arising from their work. The actions arising from the risk assessment are dictated by other more detailed regulations, eg the Control of Substances Hazardous to Health (COSHH) Regulations 2004. The following sections give guidance on risk avoidance but continuous effort on improvements to precautions and working conditions is essential for the wellbeing of the workforce. 5.1

Electric shock Contact with metal parts which are electrically live can cause injury or death because of the effect of the shock upon the body or because of a fall as a result of the reaction to electric shock. The electric shock hazard associated with arc welding may be either from the primary 230 or 460V mains supply or from the output voltage at 60-100V. Primary voltage shock is very hazardous because it is much greater than the secondary voltage of the welding equipment. Electric shock from the input voltage can occur by touching a lead inside the welding equipment with the power to the welder switched on while the body or hand touches the welding equipment case or other earthed metal. Because of such hazards, only a qualified electrician should remove the casing of a welding power source. Residual circuit devices (RCDs) connected to circuit breakers of sufficient capacity will help to protect personnel from the danger of primary electric shock.

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The transformed power is available from terminals on the front of the welding set. Heavy duty cables are attached to these terminals to carry the welding current to the torch or electrode holder and to bring a return path from the work or metal workbench to the other terminal. This return is often referred to as the earth or ground and there may be secondary earthing arranged so that the work is at zero volts. Secondary voltage shock occurs when touching a part of the electrode circuit – perhaps the jaws of an MMA electrode holder or a damaged area on the electrode cable – while another part of the body touches the other side of the welding circuit (the work or welding earth) at the same time. Whilst most welding equipment is unlikely to exceed an OCV of 100V, electric shock, even at this level, can be serious. The welding circuit should be fitted with low voltage safety devices to minimise the potential of secondary electric shock. It is important that the welding cables can carry the maximum possible output of the welding set without overheating as this can damage the insulation, leading to an increased risk of electrical shock. TWI Job Knowledge No 29, available from the TWI website (www.twi.co.uk) gives more guidance on avoiding electric shock during welding. 5.2

Heat As arc welding relies on melting metal to affect a joint, it follows that the metal will in part be very hot. All metals conduct heat to a greater or lesser degree so the area heated to a temperature that will cause skin burns is very much larger than the weld bead itself. It is a wise precaution to assume that all metal on a welding workbench or adjacent to a site weld is hot. Temperature indicating sticks should be used to check that material is cool enough to handle. Patting metal with the bare hand to check its temperature is a way of being burnt! The welding arc creates sparks with potential to cause flammable materials near the welding area to ignite and cause fires. The welding area should be clear of all combustible materials and is good practice for all personnel working in the vicinity of welding to know where the nearest fire extinguishers are and the correct type of fire extinguisher to use if a fire does break out. Welding may also produce spatter, globules of molten metal expelled from the weld area which can cause serious burns, so protective clothing, such as welding gloves, flame retardant coveralls and leathers must be worn around any welding operation to protect against heat and sparks. It is most important that traps in clothing are avoided. Trousers should not have turn-ups nor be tucked into boots – very serious injury can occur if spatter drops inside a work boot. Radiant heat from welding can be quite intense, particularly when welding at high current and duty cycle. Sufficient air movement is required to keep the welder at a sensible temperature, especially important when working in restricted access areas where reflected heat will intensify the effect. Welders should also take water regularly to avoid potential dehydration.

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5.3

Light Light radiation is emitted by the welding arc in three principal ranges: Type Infra-red (heat) Visible light Ultra-violet radiation

5.3.1

Wavelength, nanometres >700 400-700 <400

Ultra-violet radiation (UV) All arc processes generate UV and excess exposure causes skin inflammation and possibly even skin cancer or permanent eye damage. However, the main risk amongst welders and Inspectors is inflammation of the cornea and conjunctiva, commonly known as arc eye or flash. Arc eye is caused by UV radiation which damages the outmost protective layer of cells in the cornea. Gradually the damaged cells die and fall off the cornea exposing highly sensitive nerves in the underlying cornea to the comparatively rough inner part of the eyelid. This causes intense pain, usually described as sand in the eye. The pain becomes even more acute if the eye is then exposed to bright light. Arc eye develops some hours after exposure, which may not even have been noticed. The sand in the eye symptom and pain usually lasts for 12-24 hours, but can be longer in more severe cases. Fortunately, arc eye is almost always a temporary condition. In the unlikely event of prolonged and frequently repeated exposures, permanent damage can occur. Treatment of arc eye is simple: rest in a dark room. A qualified person or hospital casualty department can administer various soothing anaesthetic eye drops which can provide almost instantaneous relief. Prevention is better than cure and wearing safety glasses with side shields will considerably reduce the risk of this condition. The welder, of course, should always have a full face screen with the approved shade of protective lens for the process in hand.

5.3.2

Ultra-violet effects upon the skin The UV from arc processes does not produce an attractive browning effect of suntan; but results in acute reddening and irritation caused by changes in the minute surface blood vessels. In extreme cases, the skin may be severely burned and blisters form. The reddened skin may die and flake off in a day or so. Where there has been intense prolonged or frequent exposure, skin cancers can develop.

5.3.3

Visible light Intense visible light particularly approaching UV or blue light wavelengths passes through the cornea and lens and can dazzle and in extreme cases damage the network of optically sensitive nerves on the retina. Wavelengths of visible light approaching the infra-red have slightly different effects but can produce similar symptoms. Effects depend on the duration and intensity of exposure and to some extent, upon the individual's natural reflex action to close the eye and exclude the incident light. Normally this dazzling does not produce a long-term effect.

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5.3.4

Infra-red radiation (IR) Infra-red radiation is of longer wavelength than the visible light frequencies and is perceptible as heat. The main hazard to the eyes is that prolonged exposure (over a matter of years) causes a gradual but irreversible opacity of the lens. Fortunately, the IR radiation emitted by normal welding arcs causes damage only within a comparatively short distance from the arc. There is an immediate burning sensation in the skin surrounding the eyes should they be exposed to arc heat. The natural human reaction is to move or cover up to prevent the skin heating, which also reduces eye exposure. BS EN169 specifies a range of permanent filter shades of gradually increasing optical density which limit exposure to radiation emitted by different processes at different currents. Many welders use helmets with filter glasses that react to light and darken as soon as the arc is struck with the advantage that the welder has clear vision through non-shaded glass at all times except when the arc is struck and the protective filter is induced.

5.4

Fumes and gases Fume is a mixture of particles generated by vaporisation, condensation and oxidation of substances transferred through the welding arc. The particles are very small and remain suspended in the air for long periods, where they may be breathed. Small particles are respirable which means that they may penetrate the innermost regions of the lung where they have the most potential to do harm. If inhaled, welding fume may be hazardous to health and must be controlled to limits laid down by regulations. Toxic gases may also be generated during welding and cutting. Gases encountered in welding may be:     

Fuel gases which on combustion form carbon dioxide and if the flame is reducing, carbon monoxide. Shielding gases such as argon, helium and carbon dioxide, either alone or in mixtures with oxygen or hydrogen. Carbon dioxide and monoxide produced by the action of heat on the welding flux or slag. Nitric oxide, nitrogen dioxide and ozone produced by the action of heat or ultra-violet radiation on the atmosphere surrounding the welding arc. Gases from the degradation of solvent vapours or surface contaminants on the metal.

The degree of risk to the welder's health from fume/gases will depend on:   

Composition. Concentration. Length of exposure.

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It is essential to know the type of parent plate, together with any coating and the composition of the fume generated. This is because different fume components vary in toxicity. The limits to which welding fume and its component parts must be controlled are provided in Guidance Note EH40 Workplace Exposure Limits available from the Health and Safety Executive (HSE). This is updated annually.

In 2005, a single type of occupational exposure limit known as the workplace exposure limit or WEL was introduced. The underlying principle is to have a single criterion for exposure to airborne hazards. However, welding fume is an insufficiently precise term to be given a WEL. Individual components must be addressed. 5.4.1

What's in the fume? Exposure to fume may be measured according to the methodology defined in BS EN ISO 10882-2: 2000. Account must be taken of the exposure limits of the individual fume constituents. For example, iron oxide, limestone, titanium dioxide have WEL of 4 or 5mg/m3. They may therefore be taken to be similarly hazardous to any dust - not specifically causing a medical condition but needing control to ensure proper lung function. Some components of fume have lower WEL, manganese, trivalent chromium and soluble barium are set at 0.5mg/m3, copper at 0.2mg/m3 but hexavalent chromium compounds and nickel oxide are potential carcinogens and pose greater health risks at lower concentrations. Nickel and its water-insoluble compounds have WEL of 0.5mg/m3 and hexavalent chromium compounds only 0.05mg/m3. These exposures are over a time-weighted average reference period of 8 hours. Clearly, welding stainless steel, likely to generate both nickel and chromium in the fume, poses a very different set of conditions than welding mild steel.

5.4.2

What about gases? For gas shielded welding processes such as TIG, MIG/MAG, FCAW, shielding gases may be inert gases, such as argon, helium and nitrogen, or argon-based mixtures containing carbon dioxide, oxygen or both. Helium may be added to argon/carbon dioxide mixtures to improve productivity. Carbon dioxide (CO2) may be used, on its own, in MAG and FCAW. With the exception of CO2, these gases are not defined as hazardous to health under the COSHH Regulations but they are asphyxiants. None of the gases can be seen or smelt so their presence in hazardous concentrations is difficult to detect without prior knowledge or measuring equipment.

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The main hazard arising from exposure to shielding gases is accumulation in confined spaces. Argon is heavier than air, so tends to collect in low areas such as pits. Inhaling a gas, such as pure argon, which contains no oxygen, can cause loss of consciousness in seconds. Fatalities have occurred where welders have entered vessels or tanks where argon has accumulated. Workers should not enter an atmosphere that contains less than 18% oxygen. Carbon monoxide (CO) and CO2 may be generated in fluxed welding processes by the action of heat on flux materials such as carbonates and cellulose. In MAG welding they can both originate from CO2 in the shielding gas, CO2 undergoing reaction in the vicinity of the arc to form CO. Flame processes also generate CO and CO2. The relative amounts depend on whether the flame is oxidising or reducing, with CO present in higher concentrations when the flame is reducing. Carbon monoxide is by far the more hazardous of the two gases. It can cause a reduction in the oxygen carrying capacity of the blood that can be fatal. In lower concentrations it causes headache and dizziness, nausea and weakness. CO2 acts mainly as an asphyxiant, as indicated above. CO has a WEL of 30ppm and CO2 is listed at 5000ppm (8 hour time weighted average). However, the amounts of CO and CO2 generated by welding processes are small and, generally, they do not present an exposure problem. Nitric oxide (NO) and nitrogen dioxide (NO2) are known collectively as nitrous gases (NOx). NO is a severe eye, skin and mucous membrane irritant. NO2 is a highly toxic, irritating gas. Welding generates only small amounts of nitrous gases so exposure to NOx does not present a problem. Exposure problems may arise during cutting activities, particularly if the cutting is hand-held, as this places the operator closer to the emissions. Hotter flames generate higher concentrations of nitrous gases, so using acetylene generates more nitrous gases than using propane or natural gas. Plasma cutting with air or nitrogen generates higher levels of nitrous gases than oxy-fuel gas cutting and there is considerable risk of over-exposure. Ozone can be generated by reaction between UV light from the arc and oxygen in the air. It has a low WEL of 0.2ppm for a 15 minute reference period but in a real situation ozone generation is usually well below the exposure limit. At the levels of exposure to ozone found in welding the main concern is irritation of the upper airways, characterised by coughing and tightness in the chest, but uncontrolled exposure may lead to more severe effects, including lung damage. 5.4.3

Where is the welder's nose? No, not the obvious answer: we need to consider the relationship of the person's breathing zone to the concentration of fume and gas generated during the process. To reduce the risk of hazardous fumes and gases, keep the head out of the fume plume. As obvious as this sounds, incorrect placement of the nose within the plume is a common cause of fume and gas over-exposure because the concentration of fumes and gases is greatest in the plume. Welding position is an important variable as it affects the proximity of the fume to the welder's breathing zone and has a major effect on exposure. Welding vertically-up usually results in the welder's head being away from the path of the fume plume. Positions that place the welder closer to, or worst of all, above the plume of fume lead to highest exposures, so leaning over a flat position weld is more hazardous.

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If the welding operation is in a confined workspace accumulation of fume may be expected to increase exposure. Similarly if the duty cycle is high the concentration of fume in the vicinity and the time that the welder is exposed to it, will increase. Provision of local extraction to suck away the fume from the welder's breathing zone is an obvious remedy. It is, indeed, quite efficacious, but only when used correctly. It is most useful for fixed welding stations where repetitive jobs are carried out. Here, the extraction nozzles can be placed close to the weld and need little re-positioning. Even for applications where the welder has considerable movement, positioning of extraction nozzles will provide adequate protection if used correctly. Finally, it may be necessary to wear an approved respiratory device if sufficient ventilation cannot be provided. As a rule of thumb, if the air is visibly clear and the welder is comfortable, the ventilation is probably adequate.

5.4.4

Informing the workforce Instruction must be given to ensure that employees know:   

What they must do, the precautions that must be taken and when they must take them. What cleaning, storage and disposal procedures are in place, why they are required and when they are to be carried out. Procedures to be followed in an emergency.

Training must be provided for the effective application and use of:   

Methods of control. Personal protective equipment. Emergency measures.

To keep such records and to inform and train a workforce may seem onerous but it is the law and it is necessary to plan and implement these things effectively. Do things correctly and welding is a safe operation. Ignore the precautions and it can be very costly both for your company and your welders.

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5.5

Noise Exposure to loud noise can permanently damage hearing, cause stress and increase blood pressure. Working in a noisy environment for long periods can contribute to tiredness, nervousness and irritability. If the noise exposure is greater than 85 decibels averaged over an 8 hour period then hearing protection must be worn and annual hearing tests carried out. The employer has the responsibility of ensuring that workers wear the protection. If noise levels are between 80 and 85dB averaged over 8 hours, hearing protection must be available. Normal welding operations are not associated with excessive noise level problems with two exceptions: plasma arc welding and air carbon arc cutting. If either of these two operations is to be performed then hearing protectors must be worn. The noise associated with welding is usually due to ancillary operations such as chipping, grinding and hammering. Hearing protection must be worn when carrying out, or when working in the vicinity of, these operations.

5.6

Gas handling and storage Mostly covered in the section on gas welding as the same precautions apply to shielding gas storage and handling as for fuel gas and oxygen. The cylinders contain gas at up to 300 bar and care must be exercised that they cannot fall and sever the valve from the top. The sudden release of energy turns the cylinder into a high powered missile capable of passing through block walls. This has been demonstrated most graphically by the Discovery Channel’s Mythbusters. A video has been posted on YouTube (http://www.youtube.com/watch?v=ejEJGNLTo84). A more serious approach to this potentially lethal hazard is given in a training video clip on the same website (http://www.youtube.com/watch?v=CHDAbM09Y1o). Shielding gas cylinders must be in purpose-built cradles with secure chaining to avoid any risk of toppling. A single person should not manipulate them alone as they weigh up to 100kg and there is a real risk of loss of control. Transportation around a fabrication shop should be in a trolley designed for the purpose. Pressure regulators must be fitted to gas cylinders to extract the gas at a usable pressure and must be appropriate for the job: rated at least as high as the maximum pressure of the cylinder and designated for the specific gas. Tubes carrying the gas to the welding torch should be pressure hoses designed for the job. These should be checked for leaks by using diluted detergent around all fittings. Leakage of shielding gas is not as safety critical as leakage of fuel gas, but the weld quality can be compromised if leaks develop. For a similar reason, hoses should be purged for some minutes prior to starting work to eliminate any moisture adsorbed onto the inner wall.

5.7

Working at height and in restricted access areas Welding may be used on large civil engineering sites requiring working at considerable height. All tall buildings have a steel framework and modern structures are invariably welded. All expected precautions for working at height must be observed - correctly erected scaffolding, tied ladders, platforms and walkway boards, kick boards and handrails etc – but there are specific aspects for welding that must also be taken into account.

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There is a requirement to carry out a thorough risk assessment of any operation and this is especially required when working at height. It may be concluded that the risk of mishap when lifting, transporting and fixing gas cylinders at height is too great to allow MIG or TIG welding to be used. It is essential to know that if a welder were to receive an electric shock; his reflex reaction away from the source would not place him at risk of a fall from height. Guardrails on scaffolding are mandatory but, furthermore, should take into account the increased hazard associated with welding. Welding produces spatter and, where positional welding is required, large drops of molten metal or slag may occur. The area immediately below welding at height should be cordoned off to prevent other workers straying into the drop zone. Similarly, there are standard requirements for health and safety when working in restricted access areas, not least of which is a risk assessment and permit to work system. Here again, welding introduces additional hazards that must be considered when arranging for a person to work in limited space. Most dangerous of the hazards are those introduced by the use of gas. If gas cutting has to be used there is a risk of un-burnt fuel gas accumulating and creating an explosion risk. Increasing the concentration of oxygen in a limited space is also a risk as it marked increases the flammability of material. Shielding gases are deliberately flooded over the weld area and will remain in the vicinity in restricted space. Argon is denser than air and will fill the space from the floor upwards. Helium is less dense than air and will accumulate in the roof area. Both are asphyxiants that can easily kill an operator breathing volumes of the gases. Carbon dioxide will also suffocate a person within a few breaths. Welders working in very confined spaces should be provided with externally-fed helmets and should always be accompanied by a buddy who remains outside the danger area but in close contact with the welder. 5.8

Mechanical hazards The environment in which a welder works has a number of hazards not specific to the welding process itself. Manual handling of heavy awkward metal components is often required. Thinner, lighter metal sheet may have sharp edges. Slips, trips and falls may be more likely as welding often requires thick cables to be spread across the floor. Standard workshop safety and protection practice should be used to counter these problems. Welders need training in materials handling, both manual and with mechanical lifting assistance; protective gloves, helmets, overalls and boots must be worn; cabling on the floor should be minimised and clearly signed or marked as a trip hazard. There are hazards that are a direct result of the joining process itself. During welding, sparks and molten metal can be ejected. These are most common in arc welding but can also occur in resistance processes. In mechanised processes, guards should be used to contain the flying particles. This is not possible in manual welding so personal protective equipment (PPE) must be worn by the operator. All clothing should be fire resistant and use of leather aprons, jackets, chaps, etc is recommended.

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Grinding is commonly used in preparing metal for welding and during cleaning and rectification of deposited metal. Wheel and angle grinders are favoured tools for their speed of removal of material but create a hazard, not only for the operator but for adjacent and passing personnel, as the ejected material may be thrown some distance. Obviously the operator needs adequate protection with clothing, gloves, full-face shields and sometimes a dust mask but the whole area also needs screening with curtains to protect others. One of the more serious dangers is from the persistent use of vibrating hand tools: grinders, scaling hammers, pneumatic burrs, etc which can lead to longterm illness – hand-arm vibration syndrome, also known as white finger or dead hand. Studies of the incidence of the condition have shown that action to prevent physical damage may be required when the operator has as little as 30 minutes per day use of a chipping hammer.

Enter Course Title Enter Reference

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Section 6 Gas Welding

6

Gas Welding

6.1

Oxy-acetylene welding This is the most common gas welding process. When mixed together in correct proportions oxygen and acetylene create a flame with a temperature of about 3,200ºC. Added to this, the chemical action of the oxy-acetylene flame can be adjusted by changing the ratio of the volume of oxygen to acetylene. Three distinct flame settings are used, neutral, oxidising and carburising (also called reducing). Welding generally uses the neutral flame setting with approximately equal parts of oxygen and acetylene. The oxidising flame is obtained by increasing the oxygen flow rate while the carburising flame is achieved by increasing acetylene flow rate. Neutral flame

The neutral flame has three combustion zones. The innermost at the end of the nozzle is the cone and has a distinct contoured nucleus with a slightly rounded shape and glows white. This is surrounded by an almost colourless tongue and an outer zone which has a slightly blue coloration. Overall, the flame is mainly colourless and is characterised by a fizzling sound. Neutral flames are used for welding carbon, alloy and stainless steels and nonferrous alloys, for brazing steels and for preheating. Oxidising flame

As the name suggests, this flame requires an increased proportion of oxygen over the amount of acetylene, resulting in the innermost cone being substantially reduced in length, most often described as short and pointed. A second zone may be visible, as shown in the photograph above, but it is the overall small size and sharp delineation of the flame and the strong blue, almost violet colour, that are most noticeable.

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The oxidising flame is only used where there is a positive benefit to creating oxide during welding. It is mostly limited to welding zinc-containing alloys. Carburising (reducing) flame

The carburising flame requires an increased proportion of acetylene. As there is insufficient oxygen to burn all the acetylene, the surplus continues to burn at the outside of the flame where oxygen from the air is present, creating a flame that is very luminous and usually is yellow-white with a large inner cone and no sound. The carburising flame is used for hard surfacing as it tends to increase the carbide content in the surface layer. It is also used for welding aluminium to avoid oxide layer build-up on the weld pool surface because its reducing action stops aluminium oxide formation. 6.2

Equipment Oxy-acetylene equipment is portable, easy to use and comprises oxygen and acetylene gases stored in steel cylinders. The cylinders are fitted with regulators and flexible hoses which lead to the blowpipe. The oxygen is stored under pressure (up to 300bar) in a cylinder usually painted black and has a standard right-handed thread to the regulator and hose fittings. Acetylene cannot be stored under pressure as it is liable to explode so it is dissolved in acetone held in a porous clay/fibre/charcoal mixture within a steel cylinder usually painted maroon. It is fitted with left-handed threads to avoid any possibility of incorrect assembly. The cylinders must be held in specially designed stands or carriers to keep them upright and stable during use and storage.

Pressure reducing regulators are fitted to both the oxygen and acetylene cylinders so that the pressure and flow of the gases can be regulated to the torch. The torch itself has a flow valve for each gas so that the operator has control over flame size and composition readily to hand.

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Safety devices called flashback arrestors are fitted between the hoses and the cylinder regulators on both the oxygen and acetylene lines and are flame traps designed to prevent flames generated by a flashback from reaching the cylinders. Flashbacks can occur if the gas flow is insufficient to prevent the flame burning back into the torch, if the hoses have not been purged before ignition or if the blowpipe nozzle is overheated. Non-return valves are also fitted in the hose run to avoid any possibility of back flow due to a blocked nozzle or other flow restriction. A range of nozzles is available for the welding torch allowing choice of flame size suited to the material thickness to be welded as described below. When welding the operator must wear protective, flameproof clothing and coloured goggles. As the flame is less intense than an arc and very little ultraviolet light is emitted, general purpose tinted goggles provide sufficient protection. Operating characteristics The oxy-acetylene flame can produce a soft or harsh action on the surface of the material to be welded by varying the gas flow, but there are practical limits to the type of flame that can be used for welding. A harsh forceful flame will cause the molten weld pool to be blown away; too soft a flame will not be stable near the point of application. The blowpipe is therefore designed to accommodate different sizes of swan neck copper nozzle which allows the correct intensity of flame to be used. When carrying out fusion welding the addition of filler metal in the form of a rod can be made when required. The techniques used in oxy-acetylene welding are described by the direction of travel of a right-handed operator - leftward, rightward and all-positional rightward. Leftward welding is most commonly used and is ideally suited for butt, fillet and lap joints in sheet thicknesses up to approximately 5mm. The rightward technique finds application on plate thicknesses above 5mm for welding in the flat and horizontal-vertical position. The all-positional rightward method is a modification of the rightward technique suited to welding steel plate and pipework where positional welding, (vertical and overhead) has to be carried out. The rightward and all-positional rightward techniques enable the welder to obtain a uniform penetration bead with added control over the molten weld pool and weld metal. Moreover, the welder has a clear view of the weld pool and can work in complete freedom of movement. These techniques are very highly skilled and are less frequently used than the conventional leftward technique. Equipment safety checks Before commencing welding it is essential to inspect the condition and operation of all equipment. As well as normal equipment and workplace safety checks, there are specific procedures for oxy-acetylene. Operators should verify that:    

Flashback arrestors and non-return valves are present in each gas line. Hoses are the correct colour, blue for oxygen and red for acetylene, have no sign of wear and should be as short as possible and not taped together. Regulators are the correct type for the gas. A cylinder key is in each cylinder (unless the cylinder has an adjusting screw).

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  

All connections are tight and not subject to leaks. No oil or grease has been allowed near any part of the oxygen line or cylinder. No copper containing material is in direct contact with acetylene.

The latter two safety checks are necessary because of explosion risk. A competent inspector should check all oxy-acetylene equipment at least annually and regulators should be taken out of service after five years. Flashback arrestors should be checked regularly according to the manufacturer's instructions and, with specific designs, it may be necessary to replace the arrestor if a flashback has occurred. For more detailed information the following legislation and codes of practice should be consulted:    

UK Health and Safety at Work Act 1974. Pressure Systems and Transportable Gas Containers Regulations. British Compressed Gases Association, Codes of Practice. BOC Handbook.

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IWS Questions on Gas Welding 1

State the symptoms of a flashback and the likeliest causes.

2

State the advantages of the rightward technique over the leftward technique.

3

Describe the safety checks you would use when setting up a gas welding operation. Include the reasons why they are required.

Enter Course Title Enter Reference

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Section 7 Electricity as Applicable to Welding

7

Electricity as Applicable to Welding These notes do not attempt to give an in-depth account of electricity and electrical circuits and you are urged to research fundamental theory to acquaint yourself with those aspects of which you are unsure. There are websites, eg http://www.allaboutcircuits.com, that give easily understood information on fundamentals. Wikipedia is also useful, though the reader has to appreciate that a wiki is the product of a mass of individuals from all backgrounds and various materials interests so not all information can be relied on as accurate. A lecture from University of Manchester, Department of Computer Science is very useful: http://intranet.cs.man.ac.uk/Study_subweb/Ugrad/coursenotes/CS1222/electri city.pdf We do offer some basics about electricity as applicable to welding in the sections below.

7.1

Introduction Electricity occurs naturally in a wide range of phenomena: lightning, the sting of an electric eel, even the workings of your brain, yet it was only in the late 19th century that scientists began to understand its nature and how to use it. Some materials, eg metals, graphite, salt water, allow the passage of electricity (ie are conductors) and many, eg wood, rock, rubber, do not and are considered to be insulators. Although all materials are made of atoms, the difference between conductors and insulators lies in the strength of binding of the orbiting electrons in the atom.

7.2

Ions and electrons When chemical compounds dissolve in water or are split in a welding arc they form positively and negatively charged particles termed ions. An example of this is when common salt, sodium chloride, is dissolved in water, the sodium ion becomes positive, Na+ and is balanced by a negative charge on the chlorine ion, Cl-. As ions carry a charge they will be attracted to opposite charges and repelled by like charges. So put a positive and negative charge into a solution of salt and the Na+ will move towards the negative whilst the Cl- goes to the positive. The importance of this will become apparent when we consider the welding arc. Metals and other conductors do not form ions as such, but have charged particles than can move, electrons which are negatively charged so would be attracted to a positive, this is the basis of electricity.

7.3

Electricity generation Magnetism is also a naturally occurring phenomenon and we are familiar with the North and South Pole concept with opposites attracting and likes repelling. There is a link between magnetism and electricity as a magnet will provide the positive/negative differential required for electron movement in a conductor. The North-seeking pole is positive and will attract electrons and this is used in the dynamo principle, which was the first practical generation of electricity.

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If a metal wire, or any conductor, is moved through the magnetic field created between the two poles of a magnet, the electrons will move within the wire to try to head towards the positive pole. By winding many loops of wire and mounting the assembly on an axle, a significant amount of electron movement can be achieved. As the assembly swings through 180º and approaches the other pole of the magnet, the electron flow will be reversed. By connecting the loops of wire to individual strips of metal and contacting these only as they pass one or other of the magnet poles, we can capture electron flow as positive on one side and negative on the other, thus we have electrical current available at the contacts. The modern dynamo has many loops of wire, augmented by a soft iron core, with each loop connected to a copper strip further along the axle. Carbon brushes are held against the revolving copper strips, the commutator and leads attached to the brushes deliver a direct current. Electricity is no longer generated commercially with dynamos due to the difficulty of maintaining brushes and commutators on very large machines, but the principle of inducing a current in a moving conductor is still used in generators. 7.4

Current, voltage, watts and resistance The amount of electrons on the move defines the amount of electricity that flows, termed the current, i, measured in amps, A. Electron flow and therefore electricity, moves at the speed of light as, rather than being the movement of small solid particles, it is a form of electromagnetic wave, but as this takes us to relativity to explain, we will not offer proof here, for all practical purposes, electricity is instantaneously available throughout a circuit. The differential of the positive and negative used to attract the electrons from one to the other can be regarded as the driving force and is called the potential difference or voltage. Because of this potential there is a tendency for the electrons to move, ie there is a force, electromotive force, EMF, measured in volts, V, attempting to move them from the negative to the positive. Electricity flow has energy and is capable of doing work as passes through a conductor. Consider a light bulb, the passage of current through the conducting filament generates heat, a form of energy created by the fact that work has been done. This heat is sufficiently intense to raise the tungsten filament to well over 1000ºC at which temperature light is evolved. The amount of work depends on both voltage and current. If a light bulb intended for the UK 240V system is instead used on the American 110V mains, it will glow only dimly. Furthermore, if the current flow to a UK bulb operating on its normal 240V circuit is restricted by a dimmer switch, less light is seen. Thus it is a combination of current and voltage that gives the power consumption, measured in watts, W. Watts are the product of amps and volts, ie: W=AxV

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Different materials allow the passage of current to differing degrees. The wiring in your house between sockets is pure copper around 2mm in diameter. All of your appliances – washing machine, refrigerator, television run from this and you give no thought to the passage around your ring main. However, you may have an electric fire with a wire winding not very different in diameter and this will heat up and glow red, not what you’d want to be happening to your ring main! But the nickel-chromium alloy of the fire element passes current much less easily than copper and this causes it to heat whilst the copper does not. This reluctance to pass current is termed resistance, R and is measured in Ohms, Ω. The greater the driving force (EMF), the more current passed through the resisting material. This is Ohm’s Law, which may be expressed: V=ixR or: V=AxΩ In electrical circuitry, resistance is often required to protect components and small devices are supplied with known resistance. These are called resistors and are illustrated in circuit diagrams by a rectangular box. American and some older UK publications may show a resistor as a zig-zag line: Preferred symbol (BS EN 81714-2):

American, Japanese and superseded European symbol:

The heating effect in the electric fire is important in welding as it plays a part in raising the temperature of a current-carrying consumable wire towards melting. By experiment and measurement of the effect of changing variables, we can show that this heating is proportional to the resistance of the wire and to the square of the current it carries, often known as the i2R effect. 7.5

Direct and alternating current (DC and AC) The electricity from a dynamo always flows in the same direction in the wires attached to the brushes. This is DC. The electricity circulating in the National Grid is AC, which means that it regularly switches direction of flow. The switch is not instantaneous but builds and decays in sine wave form. A positive flow followed by a negative one constitutes one cycle.

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The number of times this happens in one second is called the frequency and this is measured in Hertz, Hz. One change of direction per second is 1Hz; 50 per second is 50Hz. The National Grids of European countries operate at 50Hz, but the US has a 60Hz supply. To pass large amounts of electricity along the distribution wires of a Grid, a high voltage (driving force) is required, usually around 400,000V. But to offer very high voltage supply to households would be dangerous. The capacity for work is a product of both voltage and current so drawing a very small current from such a high voltage supply would still amount to high energy. The voltage must therefore be changed to a lower value before the supply is connected to a consumer. 7.6

Transforming electricity We can change voltage using a transformer, a device that uses corollaries to the principle of the dynamo, viz: if a wire moving through a magnetic field creates electricity, the converse is also true, that a magnetic field moving past a wire will create electricity. Furthermore, moving electricity through a wire will create magnetism. So, if a soft iron (a good magnetic medium) in the shape of a square has a winding of wire on one side through which current is flowing, this will induce magnetism, termed magnetic flux, flowing around the iron square. Thus, if a second winding of wire is made on the opposite side of the iron square, the flow of magnetism will induce electricity in this wire even though it is not electrically connected to the first.

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The voltage generated in the second coil of wire depends on the input voltage and the ratio of turns in each of the coils. Thus if V1 and n1 are the voltage and number of turns of the input coil and V2 and n2 are for the output coil:

V1 n1  V2 n2 or

n V2  2 x V1 n1 To reduce the voltage from the high level of the grid to a lower level requires a high number of turns on the input side and low number on the output: Energy must be conserved in any system so, if we ignore losses through heat, any reduction in voltage must be accompanied by an increase in current. This may be expressed:

V1 A 2  V2 A 1 So if an input of 1000V and 2A has 100 turns on the input coil and there are 10 turns on the output coil, the output would be 100V and 20A. This simple device can transform both DC and AC supplies. Transformers are sited in the electricity supplier buildings in residential neighbourhoods with warning signs about danger of electrocution and in rural areas they may be mounted on telegraph poles. A large factory will almost certainly have a high voltage supply to its vicinity and a local transformer to supply its power needs. Welding requires relatively low voltage – arc welding may run with only 20-30V maintaining the arc - but needs high current, maybe 100-300A, to give the power to melt metal. Transformers within the power source itself generate this from the input voltage and current. Input from domestic supply (240V and typically 15A from sockets) will limit welding possibilities. Transformation of domestic mains supply to the 80V typically used for arc starting gives only 45A maximum current. Industrial supply is typically 415V with either 63 or 125A maximum, which can supply around 320A and up to 650A respectively, hence most welding workshops and power sources run on this supply. 7.7

Rectification AC power may be used in some welding processes, but most require DC. To generate DC from the AC supply requires filtering off, rectification, of one half cycle, eg the negative part, leaving all current in the one direction. The simplest form of rectification uses diodes, devices that transmit current in only one direction. The semi-conductor, silicon, is especially useful as sandwiches can be built that have this property of one-way transmission (see transistors below). Rectifiers are sometimes referred to as silicon diode rectifiers.

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The symbol for a diode is:

This shows the direction of permitted current flow – left to right, from the base of the triangle – and the blocked path – right to left, encountering the straight line. Passing a simple single-phase supply (upper graph below) through a diode will cut out the negative part of the cycle leaving the half wave in the positive direction (middle graph below). This is half-wave rectification and is a rather inefficient method of creating DC as it uses only half the energy of the input. It is possible to capture both halves of the cycle as positive output by a process called full-wave rectification and the input and output curves take the shape shown on the lower graph below.

Full-wave rectification is achieved by arranging four diodes in a square as shown below. When the input current in line A is positive, the diode in the top right allows passage of current to the positive terminal. When the input current is negative in line A, it follows that it is positive in line B and then the lower right diode allows this through to the positive terminal.

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A

B

The frequency of the pulses is now twice that of the input so, if 50Hz mains is input, full-wave rectification gives a pulsating DC at 100Hz. Three-phase rectification achieves smoother output as the cycles overlap in time, but there is still a pronounced ripple effect, as shown in the following image on next page.

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7.8

Series and parallel When an electrical circuit is built, wires or strip connectors connect components in a way to produce the appropriate electrical interaction, linked together one after another in a daisy-chain array, series connection:

The components can be connected piggy-back fashion, parallel connection.

The effect of linking components in different ways creates different overall results as seen by considering resistors and the overall resistance of the circuit: In series, resistance is additive, so the overall resistance is high, being the sum of all the individual resistors’ values: Rt R1 R2 R3

….

In parallel, the current has multiple paths to use to travel from one side of the resistor array to the other, so the overall resistance of the circuit is lower than any individual resistor, according to the formula: 7.9







Inductance Another feature of the interaction of electricity and magnetism is inductance. Current passing through a wire generates a magnetic field and the amount of magnetic flux is proportional to the current so, if the current is changing, it follows that the magnetic field intensity will also vary. Faraday found and defined in his Law that changing the field of magnetic flux induces an EMF in the wire which opposes the increase in current. Known as inductance, it is particularly useful in welding as there are instances where a very rapid rise in current can cause instability so adding inductance to the circuit can control this tendency to instability. Although inductance is generated in a straight wire, purpose-built inductors are usually wound as coils to maximise the magnetic effect. An inductor may have a ferromagnetic core that amplifies the effect and some of these cores may be moved to vary the inductive effect.

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The symbol for an inductor is:

or

7.10

Transistors and thyristors Solid state electronics was born from an observation in 1947 that when electrical contacts were placed on a crystal of germanium, the output current was greater than the input. Following this, workers at Texas Instruments developed the germanium-doped silicon transistor which can be used to increase the current through it, ie as an amplifier or as a semi-conductor, allowing it to be used as a switching device. In the early 1970s, Watkins and Needham at TWI built a welding power source based on transistors. Whilst a research tool rather than a commercial entity, it proved that sufficient current could be developed in a solid-state amplification circuit to give the high currents necessary for fusion welding. Development of commercial offerings rapidly followed and today all power sources include transistors, even if only on the control circuit. However, few simple transistors are used in a modern circuit, most use integrated circuits that contain millions, sometimes billions, of transistor functions. The thyristor is a development of the transistor principle that acts as a selflatching switch and are used in welding power sources as they are capable of handling high current switching reliably. They are not the source of power but are key components in the control system.

7.11

Inverters Inverters are fast becoming the power source of choice for welding and it is not difficult to see why, this MMA one is very small.

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Compare this with a conventional power source.

Conventional power sources are large and heavy because of the size of the transformer required to convert 415V 63 or 125A mains to an output suitable for welding. High current requires thick wires and large iron cores to avoid overheating. An inverter creates very high frequency AC (maybe 100kHz) and the transformer is much smaller than the conventional one.

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IWS questions on electricity 1

What is Ohms law?

2

How is mains AC converted to DC at current and voltage suitable for welding?

3

How is high mains voltage reduced to a safe AC welding voltage?

4

What is half wave rectification and how can full-wave rectification be achieved?

5

Explain the difference between connecting resistors in series and in parallel.

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Section 8 Power Sources

8

Power Sources

8.1

Types of power source Welding takes place at relatively low voltage compared with the input mains electricity and much higher current. The delivery of the appropriate ranges of voltage and current is the function of the power source. Welding can be achieved with DC electrode negative (DCEN), DC electrode positive (DCEP) or AC power and there are a number of ways of generating these. DC power can be generated directly from a dynamo or more likely a brushless generator as described in the section Electricity as Applicable to Welding. The motor to drive the coil through the magnetic field is an internal combustion engine so the machines are usually known as diesel or petrol generators depending on the fuel of the engine. The advantage of an engine-driven generator is that it is self-contained with no requirement of input from the electricity network so is particularly suited to site work and may be mounted on the back of a truck, or on wheels to give portability to remote sites.

A small generator welding set.

Engine-driven machines are not popular for shop fabrication as there is significant noise from the engine and rotating components and they become large and heavy when scaled to give high current capability. AC power sources are, at their simplest, transformers taking the AC input and converting it to higher current, lower voltage. The welding current available can be adjusted by adding inductance to the system, usually by placing an inductor in line. Inductance opposes the flow of current so slows the rate of growth of the current during each half cycle. With sufficient inductance the current does not reach its maximum beginning to decay towards the other half cycle. This is also known as choking and the control device may also be called a choke. The inductor - often containing capacitors as well and known as a reactor - has a means of adjustment giving current control to the operator.

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This may be by tapping the reactor at various points, giving differing numbers of turns of wire so different levels of inductance or by moving an iron core allowing a variable amount of leakage of the inductance. There is another method whereby a small DC current controls the amount of magnetism in an iron core which in turn determines the amount of inductance. This is a saturable reactor which gives fine control but is more expensive and usually only used for TIG machines. AC transformer welding sets.

For DC welding in shop fabrication conditions, the AC transformer is coupled with a rectifier, producing a heavy duty, sturdy machine noted for reliability and use in adverse conditions.

A traditional rectifier power source.

The principle of inversion gives advantages with regard to the size of transformer and as the components have become more commonplace and therefore cheaper inverter power sources are becoming very common in welding. The AC mains input is first rectified to DC, which is fed to the inverter which converts it to high frequency AC, maybe 50kHz, which means that the subsequent transformer can be very much smaller than in a conventional machine. This can be used for welding or passed through another rectifier to give a DC supply. Inverter power sources can be used for AC or DC welding and even sources with high current output are very small size.

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8.2

Power source characteristics If the input to the power source is 415V and 20A, the power consumption is 415 x 20 = 8300 watts. As energy can neither be gained nor lost, this should also be the output power, but some energy is lost due to heating components and loss of this heat to the surroundings. Effectively this machine will output 7500 watts so in theory, we could extract 300A at 25V or 3A at 250V or any other combination amounting to 7.5kW. A TIG welding power source may give around 100V maximum and operate down to maybe 10V. Over this range our 7.5kW would provide 75A up to 750A with a straight-line relationship between the current and voltage. This is not how welding power sources work as they are designed to have specific volt/amp relationships. Generally higher voltage means lower current and vice versa, but the rate of change differ according to the circuitry. The reason for different relationships lies in the processes.

8.2.1

Drooping characteristic For manual processes such as TIG and MMA welding, the arc length is dependent on how consistently the welder can hold the torch above the workpiece. Arc length is directly proportional to arc voltage, so a longer arc has a higher voltage and if the arc is shortened the voltage will decrease. Variation of arc length by 3 or 4mm can easily vary the voltage by 5V. This would vary the current between 300 and 375A in our theoretical machine. Such variation would result in significant changes in weld pool size and penetration and would make the process very difficult to control.

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By design, the TIG or MMA power source has a limited range of current and a reduced variation on changing voltage. Plotted as voltage against current this appears as:

This is termed drooping characteristic or constant current. With such a power source the variation of current over a change of 5V may be as little as 10A, giving almost imperceptible changes to the weld pool, making control much easier for the welder. With no load on the power source, ie when it is switched on but not delivering output, there is a relatively high voltage across its output terminals, the open circuit voltage (OCV). For a drooping characteristic power source, it may be 80-100V and is useful for to help initiate the arc on MMA electrodes. However, once running an arc the voltage is normally 20-35V, shown above as the normal operating range. It is the minimal variation of current over this range that gives the power source its characteristic relevant to MMA or TIG welding. 8.2.2

Flat characteristic and the self-adjusting arc MIG welding requires different characteristics from the power source. The consumable is continuously fed through the torch where it picks up current from the contact tip, at its end an arc is struck that melts the wires and transfers droplets to the workpiece. As the wire is much smaller diameter than an electrode for MMA, the current density is much higher, as is the burn-off rate. The wire feeder is set at a speed that delivers the wire at the same rate as it is melted away so a fixed arc length operating at particular values of voltage and current is established. Any variation in arc length will cause a small change of voltage as noted above for the MMA process. If the same drooping characteristic power source were to be used, the increased voltage on lengthening the arc would be accompanied by only a small lowering of the burnoff rate. With the small wire diameter of MIG, the higher voltage arc would create a much larger plasma column that would widen the weld pool. Similarly, shortening the arc would produce a much smaller weld pool. As the burns off rates are maybe twenty times as rapid as for MMA, there is insufficient time for the welder to react to these changes. If the power source is designed to give a large change of current for only a small change of voltage it is more manageable.

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The MIG power source has an operating characteristic that produces only small changes in potential (a few volts) as a result of bigger (at least one order of magnitude) changes in current.

Any small increase in arc length and thus voltage results in a large reduction in current and burn-off rate. Thus feed speed is momentarily in excess of the burn-off so the wire advances from the end of the contact tip, reducing the arc length, lowering the voltage and increasing the current, until the feed speed and burn-off are in balance and equilibrium is restored.

The opposite is also true if the torch moves towards the workpiece. The voltage drops causing a large increase in current and burn-off rate. This exceeds the feed speed so the wire burns back, automatically increasing voltage and dropping current until the equilibrium position of feed speed equalling burn-off is achieved.

The OCV of a flat characteristic power source is only a few volts above the operating range. In operation, both MIG and SAW arcs are initiated by advancing the wire until it makes contact with the base plate, creating a short circuit giving rapid heating and melting of the wire. As the molten filler drops away an arc is established. No higher voltage is needed for this to happen.

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8.3

Pulsed power There are instances where having the ability to switch off full power momentarily is advantageous. In TIG welding by pulsing between a high and a low background current, a weld pool with the penetration of full power is achieved without the overall heat normally associated with it, giving better control over side and root fusion with less danger of overheating the whole area leading to over-penetration. Pulsing the power also causes the solidification front intermittently to advance rapidly then recede which can avoid crystal growth along the weld line and in certain instances improves resistance to solidification cracking. Pulsing may be achieved in a number of ways. The earliest sets operated at mains frequency and the current was determined by chopping the full-wave rectified power:

i

t

i

t

With the advent of electronic control, rapid switching of DC became possible allowing the generation of a square wave from two base current levels: a low background and a higher peak current. In a switching circuit, the frequency is no longer dictated by the AC mains, so frequency of pulsing becomes a variable; a higher frequency of the same pulse resulting in a higher average current and therefore heat input: 8.3.1

Pulsed MIG and synergic control Flexible pulsing introduces a significant number of extra variables, but electronics can make the job of the welder much easier. For pulsed MIG welding the parameters that give stable and useful conditions for each material, wire size and shielding gas combination can be stored in software and reproduced at the touch of a button (or turn of a selector knob). Such control is known as synergic (ie working together to give a better than expected result) and is sometimes referred to as a one knob set. The power source designers realised that it was particularly useful to choose pulse MIG conditions that melted and transferred a single drop of metal from the wire and synergic power sources became synonymous with single drop transfer. Pulsed transfer is sometimes confused with single droplet transfer but a simple pulse of high current takes the process into a condition where normal spray conditions occur; it takes careful selection of pulse size, shape and frequency - different for each combination of material, wire size and gas - to achieve single drop transfer.

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8.4

Slope control and gas purging The stops and starts of weld runs can contain defects associated with establishing and extinguishing the arc. For example the start area of a TIG weld may be more prone to the formation of porosity than the stable part of the run. In most processes the final weld pool is the most susceptible to solidification cracks. For processes that involve metal transfer through the arc, there is little possibility of altering the arcing conditions to compensate as the need for transfer dictate the conditions permissible, but for TIG welding the parameters can be varied for a time at the start and end of a run to help address the problem of potential defects. At the start of a weld run, the sudden establishment of a molten pool in surrounding cold plate can make the likelihood of porosity high. If welding is started at low current, the pool is correspondingly small and insufficient gas evolution occurs to create porosity. In the middle of a run, conduction has produced a heating effect that slows the freezing of the pool and gives time for the gas to escape. TIG sets are therefore usually designed to be able to start at very low current and build over a number of seconds to the full current required for the weld run. This is called slope up and there is usually a control to adjust the time over which the build-up of current will occur. Start porosity is further aggravated by the conditions of shielding gas flow. The torch and gas delivery line will be filled with air before welding commences so the first delivery of shielding to the arc area will be inert gas diluted with air and maybe water vapour. To purge this from the system welding equipment is usually designed to allow a pre-flow of shielding gas prior to striking of the arc. When the arc is extinguished, the molten pool is subjected to rapid freezing from its perimeter inwards. This can lead to insufficient liquid being available and the final pool may have a concave top surface – often called the crater. In some instances the lack of liquid results in cracks forming in a star shape in this crater, crater cracking. TIG power sources are usually able to step down the current over time resulting in a much smaller pool for final freezing where the problem of insufficient liquid feed may be eliminated, slope out or crater fill. Gas shielding is important during this final solidification after arc extinction so shielding gas flow should not cease when the arc is extinguished. A flow is usually maintained until the pool has cooled sufficiently that severe oxidation will not place. This constitutes a post-flow of gas that may also be controlled by a timer on the welding set.

8.5

Duty cycle Section 2 stated that some energy is lost as heat and heat is generated by passing current through a conductor according to the i2R effect. Pure copper has a lower resistance than most other conductors but it will still be heated by the effect. The amount of heat generated and lost partly depends on the design of the machine and many have in-built fans to give forced air cooling. There are temperature limits on most electrical components and, in the extreme, insulation can breakdown causing shorting and even catching fire, so usage of welding equipment is kept within the heat generation that can be adequately dealt with by loss to the environment.

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The balance between heating from the passage of power and cooling by loss to the surroundings is dependent on the power passing through the circuit and the length of time for which it passes – the higher the power, the less time for critical temperatures to be reached. This presents an issue in rating a power source. A machine may be capable of delivering 400A but suffer unacceptable heating levels after only a few minutes so does this have a 400A capability? The welding equipment manufacturing industry has standardised the rating of welding machines by use of duty cycle. The duty cycle is the number of minutes, out of ten, that a machine can be continuously operated at the power output claimed. The rest of the ten minutes is for the machine to be cooling under no load. This definition is used both in USA and European standard BS EN 60974-1. The common ratings are at 35% (ie 3½ minutes running, 6½ minutes cooling); 60% (6 minutes on, 4 minutes off) and 100% (continuous running). A plate must be fixed to the machine showing its rating for it to comply with BS EN 60974:

Most manufacturers comply with the European Standard and it is compulsory that the electrical safety of a power source complies with the requirements for CE marking of electrical equipment for it to be sold in Europe. Bibliography Whilst now looking rather dated, more can be learnt of the principles of power source design by reference to: L M Gourd: Principles of Welding Technology, Edward Arnold. ISBN 0340 61399 8 A C Davies: The Science and Practice of Welding Vol 1 & 2, Cambridge University Press. ISBN 0521 43403 3 and 0521 43404 1

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IWS questions on power sources 1

Describe the operation of drooping and flat characteristic power sources.

2

Why is the MIG arc referred to as self-adjusting?

3

Explain slope-up and slope-out and their use in TIG welding.

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Section 9 TIG Welding

9

TIG Welding

9.1

Process characteristics A number of manufacturers publish very good guides covering theoretical and practical aspects of TIG welding; one is available on-line from Miller at http://www.millerwelds.com/resources/TIGhandbook/. TIG welding is a process where melting is achieved by heating with an arc struck between a non-consumable tungsten electrode and the workpiece. An inert gas shields the electrode and weld zone to prevent oxidation of the tungsten electrode and atmospheric contamination of the weld and hot filler wire (as shown below).

TIG welding.

Tungsten is used because it has a melting point of 3370°C, well above any other common metal. In the US the TIG process is also known as gas tungsten arc welding (GTAW). 9.2

Arc initiation There are three ways of striking the arc in TIG welding. Simple sets, eg hobbyist attachments to MMA equipment, rely on scratch starting, essentially the same as for MMA: the electrode is stroked on the workpiece and slightly lifted clear. A short circuit current passes whilst the electrode is touching the workpiece and as the electrode is lifted the arc is established. This method is not favoured for intricate or quality work as the tip of the tungsten is liable to be melted and transferred to the initial weld pool. These defects, whilst not the most problematic, appear vividly on radiographic inspection films as white spots (on the negative) as tungsten is very much more opaque to X- or gamma-rays than the normal engineering metals being welded.

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A variant of this which has appeared since the advent of electronically controlled power sources is the lift-arc method, which also relies on touching the electrode to the metal but the electronics are set to reduce the short circuit current to only a few amps. Thus little i2R heating occurs and the tungsten tip is not melted. As the electrode is withdrawn from the workpiece and the arc length lengthens, the current is raised by the control mechanism to the working level. The most common method of arc initiation is by using a high frequency (HF) spark. Superimposition of high voltage, but very low current, HF creates a spark between the electrode and the workpiece that will initiate the welding arc and plasma formation. For DC welding the HF only acts during initial start-up but for AC welding with a sine wave output (traditional transformed mains) the HF is run continuously to allow re-ignition as the current and voltage pass through zero each half cycle. With electronically generated square wave AC, this is not necessary as the voltage is switched instantaneously to peak value. HF is only required for the initial start in square wave AC. 9.3

Current and polarity Current determines the degree of penetration and size of the weld pool and has to be within a range suitable for the size of tungsten electrode in use. If the welding current is too high, the electrode tip can overheat and melt, leading to tungsten inclusions, if too low, the electrode tip will not be properly heated and an unstable arc may result. TIG welding is normally carried out with the electrode connected to the negative output of the power source (DCEN). Heat is generated at the anode by the impingement of electrons as we saw in the section on Arcs and Plasmas. Stripping of electrons from the cathode cools the tip of the tungsten prolonging its life. Refractory oxides formed on the surface of metals such as aluminium or magnesium can hinder fusion and require removal during welding, achieved by having the workpiece as the cathode; the emission of electrons from the surface breaks up the oxide layer, effectively cleaning the weld pool. With a DC positively connected electrode (DCEP), heat is concentrated at the electrode tip and therefore the electrode needs to be of greater diameter than when using DC negative if overheating of the tungsten is to be avoided. A water-cooled torch is recommended if DC positive is used. With the distribution of heat being only 30% at the workpiece, penetration is shallow and the process is really only useful for thin sheet material. Helium shielding gas does improve the penetration but, if thick aluminium or magnesium is to be welded, the usual choice is AC.

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Clearly, AC will combine both DCEN and DCEP operation as the current flow switches direction at each half cycle.

Current type/polarity

DCEN

AC

DCEP

Weld profile

70% at work 30% at electrode Deep, narrow

30% at work 70% at electrode Shallow, wide

Cleaning action

No

Electrode capacity

Excellent (3.2mm/400A)

50% at work 50% at electrode Medium Yes – every half cycle Good (3.2mm/225A)

Heat balance

Yes Poor (6.4mm/120A)

The recommended safe working conditions for thoriated electrodes are summarised in a TWI FAQ (available to TWI Industrial Members) and in the HSE Information Document 564/6 (see below). If not practical to ensure such conditions, the use of ceriated electrodes is recommended as a health and safety improvement.  

HSE Information document: storage and use of thoriated tungsten 564/6. TWI FAQ The use of thoriated tungsten electrodes http://www.twi.co.uk/content/faq_thoriated.html.

Further information on health and safety is available by searching the HSE website. 9.4

Preparing the tungsten electrode

9.4.1

Tungsten types TIG electrodes may be 100% tungsten but more commonly have refractory or rare earth oxides incorporated. These different types of electrodes are used to suit different applications: Pure tungsten (W) Electrodes have by a green band, are cheaper than oxide-dosed ones but generally have a shorter life. Used when welding light metals with AC because of their ability to maintain a clean, balled end, but possess poor arc initiation and arc stability in AC mode compared with other types.

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Thoriated electrodes Have a yellow or red band and contain 1 or 2% respectively of thorium oxide (thoria) to improve arc initiation. Higher current carrying capacity than pure tungsten electrodes and maintain a sharp tip for longer. Unfortunately thoria is slightly radioactive (emitting  radiation) and the dust generated during tip grinding should not be inhaled. Electrode grinding machines used for thoriated tungsten grinding should be fitted with a dust extraction system. Ceriated electrodes Have a grey band in Europe but an orange one in the US, contain nominally 2% cerium oxide and have excellent arc starting on DC even at low current, often the choice for mechanised orbital TIG welding of thin pipework and other delicate operations. Lanthanated electrodes Have a black band, 1% lanthanum oxide, perform similarly to thoriated electrodes and since lanthanum is not radioactive, are often used as direct replacements for thoriated electrodes. Zirconiated electrodes Have a white band in Europe and a brown one in the US, are alloyed with 1% zirconium oxide. Operating characteristics of these electrodes fall between the thoriated types and pure tungsten. However, since they are able to retain a balled end during welding, they are recommended for AC welding. Also, they have a high resistance to contamination and so are used for high integrity welds where tungsten inclusions must be avoided. Tungsten electrode manufacturers offer recommended current ranges for the various diameters available. A rough guide for thoriated, ceriated or lanthanated electrodes on DCEN is: Current range, A 50-150 130-250 240- 400

9.4.2

Electrode dia, mm 1.6 2.4 3.2

Grinding tungsten electrodes The end of the tungsten is ground to a point to give a concentrated area for the creation of the cathode spot. As a general rule, the length of the ground portion of the tip of the electrode should have a length equal to approximately 2-2.5 times the electrode diameter. The vertex angle is not critical but does have an effect on the bead width and penetration. A sharper, narrower electrode angle gives a wider weld bead, easier arc starting and improved arc stability. A narrower electrode is for less amperage and gives less weld penetration and shorter electrode life. A blunter, wider tungsten electrode gives a narrower weld bead that is harder to start but can handle more amperage and will provide better weld penetration. There is increased potential for arc wander, but the electrode will last longer. The sharp tip of the electrode is usually removed by grinding a small flat, shown in the centre figure below, which lowers the likelihood of melting or spitting the tip into the pool.

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For AC welding use zirconiated tungsten electrodes with a hemispherical (balled) end (see below). The ball will form naturally during AC welding as the electrode is heated on the DCEP cycle, but it is usual to pre-form it to avoid loss of molten tungsten into the weld pool. To produce a balled end grind the electrode then initiate an arc on a cold copper block and increase the current until it melts the tip of the electrode.

Electrode tip vertex angle

Electrode tip with flat end

Electrode tip with a balled end

Tips on grinding tungsten electrodes:   

   

9.4.3

Use a dedicated grinder reserved for tungsten to avoid contamination of the electrode. Grinding wheels should be made of diamond or boron nitride. Grind longitudinally and concentrically so that the lines on the ground surface are in the same direction as the electrode and the electrode has no flat spots. Never grind tungsten electrodes on belt sanders or the sides of standard grinding wheels. Do not breath grinding dust; use an exhaust system when grinding radioactive thoriated tungsten electrodes. Wear approved safety gloves and glasses. Tungsten splinters easily and can penetrate the operator’s hands and eyes. Electrodes get hot when grinding so use an electrode grinding wand to minimise burns.

Cutting tungsten electrodes Tungsten alloys are dense, very brittle and can splinter or shatter, causing fractures in tungsten electrodes which can present a laceration hazard to the operator during cutting. Even if a poorly cut electrode is apparently correctly prepared, undetected fractures can lead to arc instability or break off during welding, creating gross weld defects. When you need to cut an electrode to a specific length or remove contamination from the tip, be sure to cut electrodes correctly. For a clean, smooth cut, use a diamond wheel with the electrode secured on both sides of the cut. Incorrect cutting methods damage the integrity of the tungsten alloys, shorten arc time and increase the potential for tungsten contamination in the weld.

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Do not:   

9.4.4

Bend electrodes until they fracture. Cut tungsten electrodes with wire cutters or pliers. Notch the electrode on the grinding wheel then snap it off.

Fixing the electrode in the torch The tungsten electrode is held in a copper contact tube or collet that tightens and holds the electrode as a screw thread attached to back cap of the torch is turned. The collet also makes an electrical contact so that welding current is passed to the electrode. The electrode extends beyond the bottom of the collet and this extension is an important variable. Because the contact tube is recessed inside the gas nozzle, this parameter can be checked indirectly by measuring the stickout length (length from the end of the nozzle to the electrode tip) as below:

Electrode extension Stickout

9.5

Shielding gas In TIG welding it is important to avoid oxidation of the tungsten electrode as well as the weld pool. Gases are therefore usually inert, with argon, helium or mixtures of the two being the most widely used. Nitrogen can be used when welding copper, but is too reactive on most engineering alloys. For austenitic stainless steels, nickel alloys and cupro-nickels, argon with up to 5% hydrogen may be used to improve penetration. Argon is denser than air, whereas helium is very much less dense which means a higher flow rate of helium is needed to give good shielding except when welding in the overhead position.

9.5.1

Flow rate Whatever the gas, it is important that sufficient flow is used to give adequate shielding to the pool and adjacent hot metal. A flow meter, such as a floating ball type should be used after the pressure regulator but the flow should also be checked at the torch. Simple floating ball gauges are available which can be pressed to the upturned gas nozzle to read the flow at the torch. This can be used in conjunction with a flow meter at the cylinder to ensure that there are no significant leaks in the hose system.

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The actual flow required depends on the welding configuration and position, current, polarity and gas composition. Too low and the shielding gas cannot remove the air from the weld area and this may result in porosity and contamination - excessive tarnishing of the weld bead, disturbance of the arc or oxidation of the tungsten are indicators that the flow is too low. If the gas flow rate is too high turbulence occurs at the base of the shielding gas column, air tends to be sucked in from the surrounding atmosphere and this may also lead to porosity and contamination. This is usually rather difficult to achieve but welding outside corners do present difficulties and it is recommended that lower flow rates are used for these joints. Shielding gas flow rates are typically in the range ~10 to ~12 l/min

Flow rate too low

9.5.2

Flow rate too high

Pre- and post-flow The purpose of both pre- and post-flow is to prevent contamination of both the weld pool and tungsten electrode by the surrounding atmosphere. When the torch is not in use, air will enter the system through the nozzle, moisture in the air can condense inside the nozzle and gas hose and then cause hydrogen and oxygen contamination during initial stages of the weld. The shielding gas pre-flow will clear air and moisture from the hose and torch thus preventing this contamination. Post-flow works a little differently: immediately after the welding arc is extinguished, the weld bead, filler rod and the tungsten electrode remain hot enough to cause a chemical reaction with oxygen in the atmosphere. The result of this oxidation is obvious because it causes the weld bead, filler rod and tungsten to turn black. Proper post-flow will prevent oxidation by shielding the hot electrode and weld area and by speeding up the cooling process. If a tungsten electrode has discoloured because of oxidation it must be removed and re-prepared to eliminate all trace of oxide.

9.6

Filler wires For many applications, it is possible to use the TIG process without filler, autogenous welding. Some applications, such as the mechanised welding of Calrod for electrical heating elements, achieve high speed by using multiple inline welding heads. For most applications, however, the parent plate composition does not produce satisfactory autogenous welds. Many compositions are crack sensitive when melted and refrozen, some like aluminium alloys, absorb hydrogen when liquid and expel it as porosity on freezing and many require additional deoxidation from elements like titanium that has been added to filler wire composition to give defect-free welds.

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Filler wire is usually added manually but it is possible to set up mechanised welding with motor-fed cold wire addition from a spool. Manual TIG filler is usually sold in 1m lengths, supplied in 5kg tubes. Suppliers mark each rod at either end to minimise confusion of material at a welding station. It is good practice to store filler away from the actual welding station releasing it specifically for the job in hand as it may be difficult to distinguish between individual rods of stainless steel and nickel alloy. When working with alloys sensitive to contamination (eg by grease, machining dust, etc), it is recommended to solvent clean all working surfaces and TIG rods before welding. BS EN ISO 636 is the international standard covering TIG filler wire composition. Potential defects As well as the defects normally associated with a manual process, such as lack of sidewall fusion, poor penetration control, etc, the TIG process presents a few particular potential problems. Tungsten inclusions Any fragments of tungsten that enter a weld will show up on radiographs, white on the negative image, because of the relatively high density of this metal. There appears little demonstrated effect of even quite large amounts of tungsten in either steel or aluminium TIG welds yet most inspections standards state that they are not acceptable so measures need to be taken to avoid the incorporation of tungsten particles in the weld pool. One of the principal reasons for small particles to break from the electrode is thermal shock which can occur as full current is applied to the cold tungsten at the initiation of arcing. Modern power sources have a current slope-up device to minimise this risk which allows the current to rise to the set value over a short period so the tungsten is heated more slowly and gently. Another significant reason for tungsten loss from the electrode is oxidation from imperfect gas cover - a further reason for the need of pre-flow purging of the gas lines and torch before starting the arc.

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9.6.1

Solidification cracking Some compositions are sensitive to solidification cracking. In ferritic and stainless steels and nickel alloys, it is usually impurities such as sulphur and phosphorus that cause the problem so filler wires are designed with manganese additions as this reacts with the impurities and forms higher melting compounds less likely to give solidification cracking. Stainless steels need the presence of a small percentage (~5%) of ferrite in the austenitic matrix to avoid solidification cracking, provided by careful selection of the filler composition. The amount of dilution and composition of the parent plate have to be taken into account and then the filler composition balanced to give the required ferrite level. Fortunately diagrams exist, after Schaeffler and De Long that assist in this estimation of composition. Aluminium alloys may be sensitive merely from the percentage of alloying element present and do not require the presence of impurities to create conditions for cracking. Fillers are therefore chosen for their ability to withstand freezing without cracking, eg the eutectic composition Al 12%Si is often used, but care must be exercised to ensure that dilution will not introduce incompatible elements such as Mg together with Si. If weld metal compositions are sensitive to solidification cracking, they are likely to show it when there is insufficient liquid to back-fill incipient cracks and when the strain from shrinkage during cooling is high. These conditions apply in the final crater as the arc is extinguished and a particular type of cracking, crater cracking, is a common form of solidification cracking. As the final crater solidifies, a star-shaped crack may be formed in its centre. Modern power sources have a current slope-out device so that at the end of a weld when the welder switches off the current it reduces gradually and the weld pool gets smaller and shallower, resulting in the final crater being sufficiently small that cracking does not occur.

9.7

Advantages of the TIG process 

  

  

Does not give weld spatter which makes it particularly suitable for applications that require a high degree of cleanliness (eg pipework for the food and drinks industry, semi-conductors manufacturing, etc). A good welder can avoid inclusions and achieve fusion easily producing superior quality welds. Enables welding variables to be accurately controlled and is particularly good for controlling weld root penetration in all positions of welding. Can be used with filler metal so can weld almost all weldable metals, including dissimilar joints and is especially useful in welding reactive metals with stable oxides such as aluminium, magnesium, titanium and zirconium. The heat source and filler metal additions are controlled independently so is very good for joining thin materials. On thin sections without filler, it can produce welds at relatively high speed. Very low levels of diffusible hydrogen so there is less danger of cold cracking in ferritic steels.

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9.8

Disadvantages of the TIG process      

Gives low deposition rates compared with other arc welding processes. Need for higher dexterity and welder co-ordination than with MIG/MAG or MMA welding. Less economical than MMA or MIG/MAG for sections thicker than ~10mm. Difficult to shield the weld zone fully in draughty conditions so may not be suitable for site/field welding. Tungsten inclusions can occur if the electrode contacts the weld pool. Does not have any cleaning action so has low tolerance of contaminants on filler or parent metals.

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IWS questions on TIG 1

Describe the methods of arc initiation used in TIG welding.

2

Why is AC power selected for welding aluminium?

3

What problems exist to the use of thoriated tungsten? What alternatives might you use?

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Section 10 MIG MAG Welding

10

MIG/MAG Welding

10.1

Process characteristics The MIG/MAG welding process is versatile and suitable for thin sheet and thick section components in most metallic materials. An arc is struck between the end of a wire electrode and the workpiece, melting both to form a weld pool. The wire serves as both the source of heat (via the arc at the wire tip) and filler metal for the joint and is fed through a copper contact tube (also called a contact tip) which conducts welding current into the wire. The weld pool is protected from the surrounding atmosphere by a shielding gas fed through a nozzle surrounding the wire. Shielding gas selection depends on the material being welded and the application. The wire is fed from a reel by a motor drive and the welder or machine moves the welding torch along the joint line. Wires may be solid, (simple drawn wires of appropriate composition) or cored, (composites formed from a metal sheath with a powdered flux or metal filling). Consumables are generally competitively priced compared with those for other processes and the process offers high duty cycle and therefore productivity, because the wire is continuously fed. It is known in the USA as gas metal arc welding (GMAW). The process is shown below.

Manual MIG/MAG welding is often referred to as a semi-automatic process as the wire feed rate and arc length are controlled by the power source, but the travel speed and wire position are under manual control. The process can also be mechanised, (all parameters under control so the power source and ancillary machinery) but may still require manual adjustment during welding, eg steering of the welding head and adjustment of wire feed speed and arc voltage. Some set-ups are described as automatic when there is no manual intervention during welding. The process usually operates with the wire positively charged (DCEP) and connected to a flat characteristic (constant voltage) power source. Selection of wire diameter (0.6-1.6mm) and wire feed speed determine the welding current as the burn-off rate of the wire will be in equilibrium with the feed speed as described in the section on power sources. The self-adjusting arc is a key feature of the process.

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The feed unit for the wire may be separate or incorporated into the body of the welding set.

The wire is pulled from the reel or drum and pushed through a liner along the cable assembly connecting the feed unit to the welding torch by a set of driven rolls. For solid wires, there is usually one grooved roll and a second flat roll on top. Cored wires, having less stiffness than solid wires, may require two grooved or even grooved and knurled rolls. There are also four roll systems and for fine soft wires, such as 0.8mm aluminium, a secondary drive motor may be mounted on the torch. This is termed a push-pull system.

The umbilical connection from the welding set to the torch carries three main supplies - the wire in a liner, shielding gas in a separate hose and a welding power lead. In the torch, the liner abuts a copper contact tip that is screwed into a gas diffuser. The contact tip receives welding power when a latching trigger switch is activated, which also operates the wire drive motor. As the wire passes through the tip it picks up the welding current supply. Shielding gas passes through the diffuser and into the space inside the welding nozzle from where it flows over the weld pool, see below.

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Contact tip Gas diffuser Torch body

Liner

Gas nozzle

A number of manufacturers offer guides covering practical and theoretical aspects of the process, one such from Lincoln Electric may be found at: www.content.lincolnelectric.com/pdfs/products/literature/c4200.pdf. Such guides are strongly recommended to the engineer requiring more detail of the process and its variants. Advantages of the MIG/MAG process:           

Continuous wire feed. Automatic self-regulation of the arc length. High deposition rate and minimal number of stop/start locations. High consumable efficiency. Heat inputs in the range 0.1-2.0kJ/mm. Low hydrogen potential process. Welder has good visibility of weld pool and joint line. Little or no post-weld cleaning. Can be used in all positions (dip transfer). Good process control possibilities. Wide range of application.

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Disadvantages:         

10.2

No independent control of filler addition. Difficult to set up optimum parameters to minimise spatter levels. Risk of lack of fusion when using dip transfer on thicker weldments. High level of equipment maintenance. Lower heat input can lead to high hardness values. Higher equipment cost than MMA welding. Site welding requires special precautions to exclude draughts which may disturb the gas shield. Joint and part access is not as good as MMA or TIG welding. Solid wire consumable not tolerant to base material surface contaminants. Flux cored wires may be employed as they can tolerate greater contamination.

Transfer modes

Figure 13.1 Arc character

Spray transfer Key characteristics     

Free-flight metal transfer. High heat input. High deposition rate. Smooth, stable arc. Used on steels above 6mm thickness and aluminium alloys above 3mm thickness.

In argon when the voltage is sufficiently high, >25V for a 1mm diameter wire and the wire feed speed is adjusted to give more than 250A, the welding arc burns continuously, metal melts from the wire and passes across the arc in a series of small droplets, called spray transfer. The droplet size is typically around 0.5-1 times the wire diameter and the arc burns in a stable manner while metal transfer, becomes almost continuous.

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The wire is the anode of the electrical circuit and electron impingement heats the wire rapidly to melting. As the current is raised, the anode spot increases in size, reaches the same diameter as the wire and starts to climb up its outside. The higher the current, the larger the cylinder of wire defining the anode spot. This leads to tapering of the wire tip as the melting occurs beneath the cylindrical area of anode spot so the effective wire diameter is much reduced as current is increased and the droplets formed are correspondingly smaller. Electromagnetic forces induced by the high current density pinch the molten droplets and project them across the arc. 2% oxygen is sometimes added to the argon shielding gas for spray transfer. This diatomic gas dissociates then recombines at the anode creating more heat and giving arc stability at lower currents. 5% CO2 also has a similar effect but if CO2 greater than 20% CO2 is used spray conditions cannot be established. Spray transfer gives a large weld pool that does not lend itself to positional welding or large runs with poor toughness if not properly controlled. For this reason, some company specifications will not allow the use of solid wire MAG for critical applications. The process is considered applicable for PA and PB positions. Globular transfer Key characteristics     

Irregular metal transfer. Medium heat input. Medium deposition rate. Risk of spatter. Not widely used in the UK; can be used for mechanised welding of medium thickness steels (typically 3-6mm) in the flat (PA) position.

When helium, CO2, or argon mixtures of these gases (CO2 levels higher than 20%) are used as shielding gases, spray transfer does not occur. The anode spot does not grow so remains a small area on the wire end. Melting of the wire commences but, with the small anode spot remaining beneath the droplet, there is no direct impingement of electrons on the outside of the wire. The droplet therefore grows by conduction until its size dictates that it detaches and drops to the weld pool primarily under the action of gravity.

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The severe disturbance to the arc during this process and fall of a large globule into the weld pool causes very considerable spatter. Techniques have evolved using lower voltage settings (<20V) and pushing the arc into the weld pool. The arc force depresses the pool so that the arc is burning in a hollow (buried arc technique), cutting down the amount of spatter emitted and also minimising the UV radiation. It is cited, mostly in American literature as a means of achieving high deposition with CO2 shielded MAG but is not widely used in Europe. Globular transfer is not suitable for positional welding and is typically used on larger diameter wires and high currents. Dip transfer Key characteristics:       

Metal transfer by wire dipping or short-circuiting into the weld pool. Relatively low heat input process. Low weld pool fluidity. Used for thin sheet metal above 0.8mm and typically less than 3.2mm, positional welding of thicker section and root runs in open butt joints. Process stability and spatter can be a problem if poorly tuned. Lack of fusion risk if poorly set up and applied. Not used for non-ferrous metals and alloys.

With voltage of 16-24V, shielding gas with less than 80% argon and current below 200A, the wire feed can be set so that the end of the wire touches the weld pool and short-circuits the system, dip transfer. These short-circuits can take place 20-200 times per second.

During the short, the wire heats rapidly and fuses so that molten metal is transferred to the pool after which the arc is re-established. This re-ignition is accompanied by spatter but adjusting the inductance of the system can give a degree of control over this. Inductance When MIG/MAG welding in the dip transfer mode, the welding electrode touches the weld pool, causing a short-circuit. During the short-circuit, the arc voltage is nearly zero. If the constant voltage power supply responded instantly, very high current would immediately begin to flow through the welding circuit. The rapid rise in current to a high value would melt the short-circuited electrode free with explosive force, dispelling the weld metal and causing considerable spatter. Inductance is the property in an electrical circuit that slows down the rate of current rise. Current travelling through an inductance coil creates a magnetic field. This magnetic field generates a current in the welding circuit that is in opposition to the welding current. Increasing the inductance will also increase the arc time and decrease the frequency of short-circuiting.

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For each electrode feed rate, there is an optimum value of inductance. Too little results in excessive spatter, too much and the current will not rise fast enough and the molten tip of the electrode is not heated sufficiently causing the electrode to stub into the base metal. Modern electronic power sources automatically set the inductance to give a smooth arc and metal transfer. Pulsed transfer Key characteristics:       

Free-flight droplet transfer without short-circuiting over the entire working range. Very low spatter. Lower heat input than spray transfer. Reduced risk of lack of fusion compared with dip transfer. Control of weld bead profile for dynamically loaded parts. Process control/flexibility. Enables use of larger diameter, less expensive wires with thinner plates, more easily fed (a particular advantage for aluminium welding).

As described in the section on power sources, pulsed power can be applied to MIG welding. In its simplest form, this consists of a period at a background current that maintains the arc but does not achieve metal transfer, followed by a period of high current during which spray transfer occurs. The average current is midway between background and peak and can be well below the threshold normally associated with spray transfer. This means that the pool size is relatively small and positional welding is possible, even though the transfer mechanism is spray. Pulsing the welding current extends the range of spray transfer operation well below the natural transition from dip to spray transfer. This allows smooth, spatter-free spray transfer to be obtained at mean currents below the transition level, eg 50-150A and at lower heat inputs. Pulsing was introduced originally for control of metal transfer by imposing artificial cyclic operation on the arc system by applying alternately high and low currents.

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A typical pulse waveform and the main pulse welding variables are shown in Figure 13.2. A low background current (typically 20-80A) is supplied to maintain the arc, keep the wire tip molten, give stable anode and cathode roots and maintain average current during the cycle. Droplet detachment occurs during a high current pulse at current levels above the transition current level. The pulse of current generates very high electromagnetic forces, which cause a strong pinch effect on the metal filament supporting the droplet; the droplet is detached and is projected across the arc gap. Pulse current and current density must be sufficiently high to ensure that spray transfer (not globular) always occurs so that positional welding can be used. Pulse transfer uses pulses of current to fire a single globule of metal across the arc gap at a frequency of 50-300 pulses. Pulse transfer is a development of spray transfer that gives positional welding capability for steels, combined with controlled heat input, good fusion and high productivity. It may be used for all sheet steel thickness >1mm, but is mainly used for positional welding of steels >6mm.

Figure 13.2 Pulsed welding waveform and parameters.

Synergic A normal MIG/MAG set requires a welder to set the wire feed speed (which dictates the current) and select an appropriate voltage to suit. The two variables are dependent on the wire diameter and gas used. This requires the welder/operator to have knowledge on the relationship between current and voltage. A synergic (non-pulse) set has a one knob dial that defines the wire feed speed. The microprocessor within the equipment will select the optimum voltage from a look up table (a synergic curve) to match the given current. The synergic curve has been developed to give the best possible settings for a particular current/wire feed speed. Now the welder is not responsible to select the right voltage. A trim button can be used, which allows the user to decrease or increase the voltage by a small percentage. The trim action allows the welder to make small correction in voltage to suit the variables at the work piece. Pulse Synergic equipment will make adjustments to the pulse parameters ie pulse height, width, frequency and background current based on the wire feed speed.

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10.3

Welding parameters The primary variables in MIG/MAG welding are: Welding current/wire feed speed. Voltage. Gases. Travel speed and electrode orientation. Inductance. Contact tip to work distance. Nozzle to work distance. Shielding gas nozzle. Type of metal transfer.

        

Wire feed speed Increasing the wire feed speed automatically increases the current in the wire. Too high current/WFS, without a subsequent rise in voltage, may lead to lack of fusion and cause stubbing; essentially this is where the WFS exceeds the voltageset. Wires are generally produced in 0.6, 0.8, 1.0, 1.2, 1.4 and 1.6mm diameter. Voltage Voltage is the most important setting in spray transfer as it controls the arc length. In dip transfer it also affects the rise of current and the overall heat input into the weld. An increase of both wire feed speed/current and voltage will increase heat input. The welding connections need to be checked for soundness, as any loose connections will result in resistance and will cause the voltage to drop in the circuit and will affect the characteristic of the welding arc. The voltage will affect the type of transfer achievable, but this is also highly dependent on the type of gas being used.

  

Increasing arc voltage Reduced penetration, increased width Excessive voltage can cause porosity, spatter and undercut

Figure 13.3 The effect of arc voltage. We refer to the voltage in relation to setting the desired transfer mode and it is one of the significant parameters for the welder to adjust his welding condition. For precision work, it is common to use a portable arc monitoring system (PAMS) that will record the parameters used but these are not very helpful for the welder when setting up.

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A number of manufacturers give advice on practical solutions to correct parameter selection. The actual value of voltage will depend on the shielding gas used but one of the simplest recommendations is to be found in the Miller Welding series of articles: www.millerwelds.com/resources/articles/MIG-GMAW-welding-basics It suggests that the welder reduces voltage setting until the arc stubs into the plate and then increases it until the arc is unstable. The correct setting is midway between these! Another key parameter when welding steel is the transition current between transfer modes for those gases that support both dip and spray. The following table gives approximate values for C-steel and stainless steel.

Material

Shielding gas

Ar ≥ 10%CO2 C-steel Ar +2%O2

Stainless steel

Ar +2%O2

Wire mm 0.8 0.9 1.2 1.6 0.9 1.2 1.6 0.8 0.9 1.2 1.6

dia,

Transition current, A 155-165 175-185 215-225 280-290 130-140 205-215 265-275 120-130 140-150 185-195 250-260

The welder does not directly set the welding current in MIG/MAG welding. His control is over the wire feed speed and this is proportional to the current. The relationship is not entirely linear but is sufficiently close that, over the normal welding range, the chart below gives a good approximation.

1.6

Welding Current A

1.2 0.9 0.8

0.8 0.9 1.2 1.6

Wire Feed Speed, m/min Selecting a wire feed speed that is in excess of optimum gives a wide bead with undercut at the edges. Too low current gives an uneven, lumpy bead with poor side fusion.

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Once the welder has established a good arc condition for the application, he then has travel speed and weave to give control over the bead shape and fusion. If travel speed is too rapid the weld bead will be narrow and convex with poor fusion at the sides, too slow overheats the material, giving a wide HAZ and high distortion combined with a flat wide bead. Travel speed The faster the travel speed the less penetration, narrower bead width and the higher risk of undercut

 

Increasing travel speed. Reduced penetration and width, undercut.

Figure 13.4 The effect of travel speed.

10.4

Contact tip and nozzle set-up The contact tip to workpiece distance (CTWD) has an influence on the welding current because of resistive heating in the electrode extension. In the section on Power Sources, we saw that a constant voltage power source attempts to maintain the same arc length on altering the torch to workpiece distance (the self-adjusting arc). Thus, moving the torch away from the workpiece, results in an increased extension of the wire from tip to arc. As the wire extension increases, so does the overall resistance of that length of wire. This leads to more heating of the wire by the i2R effect. So less welding current is necessary to achieve the equilibrium rate of burn-off. Long electrode extensions can therefore cause lack of penetration as the current is lower than anticipated. Conversely, welding current increases when CTWD is reduced. This provides the experienced welder with a means of controlling the current during welding, but can result in variable penetration in the hands of an inexperienced welder.

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Contact tip

Gas nozzle

Contact tip setback Electrode extension

Nozzle-towork (stand-off)

Arc length

Contact tip-towork distance

Workpiece As the electrode extension is increased, the burn-off rate increases for a given welding current due to increased resistive heating. Increasing the electrode extension, eg in mechanised applications, is therefore one method of increasing deposition rates, as the wire feed speed is increased to maintain the required welding current. Resistive (i2R) heating depends on the resistivity of the electrode, length of the electrode extension and wire diameter. The effect is therefore more pronounced for welding materials which have high resistivity, such as steels. The electrode extension should be kept small when small diameter wires are used to prevent excessive heating in the wire and avoiding the resulting poor bead shape. At short CTWDs, radiated heat from the weld pool can cause overheating of the contact tube and welding torch, leading to spatter adherence and increased wear of the contact tube. The electrode extension should be checked when setting-up welding conditions or fitting a new contact tube. Suggested CTWDs for the principal metal transfer modes are: Metal transfer mode Dip Spray Pulse

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The contact tip may be positioned in-line with the nozzle end, protruding beyond it or recessed inside the torch. This has an effect on gas shielding efficiency and on visibility and accessibility; so, a compromise is necessary. The following gives suggested settings for the mode of metal transfer being used. Metal transfer mode Dip Spray Spray (aluminium)

Contact tip position relative to nozzle 2mm inside to 2mm protruding 4-8mm inside 6-10mm inside

The purpose of the shielding gas nozzle is to produce a laminar gas flow to protect the weld pool from atmospheric contamination. Nozzle sizes range from 13-22mm diameter. The nozzle diameter should be increased in relation to the

Contact tip extension

Contact tip recessed

Electrod e extensio

Set up for Dip transfer

Electrod e extensio

Set up for Spray transfer

size of the weld pool, larger diameter nozzles are used for high current, spray transfer application and smaller diameter nozzles for dip transfer. The flow rate must also be tuned to the nozzle diameter and shielding gas type to give sufficient weld pool coverage. Gas nozzles for dip transfer welding tend to be tapered at the outlet of the nozzle. Joint access and type should also be considered when selecting the required gas nozzle and flow rate. Too small a nozzle may cause it to become obstructed by spatter more quickly and, if the wire bends on leaving the contact tube, the shielding envelope and arc location may not coincide.

Penetration Shallow Excess weld metal

Deep

Moderate

Maximum

Moderate

Figure 13.5 The effect of torch angle.

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10.5

Shielding gases and nozzles The purpose of the shielding gas nozzle is to produce a laminar gas flow in order to protect the weld pool from atmospheric contamination. Nozzle sizes range from 13-22mm diameter. The nozzle diameter should be increased in relation to the size of the weld pool. Therefore, larger diameter nozzles are used for high current, spray transfer application and smaller diameter nozzles for dip transfer. The flow rate must also be tuned to the nozzle diameter and shielding gas type to give sufficient weld pool coverage. Gas nozzles for dip transfer welding tend to be tapered at the outlet of the nozzle. Joint access and type should also be considered when selecting the required gas nozzle and flow rate. Use of too small a nozzle may cause it to become obstructed by spatter more quickly and, if the wire bends on leaving the contact tube, the shielding envelope and arc location may not coincide. Shielding gas composition plays an active role in the formation and properties of the arc and plasma and also affects the metal transfer characteristics in MIG/MAG and helps determine weld pool shape. A number of physical properties of gases create these welding differences. Ionisation energy and arc voltage Ionisation energy or ionisation potential, measured in electron volts (eV) or kJ/mol, determines how easily the shielding gas can form electrically conducting plasma. Helium has a high ionisation energy (~25eV) as does CO2, but argon is significantly lower at 14.7eV. So the voltage required to maintain an arc in Ar is significantly lower than for He or CO2 and this is reflected in the welding conditions. Higher arc voltage tends to give a wider plasma at the workpiece and so a wider weld. Argon shielded MIG typically gives a fairly narrow weld bead with a deep finger penetration. Helium gives a much wider, rounder bead shape. Carbon dioxide, being a multiple atom molecule, dissociates in the arc as well as ionising. On re-combination heat is released that increases the effective melting of the weld pool giving a deep and wide bead.

Ar

He

Ar-He

CO2

The addition of some helium to argon gives a more uniform heat concentration within the arc plasma and this affects the shape of the weld bead profile. Argon-helium mixtures effectively give a hotter arc and so are beneficial for welding thicker base materials and those with higher, thermal conductivity, eg copper or aluminium. For welding steels, all grades, including stainless steels, a controlled addition of oxygen or CO2 helps to generate a stable arc and give good droplet wetting. Because these additions react with the molten metal they are referred to as active gases hence the name (metal active gas) MAG welding is the technical term that is used when referring to the welding of steels.

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100%CO2 CO2 is a relatively low cost gas so is an attractive consumable. In practice it is oxidising and can also transfer carbon to the weld metal so is only applicable to welding ferritic steels. It cannot sustain spray transfer as the ionisation potential of the gas is too high so is normally restricted to dip transfer welding. As noted above, it is possible to use higher current with a globular transfer but this is not popular. 100% Ar Argon is inert but as noted above has sufficiently low ionisation energy to maintain a stable arc. This is, however, relative. MIG welding of non-ferrous alloys, eg aluminium, copper or nickel alloys, is acceptable under Ar shielding but the characteristics can be improved by using gas mixtures. Pure Ar shielding of aluminium benefits from the presence of oxide which helps to give a strong, deeply penetrating arc. Nickel alloys are notoriously sluggish and, together with copper alloys benefit from the addition of helium to the shielding gas. Ar/He mixtures Helium is more expensive than argon, making mixtures higher priced. The advantage of adding He to the shielding is the increased arc stiffness and greater heat transfer leading to a deeper, more rounded bead cross-section. Helium addition also increases the operating voltage giving a wider bead. Although pure He will not support spray transfer, addition of over 20% Ar produces stable spray conditions. The mixtures are fully inert so can be used on reactive metals such as titanium. Mixtures containing 70%Ar and 30%He are often selected for welding non-ferrous alloys but up to 75%He with 25%Ar is recommended for welding heavy sections as the high helium content gives much greater depth of penetration. Ar + 5 to 20%CO2 An Ar/CO2 mixture is a common shielding gas for spray transfer welding of ferritic steels. Oxygen may be present at around 2%. The percentage of CO2 depends on the type of steel being welded and the mode of metal transfer required. Ar + 5%CO2 is better for spray but 18-20%CO2 offers the prospect of operating both in spray and dip conditions. The welding arc and pool gain the benefit of both gases, ie good penetration with a stable arc and very little spatter in spray. In dip transfer with mixed gas the spatter is much reduced compared with 100%CO2. Industrial gas suppliers offer a range of gas mixtures that they claim are designed for particular steels and thickness ranges, but all are essentially argon rich with or without a small amount of oxygen and 525%CO2.

Increased extension

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Ar + 1 to 5% O2 The addition of oxygen acts in a similar way to CO2 in that it helps to give a strong stable spray arc. Carbon steels are often welded with 5%O2 as this gives a fluid pool that wets the sidewalls easily. This mixture is significantly oxidising and only suitable for carbon and C-Mn steels. Stainless steels may be welded with 1 or 2%O2 mixtures, preferred to CO2 containing mixtures to avoid carbon pick-up by the stainless steel. The 2% mix gives better wetting but does tend to produce oxide that appears as a black powder alongside the weld bead. A summary table of shielding gases and mixtures used for different base materials is given in below.

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Summary of shielding gas mixtures for MIG/MAG welding. Reaction Shielding behaviou Metal gas r Characteristics Carbon Argon-CO2 Slightly Increasing CO2 content gives hotter arc, steel oxidising improved arc stability, deeper penetration, transition from finger-type to bowl-shaped penetration profile, more fluid weld pool giving flatter weld bead with good wetting, increased spatter levels, better toughness than CO2. Minimum 80% argon for axial spray transfer. General-purpose mixture: Argon-10-15%CO2. Argon-O2 Slightly Stiffer arc than Ar-CO2 mixtures oxidising minimises undercutting, suited to spray transfer mode, lower penetration than ArCO2 mixtures, finger-type weld bead penetration at high current levels. General-purpose mixture: Argon-3% CO2. Ar-He-CO2 Slightly Substitution of helium for argon gives oxidising hotter arc, higher arc voltage, more fluid weld pool, flatter bead profile, more bowlshaped and deeper penetration profile and higher welding speeds, compared with Ar-CO2 mixtures. High cost. CO2 Oxidising Arc voltages 2-3V higher than Ar-CO2 mixtures, best penetration, higher welding speeds, dip transfer or buried arc technique only, narrow working range, high spatter levels, low cost. Stainless He-Ar-CO2 Slightly Good arc stability with minimum effect on steels oxidising corrosion resistance (carbon pick-up), higher helium contents designed for dip transfer, lower helium contents designed for pulse and spray transfer. Generalpurpose gas: He-Ar-2%CO2. Argon-O2 Slightly Spray transfer only, minimises oxidising undercutting on heavier sections, good bead profile. Aluminium, Argon Inert Good arc stability, low spatter and copper, general-purpose gas. Titanium alloys nickel, require inert gas backing and trailing titanium shields to prevent air contamination. alloys ArgonInert Higher heat input offsets high heat helium dissipation on thick sections, lower risk of lack of fusion defects, higher spatter, higher cost than argon.

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10.6

Solid wire consumables The wire is usually supplied layer-wound on a wire basket. Occasionally plastic spools are used.

Both methods feed well, though personal preference may be the cause of considerable debate amongst welders on the merits and shortcomings of wire winding! For heavy wire usage, especially for automated stations, drums of wire up to 350kg may be used.

To feed wire from these large packs without it twisting on exiting the welding torch, loading into the drum has to be done with a preset opposite twist. Smooth feeding is an essential part of MIG/MAG welding, especially mechanised or automated. Wire appearance is the most obvious differentiator to the welder but is not a good indicator of feeding characteristics. Solid C-Mn wires are traditionally copper-coated, variously thought to help feeding, improve current pick-up, slow contact tip wear and slow rusting of the wire in storage. It is difficult to prove any of these attributes. Bare wires became available in the 1980s and proved just as able to run on automated equipment. It seems that the important characteristics when considering feeding are:

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Cast Most easily demonstrated by pulling a metre of wire from the reel or drum and tossing it onto the floor. The diameter of the loop formed is the cast. If too small the wire has a tendency to rub the walls of the liner with some pressure and can give juddering during feeding. Helix The loop used to demonstrate cast also shows helix. If the loop is clipped to be a single circle and is hung over a horizontal bar, the offset between the ends is the helix. Excessive helix can give feeding issues, mostly with wear of the contact tip and wander of the wire tip and therefore arc across the bead. Lubrication Welding wires need a thin layer of lubricant to give efficient feeding through the liner. Fortuitously, drawn wire has a persistent film of oil left from the drawing process. Some manufacturers deliberately control the lubrication of the final stages of drawing and winding with a view to improving feeding. Stiffness This is more an issue between alloy types. All C-Mn steel wires are likely to be in a cold-drawn state. Some alloys are very difficult to draw to welding wire sizes and may be annealed just prior to final drawing. Aluminium alloys, even in a cold-drawn condition, will not rival steel for stiffness. These are notoriously difficult to feed through a welding torch and may need a plastic liner and even a two motor, push-pull feeding system. 10.7

Important Inspection Points/Checks When MIG/MAG Welding Welding equipment A visual check should be made to ensure the welding equipment is in good condition. Electrode wire The diameter, specification and quality of wire are the main inspection headings. The level of de-oxidation of the wire is an important factor with single, double and triple de-oxidised wires being available. The higher the level of de-oxidants in the wire, the lower the chance of occurrence of porosity in the weld. The quality of the wire winding, copper coating and temper are also important factors in minimising wire feed problems. Quality of wire windings and increasing costs (a) Random wound. (b) Layer wound. (c) Precision layer wound. Drive rolls and liner Check the drive rolls are the correct size for the wire and that the pressure is only hand tight, or just sufficient to drive the wire. Any excess pressure will deform the wire to an ovular shape, making the wire very difficult to drive through the liner, resulting in arcing in the contact tip and excessive wear of the contact tip and liner.

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Check that the liner is the correct type and size for the wire. A size of liner will generally fit 2 sizes of wire ie 0.6 and 0.8, 1.0 and 1.2, 1.4 and 1.6mm diameter. Steel liners are used for steel wires and Teflon liners for aluminium wires. Contact tip Check that the contact tip is the correct size for the wire being driven and check the amount of wear frequently. Any loss of contact between the wire and contact tip will reduce the efficiency of current pick. Most steel wires are copper coated to maximise the transfer of current by contact between two copper surfaces at the contact tip but this also inhibits corrosion. The contact tip should be replaced regularly. Connections The length of the electric arc in MIG/MAG welding is controlled by the voltage settings, achieved by using a constant voltage volt/amp characteristic inside the equipment. Any poor connection in the welding circuit will affect the nature and stability of the electric arc and is thus a major inspection point. Gas and gas flow rate The type of gas used is extremely important to MIG/MAG welding, as is the flow rate from the cylinder, which must be adequate to give good coverage over the solidifying and molten metal to avoid oxidation and porosity. Other variable welding parameters Checks should be made for correct wire feed speed, voltage, speed of travel and all other essential variables of the process given on the approved welding procedure. Safety checks Checks should be made on the current carrying capacity or duty cycle of equipment and electrical insulation. Correct extraction systems should be in use to avoid exposure to ozone and fumes. A check should always be made to ensure that the welder is qualified to weld the procedure being employed. Typical welding imperfections:    

10.8

Silica inclusions (on ferritic steels only) caused by poor inter-run cleaning. Lack of sidewall fusion during dip transfer welding thick section vertically down. Porosity caused by loss of gas shield and low tolerance to contaminants. Burn through from using the incorrect metal transfer mode on sheet metal.

Summary of solid wire MIG/MAG GMAW Equipment requirements        

Transformer/rectifier (constant voltage type). Power and power return cable. Inert, active or mixed shielding gas (argon or CO2). Gas hose, flow meter and gas regulator. MIG torch with hose, liner, diffuser, contact tip and nozzle. Wire feed unit with correct drive rolls. Electrode wire to correct specification and diameter. Correct visor/glass, all safety clothing and good extraction.

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Parameters and inspection points           

Wire feed speed/amperage. Open circuit and welding voltage. Wire type and diameter. Gas type and flow rate. Contact tip size and condition. Roller type, size and pressure. Liner size. Inductance settings. Insulation/extraction. Connections (voltage drops). Travel speed, direction and angles.

Typical welding imperfections   

Silica inclusions. Lack of fusion (dip transfer). Surface porosity.

Advantages and disadvantages Advantages High productivity Easily automated All positional (dip, pulse and FCAW) Material thickness range Continuous electrode 10.9

Disadvantages Lack of fusion (dip transfer) Small range of consumables Protection for site working Complex equipment High ozone levels

Flux-cored arc welding In the mid-1980s the development of self- and gas-shielded FCAW was a major step in the successful application of on-site semi-automatic welding and has also enabled a much wider range of materials to be welded. The cored wire consists of a metal sheath containing a granular flux. This can contain elements which normally used in MMA electrodes so the process has a very wide range of applications. Gas producing elements and compounds can be added to the flux so the process can be independent of a separate gas shield which restricts the use of conventional MIG/MAG welding in many field applications. A further advantage is the increased deposition rate compared with solid wires. The core tends to be non-conducting and with metal cored wires the resistivity of the powder is much higher than solid metal, so, in essence, the current is carried by the sheath. This has a smaller cross-sectional area than solid so, at the same amperage, the current density is higher.

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Higher current density creates faster burn-off, so more material is transferred in unit time. Manufacture There are two main methods of producing cored wire. The main method starts with a strip of metal rolled to a U shape, filled with powdered flux or metal, closed to a tube then drawn to size. Wires are usually supplied in sizes 0.82.4mm diameter.

Manufacturers have worked with several forms of closure - overlaps, interlocking edges, etc, but insufficient advantage was found for these forms to become common. The usual closure simply butts the edges together. There is some concern that the seam can present access for moisture to enter the flux but in practice this is very unlikely. What is a potential problem is the use of drawing compounds (soaps) as the tube is reduced to final size. These can easily be squeezed into the tube during the drawing process and, being hydrogen containing, can be a source of potential hydrogen during welding. Techniques involving baking the wires partway through manufacture and finishing with lightly lubricated diamond dies have been developed to counter this. Modern cored wires can easily reach <5ml H2/100g.

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The second filling method overcomes this issue as the wire is seamless. A long coil of seamless tube is mounted on a vibrating pad and powdered flux is poured into one end whilst the coil is vibrated to encourage the filling to move through the coil and form a central core with no voids. Once filled, the coil is drawn to size, but, as there is no seam, die lubrication can be similar to that for solid wire. These wires can reach very low hydrogen levels. The method requires very careful control on particle size and shape to avoid segregation during filling. Cored wires are available in all the packages used for solid wire - layer or nearlayer wound reels are most common, but loose coils, drums and Marathon Pac style bulk supplies are all used. Types of wire Wires are described by the type of core with the two main categories being gas and self-shielded. Gas-shielded flux compositions are formulated for weld composition, arc characteristics, positional welding ability and mechanical properties. Self-shielded wires have the additional attribute of creating gasshielding in a similar fashion to MMA electrodes. There is a finite space within the core of a wire and if self-shielding is a feature the possibilities for compositional and mechanical property control are more limited than for gasshielded wires. Nonetheless, self-shielded wires may be as diverse as 55%Ni-45%Fe for cast iron welding and all-positional, high toughness C-Mn-Ni steel for offshore jacket construction. Gas-shielded wires are common in three alloy groups – ferritic steels for general and high mechanical property applications, stainless steels and hardfacing alloys. All may be formulated in one of three fluxing systems: Rutile Give good bead shape and wide ranging positional capabilities. Basic Excellent positional capability and mechanical properties, but less smooth bead shape and poorer slag release than rutile types. Metal cored Very little fluxing, designed for higher productivity, some having excellent root run capabilities. Note: Unlike MMA electrodes, the potential hydrogen levels and mechanical properties of welds with rutile wires can equal those of the basic types.

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10.10

Process variants Electrogas A vertical butt welding process for Carbon steel that resembles electroslag welding (see Submerged Arc Welding) but uses MIG/MAG welding principles. It is appropriate for thick plate and completes the joint in a single pass. The parent plates have no edge preparation, are aligned with a gap of around 2540mm. They are clamped into position resting on a small steel start pad. Water-cooled copper shoes are clamped either side of the gap to make a rectangular shaped well at the bottom of the plates. Any small areas with imperfect fit are packed with ceramic putty to give a receptacle that will hold molten metal.

Parent plate

Water-cooled shoe

Start pad

A modified MIG torch is used to blow inert gas into the well and feed wire to the bottom of the well where the arc is initiated. As the weld pool grows and fills the gap between the parent plates, the torch is mechanically slowly withdrawn allowing the bottom of the pool to solidify and the weld to progress slowly up the gap. As the molten pool approaches the top of the water-cooled shoes, a second pair is attached above the originals. Once the weld has solidified above the top of the first pair, they are removed and placed above the working set. Electrogas welding is an efficient method of making large vertical welds in thick plate but the mechanical properties are limited. The weld bead is hot for a very long time so microstructures are near equilibrium – ferrite and pearlite in hypoeutectoid steels – giving little flexibility to optimise toughness etc. Tandem wire A method of increasing deposition by using two wires, each with its own power supply, running into a single weld pool. Some manufacturers offer a special torch with two electrically isolated contact tips within a single gas nozzle. The arrangement seeks to provide faster travel speed and therefore improved productivity.

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The wires are arranged one behind the other creating a very elongated weld pool. As there are separate controls, it is possible to run both arcs in the spray condition, one spray and one pulsed, or both in pulsed mode. In pulsed mode the two wires are alternately pulsed to avoid magnetic interaction between the two arcs. The twin spray technique may be used for welding thick material requiring deep penetration. The twin pulsed condition allows very high speed welding of sheet material. Due to the two arcs operating simultaneously the level of UV radiation emitted is very high, combined with being heavy and difficult to manoeuvre and the process requiring high travel speeds means that it is almost exclusively used as a mechanised process. Controlled short circuit transfer With modern power sources it is possible to detect small changes to condition instantly and apply a correction to the current waveform. Several manufacturers make use of this for low current applications. Examples are Lincoln Electric’s Surface Tension TransferTM (STT) system and the Fronius cold metal transfer (CMT) system, which also uses a synchronised pulsed wire feed to aid droplet detachment. This attempts to control dip transfer to achieve consistent controlled metal transfer without spatter. A background current produces a molten end to the wire which grows until it touches the surface of the pool as is normal in dip transfer. Immediately the short circuit is recognised by the software a high current is applied to create the pinch effect normally associated with spray transfer. This necks the droplet at the solid wire interface. This is detected by the system and at this point, near detachment of the droplet, the current is lowered to below background level so that the droplet collapses into the pool with no violent recreation of the arc as is in dip transfer. The system immediately applies a high peak current to re-establish the arc and commence wire melting once more. After a short time the current is slowly decayed back to the background level and the cycle commences once more.

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IWS Questions MIG/MAG 1

Explain the options available for shielding gas and how you would choose the right gas for a particular application.

2

Why can’t you weld vertically with a spray condition? What would you choose instead?

3

What is a push-pull gun and when is it used?

IWT Questions MIG/MAG 1

Explain why there are different transfer modes in MIG/MAG. Give some examples of when you would choose one over another.

2

What factors influence the ease of feeding wire? Comment on both equipment and consumable factors.

3

Why might you use cored wire consumables for MIG/MAG welding?

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Section 11 Manual Metal Arc (MMA) Welding

11

Manual Metal Arc (MMA) Welding

11.1

History Manual metal arc welding (MMA) had its origins in the last decade of the 19th century as experimenters tried using metal rods to replace the hand-held electrode in the recently invented Bernados carbon arc process. Originally the metal rod was bare, with no flux coating to give a protective gas shield so welds were poor quality with gross porosity and oxide inclusions. Improvements were made by dipping the rods into a lime wash but it was not until the early 1900s, when the Kjellberg process was patented in Sweden that coated electrodes appeared and almost simultaneously, the Quasi-Arc method was introduced in the UK. MMA welding is the most versatile of the welding processes and is suitable for welding most non-ferrous metals as well as steels, over a wide range of thicknesses, can be used in all positions, with reasonable ease of use and relatively economically. The final weld quality is primarily dependent on the skill of the welder. The process was for many years the most common but has been overtaken in the last twenty years by MIG/MAG, especially as power source control and pulsed power have developed. Some materials, like aluminium, magnesium and titanium, are rarely welded by MMA now and the usage of stainless steel MMA is declining in favour of MIG with solid or cored wires.

11.2

Process characteristics The electrodes are produced in lengths, usually 300-450mm long and the core wire diameter is typically 2-6mm with a flux covering that might double the overall diameter. However, variants of the process over the years have used electrodes well outside these ranges. When an arc is struck between the coated electrode and workpiece, both melt to form a weld pool. The temperature of the arc is reported to be a minimum of 6000°C, sufficient to melt the parent metal, consumable core wire and flux coating simultaneously. The flux forms gas and slag which protect the weld pool from oxygen and nitrogen in the surrounding atmosphere. The molten slag solidifies and cools and must be chipped off the weld bead once the weld run is complete (or before the next weld pass is deposited where multi-run welding is necessary). The process allows only short lengths of weld to be produced before a new electrode needs to be inserted in the holder. The presence of the slag changes the simple principles of anode heating and cathode cooling explained in the section on Arcs and Plasmas. In general, DCEP results in deeper penetration and DCEN has a higher burn-off for a given current resulting in better deposition rate.

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Electrode angle 75-80o to the horizontal Consumable electrode Filler metal core Flux coating Direction travel Solidified slag

Arc

of

electrode

Gaseous shield Molten weld pool

Parent metal Weld metal The manual metal arc welding process.

A wide range of alloying can be achieved by additions to the flux coating. Many steel electrodes have the same low C, low Mn steel core wire and flux additions produce the high toughness, higher Mn weld metal. For more information on MMA see

http://www.twi.co.uk/content/tec_index.html

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11.3

MMA basic equipment requirements

1

10

2

9

3 8

4

7

6

1 2 3 4 5 6 7 8 9 10

5

Power source transformer/rectifier (constant current type). Holding quiver (holds at temperatures up to 150°C). Inverter power source (more compact and portable). Electrode holder (of a suitable amperage rating). Power cable (of a suitable amperage rating). Welding visor (with correct rating for the amperage/process). Power return cable (of a suitable amperage rating). Electrodes (of a suitable type and amperage rating). Electrode oven (bakes electrodes at up to 450°C). Control panel (on/off/amperage/polarity/OCV).

In the chapter on power sources it said that MMA requires a constant current power source so that the unsteadiness of the welder’s hand has only a limited effect on the current and thus the fusion characteristics.

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As the process can be used DCEN, DCEP or AC, all types of power source are used for MMA. Inverter sets are very popular as they give a wide range of current from lightweight, portable units. MMA is still used extensively for site work as it can operate successfully from engine-driven generators. When used in manufacturing, large transformer or transformer rectifier sets are used, often running several operators from the same machine. 11.4

Electrode types As described in Section 1 History, there are many types of electrode coating and manufacturers are prone to claiming particular attributes for their formulae, but they can be grouped into types.

11.4.1 Cellulosic A strong arc action and give deep penetration, AWS E6010 types are DC operating and E6011 run on AC. The gas shield generated is principally hydrogen which gives good protection but high diffusible hydrogen in the weld metal and HAZ. Cellulosic coatings are only used on carbon and C-Mn steels and are noted for their ability to weld in the vertically down position known as stovepiping. In fact E6010 electrodes are often known as stovepipe rods. 11.4.2 Rutile The coating of rutile electrodes has a high proportion of titanium dioxide. AWS type E6012 electrodes are DC operating and E6013 run on AC. Early rutile electrodes for steel were for welding in the flat position. They have fluid slag that solidifies just after the metal giving a smooth bead surface and easy slag removal. E6013 electrodes may be for welding in the flat position, but many followed the lead of Murex Welding’s Vodex (vertical, overhead, downhand plus – ex from Murex) in offering all-positional capability. E6013 electrodes remain the welders’ choice for general purpose welding having a smooth arc action and good slag release. 11.4.3 Rutile high recovery The addition of significant proportions of iron powder to a rutile coating has advantages. The recovery is much greater so more weld metal is laid at the same current. The coating is much thicker so forms a deeper cup in which the arc burns, this is sufficiently recessed to allow the end of the coating to be rested on the workpiece without risk of extinguishing the arc, making guiding the rod easier, even novices can handle this touch welding technique. The slag is also readily released, sometimes self-releasing behind the welder as he progresses along the joint. The downside is that these rods can only be used in the flat position, but for flat butt or fillet work, these AWS E7024 electrodes are a good choice. Manufacturers offer rods with 150-180% recovery, though some have tried up to 240%. Recovery is calculated as:

Recovery % 

weight of weld metal x 100 weight of core wire used

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11.4.4 Basic The original coatings applied to electrodes by Kjellberg were basic coatings, little more than ground limestone and clay bound by silicate, but these ingredients are still used today. Electrodes classified as E7015 in the AWS system were the first modern basic rods, are for DC operation and have generally been superseded by E7016 or E7018 types that can operate on AC and DC. The main difference between E7016 and E7018 electrodes is the iron powder content in the latter. Both give good mechanical properties, especially toughness and low hydrogen weld metal. 11.5

Setting up for welding As MMA electrodes produce a slag, it is often assumed this will take care of any oxide on the surface or inclusions in the material. This may be true to an extent but there is no substitute for correct preparation prior to welding. All materials, including mild steel, should be ground, brushed or otherwise cleaned of oxide from the joint area. Edge preparations must be cut for all but the thinnest butt welds. Straight sided V preparations are set up with an included angle of 60-70° for ferritic steels, 70-80° for stainless and copper alloys and 90° for nickel alloys. Good connection between the workpiece and the earth return to the power source is essential for MMA welding. If the current route is changing across the workpiece, a DC arc may be deflected; residual magnetism in the material may give similar deflection. This arc blow is at its worst when depositing root runs in magnetic material where each plate forms a magnetic pole along its edge so the preparation has a highly confused magnetic field that deflects the arc very significantly. As the root run is laid, a metallic bridge is formed that removes the effect of the poles but the quality of the run may have been compromised before this happens.

11.6

Welding parameters

11.6.1 Current (amperage) The current range required depends on the diameter and type of electrode. Always follow the manufacturer’s recommendation as variation can occur as shown in the table below which cites a few specific electrodes.

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If the current selected is higher than recommended, the electrode will overheat and towards the end of the run may begin to glow red. Weld quality is affected by incorrect current settings: 



Current too low Poor fusion or penetration, irregular weld bead shape, slag inclusions, unstable arc, porosity. Current too high Excessive penetration, burn through, undercut, spatter, porosity, deep craters, electrode damage due to overheating, high deposition making positional welding difficult.

11.6.2 Voltage For current to flow through the circuit there needs to be a potential difference or voltage (V). For MMA welding the voltage required to initiate the arc is called the open circuit voltage (OCV), which is the voltage measured between the output terminals of the power source when no current is flowing through the welding circuit. For safety reasons the OCV should not exceed 100V and is usually between 50-90V. Immediately the arc is established a working (arc) voltage of 20-30V is adopted. Arc voltage is a function of arc length which with MMA is controlled by the welder. Arc voltage controls weld pool fluidity. The effects of the wrong arc length and therefore arc voltage can be: 

Arc voltage too low Poor penetration, electrode stubbing, lack of fusion defects, slag inclusions, unstable arc condition, irregular weld bead shape.



Arc voltage too high Excessive spatter, porosity, arc wander, irregular weld bead shape, slag inclusions, fluid weld pool making positional welding difficult.

11.6.3 Travel speed Travel speed (S) is the rate of weld progression, the third factor that affects heat input and therefore metallurgical and mechanical conditions. Potential defects associated with incorrect welding speeds when using the MMA welding process are: 

Travel speed too fast Narrow thin weld bead, fast cooling, slag inclusions, undercut, poor fusion/penetration, insufficient heat input giving high hardness structures.



Travel speed too slow Cold laps, excess weld deposition, irregular bead shape, undercut, excess heat input making the development of high toughness impossible.

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11.6.4 Heat input Heat input is a calculation of the total energy passed into the weld bead in unit time. It is calculated as: i.V.k  kJ  Heat input     mm  S.1000

Where: I V S K

= = = =

current in Amps. voltage in Volts. travel speed in mm/sec. thermal efficiency factor.

The thermal efficiency factor is taken as 0.8 for MMA, MIG/MAG and FCAW, 0.6 for TIG and plasma and 1 for submerged arc. 11.6.5 Polarity (type of current) Polarity affects the distribution of heat differently in MMA than in MIG or TIG, presumed to be because of the effects of the compounds in the flux. There is evidence that DCEN can result in higher melting rates in MMA, completely different from MIG or TIG. The preferred polarity of the MMA system depends upon the electrode being used and the desired properties of the weld. Manufacturers have developed coating systems that stabilise the arc in AC, DCEP or DCEN. Many electrodes work on more than one polarity and some work successfully on all three. 11.7

Practical aspects of MMA These paragraphs are not attempting to give tuition in the practice of MMA welding but to look at the techniques available and their effect on weld quality.

11.7.1 Stringer or weave As well as controlling the run-out length by moving his hand faster or slower, the welder can make a slight lateral, side-to-side motion. This weaving can be useful as the welder briefly points the electrode tip at the sidewall thus assisting fusion but it means that the run-out is shorter so heat input is higher. Heat input dictates the cooling rate of the weld bead and for ferritic steels, the transformation products. To develop the best toughness requires low heat input. Weaving slows the cooling rate and tends to lead to larger grained microstructure with poorer toughness and yield strength. Running the weld bead in a straight line along the preparation is called stringer bead technique and can achieve lower heat input per unit length. It is possible to lay stringer beads at heat input that is too low resulting in the formation of martensite in ferritic steel with a consequent loss of toughness. 11.7.2 Multi-pass or block welding In a butt weld in thick material a weld bead laid with typical parameters is not going to fill the groove. The welder can attempt to move more slowly allowing the metal to build but in the flat position there is a limit to how much fill can be achieved in a single pass.

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If the butt weld is in the vertical position the welder can work a triangular weave into the root along one edge of the preparation, out along the other, then across the face. In this block welding manner fewer runs are needed to fill a thicker section joint. As the deposition rate of an electrode is controlled by welding current (amps) the volume of metal deposited over a given time hence the joint completion time, will be virtually the same regardless of whether a weave or stringer technique is used. Some reduction in time may be achieved by weaving as fewer runs means that less time spent in inter run cleaning. Block welding creates very high heat input with correspondingly poor mechanical properties and is not recommended for quality work and is often banned as a technique. Some specifications place a limit on weave width to avoid overly thick, near block welding. The usual technique for filling deep and wide grooves is multiple layers - multipass welding requiring full removal of slag from underlying beads. If the lower bead has been laid with a convex profile, it is possible for slag to be trapped in the toes which needs removal by grinding and brushing before another layer is laid otherwise there is a strong possibility of leaving a string of slag inclusions. Multi-pass welding can result in excellent mechanical properties as each bead gives an amount of heat treatment to the one below which can give areas of very fine-grained recrystallised material with high toughness. 11.7.3 Skip welding or back stepping A technique used to minimise distortion, particularly when welding thin material with long lengths to be completed. A very short, 30-50mm weld is made then the welder moves maybe 150mm along the seam and lays another short run. This continues until the end of the seam is reached. He then returns to the start and makes further 30-50mm welds in the gaps and repeats the procedure until the seam is complete. The disadvantage is that it requires a large number of starts and stops, the areas most prone to defect formation like porosity or solidification cracking. 11.7.4 Preheat When welding ferritic steels you have to guard against hydrogen diffusing through the weldment and inducing cracking, one method is to apply preheat to slow the cooling rate of the weld bead, giving the hydrogen time to be released. 11.7.5 Interpass When multi-pass welding it may be necessary to avoid heat build-up as excessive heating of the weld metal can lower its strength and reduce toughness, so a maximum interpass may be specified. If preheat is applicable to the situation, this still applies in a multi-pass weld, there may be a minimum interpass temperature (equivalent to the original preheat) and a maximum.

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11.7.6 Operating factor Because the electrodes are lengths of coated wire the welder cannot keep the arc burning indefinitely – he needs to change rods, deslag the weld bead, grind any imperfections, may be required to check and observe interpass temperatures and/or call an inspector to check his work and on long runs will need to reposition himself for ease of operating. All of which reduce the amount of time weld metal is being deposited. The percentage of arc time to total time is called the operating factor and for MMA this is rarely above 30% and for site work of heavy fabrications it is often about 15%. MIG/MAG can achieve 60% and fully automated welding may reach 90% on jigs with simple loading and unloading. Operating factor is sometimes referred to as duty cycle but this is confusing as that term is used for power source capability as described in the section on power sources. 11.8

Storage and handling MMA electrodes are packed in a variety of forms, most common is packaging of around 5kg of electrodes in thin card packs wrapped in polyethylene film, adequate protection in storeroom conditions but permeable to moisture so deterioration can take place in wet environments. Electrodes in packs should always be stored in warm dry stores and manufacturers give specific recommendations on their packages and data sheets. Electrodes that have become damp can be returned to expected performance as noted in 11.9 Baking electrodes, but if the flux has become discoloured or pieces have broken away, the rods should be discarded. For pipeline welding, packaging in tins is a favourite as the hermetic seal gives long-term protection. Cellulosic electrodes are often packed in up to 25kg amounts in tins and may be used directly from such packaging and do not need further drying. Basic electrodes are available from many manufacturers in vacuum packs, packed under careful control directly from the baking oven as they are manufactured and hermetically sealed under vacuum so there is no possibility of moisture pick-up. Manufacturers offer a guarantee of low hydrogen performance straight from the package and may give an exposure that is permissible after opening the package whilst still maintaining adequate hydrogen control but once this exposure is exceeded, the rods should be discarded.

11.9

Baking electrodes The oven and quiver shown in the photograph of welding equipment are required for electrodes where moisture is a problem, usually basic electrodes for use on ferritic steels liable to hydrogen cracking. When aluminium was welded with MMA, it was necessary to dry such rods but Al alloys are now exclusively welded by MIG or TIG. Copper alloys are usually no problem but stainless steel and nickel alloys need dry electrodes to avoid porosity formation. Cellulosic Should not be dried as they rely on a hydrogen atmosphere to create the shielding and should be used directly from the manufacturer’s packaging. If electrodes have been left exposed and become soaked they should be discarded.

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Rutile Also require an amount of moisture in them to run correctly as dried rutile rods have a very poor arc action and shielding. If rutile rods become inadvertently wet they can be returned to condition by holding for an hour at around 80°C. Some texts have suggested 120°C but excessive time at such a temperature can easily over-dry the flux. Basic These coatings produce CO and CO2 as the limestone in their formulation breaks down under heating. These gases generate good shielding and arc force and do not require hydrogen or moisture. These can be baked totally dry and in manufacture they may be produced at 450°C, so temperatures up to this may be used to restore them. To keep them in good condition after baking in an oven, they may be held in a heated quiver beside the welder and used directly from this.

Vacuum-packed basic Basic electrodes can now be put into hermetically sealed vacuum packs by directly after baking by the manufacturer. With help from the formulation, using silicates with a low tendency to absorb moisture, these electrodes do not need baking to achieve low hydrogen levels. Manufacturers now offer guarantees that, at known humidity and temperature, vacuum-packs may be opened at the start of a shift and the electrodes used throughout that shift without the need to bake. 11.10

Electrode classification Many national and international specifications cover MMA electrodes and the detail in them is too much to be covered here so the student is advised to seek the relevant specifications directly from the national standards office. TWI published a series of Job Knowledge articles that make excellent additional reading to these notes and the article on BS EN and AWS systems (www.twi.co.uk/content/jk84.html ) is well worth studying.

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IWS Revision questions 1

Describe the basics of the MMA process.

2

Why are inverter power source finding increasing favour for MMA?

3

What types of MMA consumable are available for all-positional welding and which gives the lowest weld metal hydrogen level?

4

What is arc blow and how do you deal with it?

5

How should you control multi-pass weave welding?

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Section 12 Welding Consumables

12

Welding Consumables Welding consumables are defined as all that is used up during the production of a weld. This list could include all things used up in the production of a weld, however, we normally refer to welding consumables as those items used up by a particular welding process. These are: Wires

Fluxes

Gases

E 8018

Electrodes

SAW FUSED Flux

When inspecting welding consumables arriving at site it is important that they are inspected for the following:    

Size. Type or specification. Condition. Storage.

The checking of suitable storage conditions for all consumables is a critical part of the welding inspector’s duties.

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12.1

Consumables for MMA welding Welding consumables for MMA consist of a core wire typically between 350 and 450mm length and 2.5-6mm diameter. Other lengths and diameters are available. The wire is covered with an extruded flux coating. The core wire is generally of low quality rimming steel as the weld can be considered as a casting and therefore the weld can be refined by the addition of cleaning or refining agents in the flux coating. The flux coating contains many elements and compounds that all have a variety of jobs during welding. Silicon is mainly added as a de-oxidising agent (in the form of ferro-silicate), which removes oxygen from the weld metal by forming the oxide silica. Manganese additions of up to 1.6% will improve the strength and toughness of steel. Other metallic and non-metallic compounds are added that have many functions, some of which are:        

Aid arc ignition. Improve arc stabilisation. Produce a shielding gas to protect the arc column. Refine and clean the solidifying weld metal. Form a slag which protects the solidifying weld metal. Add alloying elements. Control hydrogen content of the weld metal. Form a cone at the end of the electrode, which directs the arc.

Electrodes for MMA/SMAW are grouped depending on the main constituent in their flux coating, which in turn has a major effect on the weld properties and ease of use. The common groups are: Group Rutile

Constituent Titania

Shield gas Mainly CO2

Basic

Calcium compounds Cellulose

Cellulosic

AWS A 5.1 E 6013

Mainly CO2

Uses General purpose High quality

Hydrogen + CO2

Pipe root runs

E 6010

E 7018

Some basic electrodes may be tipped with a carbon compound, which eases arc ignition.

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The electrode classification system of EN 499. EN ISO 2560 2005 (supersedes BS EN 499 1994) Classification of Welding Consumables for Covered Electrodes for Manual Metal Arc (111) Welding of Non-alloy and Fine Grain Steels. This standard applies a dual approach to classification of electrodes using methods A and B as is indicated below: Classification of electrode mechanical properties of an all weld metal specimen: Method A: Yield strength and average impact energy at 47J Example

ISO 2560 – A – E XX X XXX

X

X

X

Mandatory designation: Classified for impacts at 47J + yield strength Covered electrode Minimum yield strength Charpy V notch minimum test temperature °C Chemical composition Electrode covering Optional designation: Weld metal recovery and current type Positional designation Diffusible hydrogen ml/100g weld metal

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Typical example:

ISO 2560 – A – E 43 2 1Ni RR 6 3 H15

Method B: Tensile strength and average impact energy at 27J Example

560 –

B –E XX XX XXX X X HX

Mandatory designation: Classified for impacts at 27J + tensile strength Covered electrode Minimum tensile strength Electrode covering Chemical composition Heat treatment condition Optional designation: Optional supplemental impact test at 47J at same test temperature given for 27J test Diffusible hydrogen ml/100g weld metal Typical example: ISO 2560 – B – E 55 16 –N7 A U H5 Classification of tensile characteristics Method A Symbol

Minimum yield a, Tensile strength, N/mm2 N/mm2 35 355 440-570 38 380 470-600 42 420 500-640 46 460 530-680 50 500 560-720 a Lower yield Rel shall be used. b Gauge length = 5 x 

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Minimum E% b, N/mm2 22 20 20 20 18

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Method B Symbol 43 49 55 57

Minimum tensile strength, N/mm2 430 490 550 570

Other tensile characteristics ie yield strength and elongation % are contained within a tabular form in this standard (Table 8B) and are determined by classification of tensile strength, electrode covering and alloying elements, ie E 55 16-N7. Classification of impact properties Method A Symbol Z A 0 2 3 4 5 6

Temperature for the minimum average impact energy of 47J No requirement +20 0 -20 -30 -40 -50 -60

Method B Impact or Charpy V notch testing temperature at 27J temperature in method B is again determined through the classification of tensile strength, electrode covering and alloying elements (Table 8B) ie a E 55 16-N7 which must reach 27J at -75°C. Classification of electrode characteristics and electrical requirements varies between classification methods A and B as follows: Method A This method uses an alpha/numerical designation from the tables as listed below: Symbol A C R RR RC RA RB B

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Electrode covering type Acid Cellulosic Rutile Rutile thick covering Rutile/cellulosic Rutile/acid Rutile/basic Basic

Symbol 1 2 3 4 5 6 7 8

12-6

Efficiency, % < 105 <105 >105-<125 >105-<125 >125-<160 >125-<160 >160 >160

Type of current AC or DC DC AC or DC DC AC or DC DC AC or DC DC

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Method B This method uses a numerical designation from the table as listed below Symbol 03 10 11 12 13 14 15 16 18 19 20 24 27 28 40

Covering type Rutile/basic Cellulosic Cellulosic Rutile Rutile Rutile + Fe powder Basic Basic Basic + Fe powder Rutile + Fe oxide (Ilmenite) Fe oxide Rutile + Fe powder Fe oxide + Fe powder Basic + Fe powder Not specified

Positions Allb All All Allb Allb Allb Allb Allb Allb Allb

Type of current AC and DC +/DC + AC and DC + AC and DC AC and DC +/AC and DC +/DC + AC and DC + AC and DC + AC and DC +/-

PA/PB AC and PA/PB AC and PA/PB only AC and PA/PB/PC AC and As per manufacturer’s recommendations 48 Basic All AC and bAll positions may or may not include vertical-down welding

DC DC DC DC

+/+

DC +

Further guidance on flux type and applications is given in the standard in Annex B and C. Hydrogen scales Diffusible hydrogen is indicated in the same way in both methods, where after baking the amount of hydrogen is given as ml/100g weld metal ie H 5 = 5ml/100g weld metal.

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12.2

AWS A 5.1- and AWS 5.5A typical AWS A5.1 and A5.5 Specification Reference given in box letter:

A) Tensile + yield strength and E% Code Min yield Min tensile PSI x 1000 PSI x 1000

Min E % In 2” min

General E60xx E 70xx E 80xx E 100xx

17-22 17-22 19-22 13-16

48,000 57,000 68-80,000 87,000

60,000 70,000 80,000 100,000

E 80 1 8 G A) B) C) (D For A5.5 only) B) Welding position 1 All positional 2 Flat butt & H/V fillet welds 3 Flat only

Specific electrode information for E 60xx and 70xx E 6010 48,000 60,000 22

Note: Not all Category 1 electrodes can weld in the vertical down position. V notch impact Radiographic Izod test (ft.lbs) standard 20 ft.lbs at –20F Grade 2

E 6011

48,000

60,000

22

20 ft.lbs at –20F

Grade 2

E 6012

48,000

60,000

17

Not required

Not required

E 6013

48,000

60,000

17

Not required

Grade 2

E 6020

48,000

60,000

22

Not required

Grade 1

E 6022

Not required

60,000

Not required

Not required

Not required

E 6027

48,000

60,000

22

20 ft.lbs at –20F

Grade 2

E 7014

58,000

70,000

17

Not required

Grade 2

E 7015

58,000

70,000

22

20 ft.lbs at –20F

Grade 1

E 7016

58,000

70,000

22

20 ft.lbs at –20F

Grade 1

E 7018

58,000

70,000

22

20 ft.lbs at –20F

Grade 1

E 7024

58,000

70,000

17

Not required

Grade 2

E 7028

58,000

70,000

20

20 ft.lbs at 0F

Grade 2

Code

Coating

Current type

Exx10 Exx11 Exx12 Exx13 E xx14 E xx15 E xx16 E xx18 E xx20 E xx24 E xx27 E xx28

Cellulosic/organic Cellulosic/organic Rutile Rutile + 30% Fe powder Rutile Basic Basic Basic + 25% Fe powder High Fe oxide content Rutile + 50% Fe powder Mineral + 50% Fe powder Basic + 50% Fe powder

DC + only AC or DC+ AC or DCAC or DC+/AC or DC+/DC + only AC or DC+ AC or DC+ AC or DC+/AC or DC+/AC or DC+/AC or DC+

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D) AWS Symbol A1 B1 B2 B3 B4 B5 C1 C2 C3

A5.5 low alloy steels Approximate alloy deposit 0.5%Mo 0.5%Cr + 0.5%Mo 1.25%Cr + 0.5%Mo 2.25%Cr + 1.0%Mo 2.0%Cr+ 0.5%Mo 0.5%Cr + 1.0%Mo 2.5%Ni 3.25%Ni 1%Ni + 0.35%Mo + 0.15%Cr D1/2 0.25-0.45%Mo + 0.15%Cr G 0.5%Ni or/and 0.3%Cr or/and 0.2%Mo or/and 0.1%V For G only 1 element is required

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12.3

Inspection points for MMA consumables Size

Wire diameter and length

Condition Cracks, chips and concentricity

All electrodes showing signs of the effects of corrosion should be discarded. Type (specification) Correct specification/code 1.1.1 3B Storage

E 46

Suitably dry and warm (Preferably 0% humidity)

Checks should also be made to ensure that basic electrodes have been through the correct pre-use procedure. Having been baked to the correct temperature (typically 300-350C) for 1 hour and then held in a holding oven (150C max) basic electrodes are issued to the welders in heated quivers. Most electrode flux coatings will deteriorate rapidly when damp and care should be taken to inspect storage facilities to ensure that they are adequately dry and that all electrodes are stored in conditions of controlled humidity. Vacuum packed electrodes may be used directly from the carton only if the vacuum has been maintained. Directions for hydrogen control are always given on the carton and should be strictly adhered to. The cost of each electrode is insignificant compared with the cost of any repair, thus basic electrodes that are left in the heated quiver after the day’s shift may potentially be re-baked but would normally be discarded to avoid the risk of H2 induced problems.

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Section 13 Submerged Arc Welding

13

Submerged Arc Welding

13.1

History Union Carbide (Linde Division) described a fully submerged (invisible) arc process in 1935 patents and the process was licensed to users as the Union melt process. The original Union melt process used a pre-fused flux, based on manganese oxide and silicon dioxide which gave a manganese silicate glass that could be crushed and ground to a coarse powder. The flux was sieved to give different particle size distribution as operators found that different current ranges favoured selection of different density flux. Union melt 20, the original formula, was designed to operate at up to 2000A. It continues to be sold as OK Flux 20 in US by ESAB.

Lincoln Electric tried to introduce their version but were sued by Union Carbide. Several of their offerings were found to infringe the patents but they were able to replace the flux with two new formulae, Lincoln 770 and 780, which were novel. Lincoln 780, still sold today, is a bonded or agglomerated, flux. The ingredients are not fused together but mixed as dry powders then bonded together with small amounts of silicate, similar in principle to an electrode coating. Thus they were able to incorporate deoxidants and alloying; something impossible with fused fluxes. Lincoln became the best known manufacturer of high quality fluxes from the 1950s onward and popularised the process name as submerged arc welding (SAW). Many other manufacturers then became consumable suppliers. Consumables were developed for hardfacing applications and SAW of stainless steel and nickel alloys became possible. Developers devised ways of using multiple wires and iron powder addition for high productivity welding and SAW quickly became the process with more variants than any other.

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13.2

Process characteristics Submerged arc welding is where an arc is struck between a continuous bare wire and the parent plate. The arc, electrode end and molten pool are submerged in an agglomerated or fused powdered flux which turns into a gas and slag in its lower layers when subjected to the heat of the arc, protecting the weld from contamination. The wire electrode is continuously fed by a feed unit of motor driven rollers, usually voltage-controlled to ensure an arc of constant length. The flux is fed from a hopper fixed to the welding head and a tube from the hopper spreads the flux in a continuous elongated mound in front of the arc along the line of the intended weld and of sufficient depth to submerge the arc completely so there is no spatter, the weld is shielded from the atmosphere and there are no UV or infra-red radiation effects. Unmelted flux is reclaimed for use. The use of powdered flux restricts the process to the flat and horizontalvertical welding positions.

Submerged arc welding is noted for its ability to use high weld currents giving deep penetration and high deposition rates. Generally DCEP is used up to about 1000A because it produces a deep penetration. On some applications (eg cladding operations) DCEN is chosen to reduce penetration and dilution. At higher currents or in case of multiple electrode systems, AC is often preferred to avoid the problem of arc blow. On multiple electrode systems, DCEP is generally used for the lead arc and AC for any trailing arcs. Difficulties sometimes arise in ensuring conformity of the weld with a predetermined line owing to the obscuring effect of the flux. Where possible, a guide wheel to run in the joint preparation is positioned in front of the welding head and flux hoppers. Submerged arc welding is widely used in the fabrication of ships, pressure vessels, line pipe and railway carriages - anywhere that long welds are required. It can be used to weld thicknesses from 3mm upwards, although its main use is for section thicknesses greater than this.

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13.3

Power source DC SAW can be carried out with either constant potential/voltage (CP) or constant current (CC) power sources. CP power sources are the accepted norm as the control to maintain arc length on a CV set traditionally required rapidly acting, low speed, low inertia motors that were expensive and difficult to build with sufficient accuracy. With microprocessor control, fine adjustment is easy and cost-effective so there is less reason to specify CP. AC power is also usable for SAW. As noted above it is necessary to use AC when there is more than one wire being used. Wires carrying several hundred amps DC produce substantial magnetic fields that will deflect any subsidiary arc in the vicinity. The normal method is to run the first wire on DCEP to give deep penetration, followed by up to four AC wires to give extra weld metal into a single elongated weld pool. Each wire has its own power source and control, making set-up of optimum conditions particularly difficult. For repetitive production where high speed is crucial, such multi-wire arrays are common. The production of welded pipe, either spirally welded or longitudinal seamed, is a typical application.

13.4

Equipment The size and layout of a submerged arc installation can vary, but Lincoln Electric has for many years marketed a hand held gun for SAW.

It is more usual to see a mechanised or automated machine. Small, mobile tractor units are available and particularly useful for working inside pipes. Column and boom system are also popular alternatives allowing the positioning of the welding head above or within the component to be welded, the component is then manipulated beneath the head as welding progresses. Large-scale production of repetitive shapes, eg ships plate or longitudinally welded pipe, can justify the installation of major gantry systems with several welding heads held on a cross beam that travels over the workpiece.

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13.5

Consumables

13.5.1 Wires Wires are usually available in 2-5mm diameter, though for special applications diameters both below and above this range have been used. Traditionally they are solid wires cold-drawn to size, cleaned and copper plated prior to spooling. Spooling is most frequently as 25-30kg coils. The wire is relatively stiff and requires a substantial feed motor and set of rolls to give smooth delivery to the contact tip at the welding head so is wound on a larger diameter reel than for MIG.

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Tubular, mainly flux cored, wires are also produced for the SAW process and can result in an increase in deposition rate and can be used to produce small quantities of low alloy compositions using a carbon steel sheath and are often used for surfacing and hardfacing applications, as well as welding of high strength low alloy (HSLA) steels. 13.5.2 Fluxes Flux may be categorised in two ways: by method of manufacture (fused or agglomerated) or by its activity (neutral, active or alloying). Within these broad groupings fluxes may be classified further by constituents, silica, manganese oxide, calcium fluoride, etc. Fused fluxes are produced by mixing the ingredients then melting them in an electric furnace to form a chemically homogeneous product, cooled and ground to the required particle size. Fused fluxes are limited in composition, primarily manganese silicates so are relatively neutral in their reaction with weld metal although pick-up of Mn and Si may be detected. The main benefits are that they are entirely homogeneous so recycled flux is of the same composition as the original. They also are non-hygroscopic so do not pick up moisture in storage and need baking before use. Because of the temperature of their manufacture they have compositional stability up to melting and can accept very high current arcs. Smooth stable arcs, with welding currents up to 2000A and consistent weld metal properties, are the main attraction of these fluxes. Agglomerated fluxes have more flexibility of composition, may be classified as acidic, neutral, basic and alloyed and can be formulated to give deliberate addition to the weld metal composition or to deoxidise and nucleate fine grained structures for high toughness. Acidic or active fluxes (though in truth all fluxes are active), transfer Mn and Si to the weld composition which helps with weld metal strength but must be kept within limits to avoid cracking. Some of these fluxes are recommended for single pass, or maximum two-layer, work. Neutral fluxes have been balanced to minimise the Mn and Si pick-up and is required. They will achieve reasonable toughness weld metal but for maximum properties basic fluxes should be used. Basic fluxes, like MMA electrode flux compositions, use fluorspar, to create the molten slag and may also contain limestone, alumina and manganese oxide. During manufacture, they are baked at maybe 500°C but it is still possible to add and retain deoxidants such as titanium, aluminium or magnesium powder. The principle of adding metals and alloying through the flux is used to great advantage for welding stainless steel and hardfacing. When welding stainless steel, the high reactivity of chromium results in it oxidising and being absorbed into the flux, quite significant reductions in Cr content may ensue, but this is compensated for by adding Cr, usually as ferro-chrome, to the flux. Welding engineers should be aware of this deliberate addition and not attempt to use flux formulated for welding stainless steel on C-Mn steel. A disadvantage of agglomerated fluxes is they are prone to picking up moisture so should therefore not be left in the dispensing hopper overnight and storage should be in a dry, warm store room. The flux can be baked prior to use but it needs to be spread thinly on trays in the baking oven or agitated repeatedly in order for moisture to be released from within its bulk.

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The individual particles of an agglomerated flux, particularly one with loading of ferro-alloys, do not break down to consistent composition fines so recycling agglomerated fluxes needs more care than for fused fluxes. The most practical arrangement is to sieve all reclaimed flux, ie the material sucked from the bead surface after the passing of the weld pool and to reject the fines before returning the undamaged flux to the hopper for re-use. 13.6

Welding parameters

13.6.1 Flux depth

The flux depth is often poorly controlled in practice and the powder simply heaped around the wire until the arc is completely covered. For optimum results, the flux depth should be just sufficient to cover the arc although, at the point where the electrode enters the flux cover, light reflected from the arc should be just visible. Too shallow and the arc may flash through and can cause porosity and a rough surface because of inadequate protection of the molten metal. Too deep can give poor bead appearance and lead to spillage on circumferential welds. On deep preparations in thick plate it is particularly important to avoid excessive flux cover as weld bead shape and slag removal can be unsatisfactory. 13.6.2 Arc voltage Arc voltage has an important effect on the weld bead shape and penetration depth; the precise effect being dependent on the joint preparation. Bead-onplate and square edge close butt welds have increased bead width and dilution as the arc voltage increases, although the depth of penetration is relatively unaffected 13.6.3 Wire diameter and welding current

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The preferred wire diameter is governed by the welding current required for a particular application. Commonly used SA wire diameters are 2-6mm. For a given wire diameter, the deposition rate and depth of penetration increase with increasing welding current. Excessive current causes the electrode wire to overheat causing arc instability, a deterioration in weld profile and, sometimes, undercutting. Below a minimum current level, arc instability will also occur, giving arc wander and poor penetration. For single pass (and one pass either side) procedures the current should be sufficient to achieve the required depth of penetration without burn-through. For multi-pass welding, the current should be selected to give the required weld bead size whilst ensuring adequate fusion to the underlying material. In the case of circumferential joints, the selection of welding current will also be affected by the diameter of the workpiece. 13.6.4 Travel speed Bead size is inversely proportional to welding speed at the same current. Higher speeds reduce bead width, increase the likelihood of porosity and if taken to the extreme, produce undercutting and irregular beads. At high welding speeds the arc voltage should be kept low to minimise the risk of arc blow. If the welding speed is too low, burn-through can occur. A combination of high arc voltage and low welding speed can produce a mushroom-shaped weld bead with solidification cracks at the bead sides. Excessive travel speed can also produce centreline solidification cracking.

For a given arrangement of wires and wire diameters, welding speed is limited by the welding current tolerated by the flux: Some fluxes are specially formulated to allow high speed operation and higher speeds are possible with multiple wire operation or by holding a more acute electrode angle. 13.6.5 Electrode positioning As the angle between the electrode and the plate determines the point of impingement and direction of the arc force, it has a critical effect on the weld bead profile and depth of penetration. Welding can be carried out with the electrode wire leading, trailing or normal to the plate surface and the effects on weld shape, penetration and undercut are shown below:

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For most applications weld with the electrode wire pointing forwards, ie leading by 10° to obtain the best combination of bead shape, penetration and resistance to undercut. 13.7

Potential defects

13.7.1 Porosity Porosity is a fairly common defect which can be influenced by many factors. Sometimes clearly visible as pinholes or larger voids at the weld surface, other times it is below the surface and revealed only by X-ray examination or ultrasonic testing. Unless it is gross or preferentially aligned, porosity is unlikely to be harmful.

Common causes of porosity are:



Contamination of joint surfaces with oil, paint, grease, hydrated oxides, etc which decompose in the arc giving gaseous products which can cause elongated wormhole porosity often located along the centreline of the weld.



Damp flux: flux should be kept dry. It is good practice to dry all fluxes before use and store them in a heated hopper. The manufacturer's recommendations regarding drying temperatures should be observed. If a flux recovery unit driven by compressed air is used the compressed air should be dried thoroughly.



Insufficient flux burden can expose the arc and molten weld pool to atmospheric contamination.



When welding stainless or duplex steels by SAW, the voltage needs very careful setting up, as incorrect voltage can cause porosity in these materials.



The surface of a weld may sometimes contain small depressions known as surface pocking or gas flats. These are harmless and while the exact cause is not fully understood it is linked to conditions which cause generation of gas or make it difficult for gas to escape; for example, moisture or lack of deoxidants and too many fines in the flux to allow gas to pass readily.

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13.7.2 Solidification cracking Because of the large weld pools and high welding speeds often associated with submerged arc welds, solidification or hot cracking may be encountered and is usually found along the centreline of the weld. Solidification cracking is controlled by the composition of the weld, its solidification pattern and the strain on the solidifying weld metal. The problem is aggravated by the presence of phosphorus, sulphur and carbon and if these elements are known to be in the parent material in higher amounts than usual, a change should be made to a wire with higher manganese content and steps taken to minimise dilution and ensure good weld bead profiles. The most dangerous element is carbon which, if other considerations allow, can be kept low in the weld by use of high silica fluxes, ie manganese and calcium silicate types. If the carbon level is not too high, a basic flux would be more preferable as this can help to reduce weld metal sulphur levels. Sometimes improvement to the weld metal composition can be obtained by selecting a wire particularly low in carbon, sulphur and phosphorus, so as to reduce the risk of cracking. The weld bead shape also has a critical effect. Deep narrow welds, with high depth to width ratios, are prone to centreline cracking,

W > d tendency for surface cracks

W < d tendency for centreline cracking

W/d

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Cracking can be a problem in root runs where dilution of parent plate into the weld is high giving excessive carbon content. Long and deep weld pools, welds made at high welding speeds or with high restraint and large gaps, accentuate the problem. Conversely, a combination of high arc voltage and slow welding speed can produce a mushroom-shaped weld bead with solidification cracks at the weld bead sides.

In the root beads of a multi-run weld

Caused by high speed giving a long deep weld pool in first pass.

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Mushroom shaped weld penetration resulting from high voltage combined with low speed Occasionally a groove may be found on the surface running along the centre of the weld. This may be caused by shrinkage and although it is sometimes mistaken for incipient solidification cracking it is actually only superficial. 13.8

Classification of consumables As with MMA welding, Gene Mathers has written a series of articles on submerged arc welding (www.twi.co.uk/content/jk87.html; www.twi.co.uk/content/jk88.html; www.twi.co.uk/content/jk89.html). We recommend these articles for those wishing to understand the classification schemes without the need for detailed study of the specifications themselves.

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IWS Revision Questions 1

Describe the basics of the SAW process, including the use of different polarity power.

2

Describe the various types of flux and the typical use.

3

Why is travel speed an important variable? What problems may occur if it is not optimum?

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Section 14 Electroslag Welding

14

Electroslag Welding

14.1

History Electroslag welding (ESW) is a very efficient, single pass process carried out in the vertical or near vertical position for joining steel plates/sections of 25mm and above and was developed into a viable welding process by the Paton Institute in the Ukraine in the early 1950s. The process was used extensively in the US for welding thick structural steel members in the 1960s and 70s but the Federal Highways Agency decided on the basis of laboratory tests that the very high heat input of ESW gave dangerously low toughness which led to a ban in US of the use of ESW for many applications. The Northridge earthquake in 1994 tested the welds in highway bridges and structural steel work and revealed that repairs to self-shielded welds in structural steel cost over £1bn, but that not one ESW weld had required a repair so the FHA ban was rescinded in 2000.

14.2

Process characteristics Unlike other high current fusion processes, ESW is not an arc process. Heat required for melting both the welding wire and plate edges is generated through the molten slag's resistance to the passage of an electric current. In its original form plates are held vertically, approximately 30mm apart, with the edges of the plate cut normal to the surface and a bridging run-on piece of the same thickness is attached to the bottom of the plates. Water cooled copper shoes are placed each side of the joint, forming a rectangular cavity open at the top. Filler wire, which is also the current carrier, is fed into this cavity, initially striking an arc through a small amount of flux. Additional flux is added which melts forming a flux bath which rises and extinguishes the arc. The added wire melts into this bath sinking to the bottom before solidifying to form the weld. For thick sections, additional wires may be added and an even distribution of weld metal is achieved by slowly oscillating the wires across the joint. As welding progresses, both the wire feed mechanism and the copper shoes are moved progressively upwards until the top of the weld is reached.

Figure 14.1 Electroslag welding.

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The consumable guide variant uses a simpler set-up and equipment arrangement which does not require the wire feed mechanism to climb. The wire is delivered to the weld pool down a consumable, thick walled tube which extends from the top of the joint to the weld pool. The original consumable guides were flux-covered which helped avoid any shorting on to the preparation sides and topped up the flux bath as material was lost by sticking to the copper shoes. This process was patented by the Linde Division of Union Carbide so is subject to royalty payment so alternatives were tried.

At TWI in the mid 1960s experiments with bare guide tubes were successful provided the guide did not touch the wall during any part of its oscillation. One simple and cheap guide tested consisted of four straight lengths of rod tacked together in a square format with sufficient space in the centre for the wire to be pass down it which worked well if the gap was sufficiently wide but was prone to arcing on to the side. Consumable guide ESW is often carried out without oscillation. The tubular guides can be further supplemented by additional consumable plates attached to the tube. Generally, as the thickness of plate increases, the number of wires/guides increases, approximately in the ratio of one wire per 50-75mm of thickness. Support for the molten bath is provided by two pairs of copper shoes which are moved upwards, leapfrogging as welding progresses. An operator is required to observe the flux bath and add more flux as the bath thins. The flux is very similar to submerged arc flux and is usually agglomerated. Slight changes in composition give the flux more fluidity so that it floods the initial start-up arc and extinguishes it. After that heating and melting continue due to the resistive heating of the current flow through the molten flux bath.

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14.3

ESW materials other than steel

14.3.1 Aluminium Uttrachi (www.netwelding.com/serv04.htm#Aluminum Electroslag) describes work at Union Carbide, Linde Division and latterly at WA Technology that demonstrated the possibility of ESW being used on aluminium alloys. His narrative from the website is reproduced below.

The Consumable Guide Aluminum Electroslag Welding process was developed in the Laboratory and produced welds in 2 inch thick (50mm) and 4 inch thick (100mm) busbar material. Welds were made at a very rapid rate of vertical travel speed not possible with steel welding. A sample of a weld made with the process is shown on the left. Unfortunately the main application for the process was for joining heavy aluminum busbars. These are mostly employed in aluminum production facilities and the market for aluminum had significantly deteriorated. The development work was therefore terminated and the process was not commercialized. The demand for aluminum is now high and new plants are under construction. A company who works in the area asked if it were possible to weld over 10 inch thick by 4 foot high busbars by completing the early development work and extending it to these much thicker sections. After considerable additional development work and cost, refining the flux, welding parameters and equipment; the objective was achieved. The process was used on a production application over 10 inches thick with welds made at very high vertical travel speeds.

The photo left shows the equipment system welding a >10 inch thick section. The centre photo is the finished weld. Welding speeds were very high, much higher than in steel welding. Weld surface is excellent. The photo right is a cross section showing good fusion and defect free weld.’

14.3.2 Titanium A team working with Prof Eager of MIT demonstrated ESW thick Ti -6Al -4V alloy using a consumable guide technique described in a research paper published online at http://eagar.mit.edu/EagarPapers/Eagar089.pdf. In this paper they refer to early work in USSR that developed the principle. Eager’s team showed that pure calcium fluoride was needed as flux and must be kept free from moisture. They found AC power was necessary but reported the successful completion of welds in both 25 and 50mm plate.

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14.4

Stainless steel and nickel alloys The Paton Institute in Kiev welded many materials by ESW during the early development of the process. Reference can be found to the possibility of welding both austenitic stainless steel and nickel alloys but there are no examples of its commercial use to cite other than as a surfacing technique.

14.5

Current status Electroslag welding is not one of the major welding processes because the high heat input generates large, coarse grains in the weld metal and HAZ that lead to poor fracture toughness properties in these areas. Toughness improvements can only be achieved by post-weld normalising treatment. Additionally, the near parallel-sided geometry of the weld, combined with the coarse grains, can make it difficult to identify defects at the fusion boundary by standard ultrasonic NDT techniques. Considerable interest was shown in ESW during the 1970s when ideas for increasing welding speed, such as narrow gap welding, were investigated. This was seen as an important parameter for increasing productivity and to reducing heat input to improve HAZ and weld metal impact properties. However since then little development has taken place, limited to the tuning of parameters and tailoring techniques for specific applications. ESW has considerable potential for increasing productivity but its use has been limited because of relatively poor understanding of the process and, for specific applications, the significance of the fracture toughness values. As a result, use has been restricted to a few niche applications. In the fabrication industry, the process continues to be used for thick walled pressure vessels which are post-weld normalised and for structures such as blast furnace shells and steel ladles used at above ambient temperatures. The process is also extensively used for the welding railway points. It is most commonly used now with strip electrode as a surfacing technique and is described in more detail in the section on surfacing.

14.6

Benefits and disadvantages The principal benefits are:       

Speed of joint completion; typically 1hr/m of seam irrespective of thickness. Lack of angular distortion. Lateral angular distortion limited to 3mm/m of weld. High quality welds produced. Simple joint preparation, ie flame-cut square edge. Major repairs can be made by cutting out total weld and re-welding. Can be modified for use as a cladding technique.

The main disadvantages are:   

Grain growth giving very large grains due to very high heat input and slow cooling - poor toughness. Limited to vertical or near vertical position. Difficult to examine with NDT.

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Section 15 Thermal Cutting IWS

15

Thermal Cutting and Gouging

15.1

Introduction Thermal cutting normally refers to the severing of metal, creating two pieces or a specific shaped single piece. Gouging is a particular form of cutting where the aim is to remove metal in a controlled manner to leave a groove that will act as the basis of weld preparation. In terms of the process and fundamental principles, they are the same; only the details of the torch and the parameters vary. Thermal cutting and gouging are essential parts of welding fabrication. Used for rapid removal of unwanted metal, the material is locally heated and molten metal ejected, usually by blowing it away. Flame, laser or arc processes can be used to produce rapid melting and metal removal. Thermal processes, operations and metals which may be gouged or otherwise shaped: Thermal process

Process operations

Metals

Primary

Secondary

Oxyfuel gas flame Manual metal arc Air carbon arc Plasma arc

Cutting Gouging

Grooving washing chamfering Grooving chamfering

Ferritic steels, cast iron

Gouging

Grooving Chamfering

Cutting Gouging

Laser

Cutting

Chamfering grooving washing Chamfering drilling

Ferritic steels, cast iron, nickel-based alloys, copper and copper alloys, copper/nickel alloys, aluminium Aluminium, stainless steels

Gouging

Ferritic steels, stainless steels, cast iron, nickel-based alloys

Ferritic steels, stainless steels, C-Mn steels, aluminium, other non-ferrous metals, thermoplastic materials Note: All processes capable of cutting/severing operations. Preheat may be required on some metals prior to gouging.

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15.2

General safety

Because cutting and gouging rely on molten metal being forcibly ejected, often over large distances, the operator must take appropriate precautions to protect himself, other workers and his equipment. Sensible precautions include protective clothing for the operator, shielding inside a special enclosed booth or screens, adequate fume extraction and removal of all combustible material from the immediate area. 15.2.1 Gouging applications Thermal gouging was developed primarily for removal of metal from the reverse side of welded joints, tack and temporary welds and weld imperfections.

Typical back-gouging applications carried out on arc welded joints.

Imperfection removal in preparation for weld repair.

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Applications include: 

  



15.3

Repair and maintenance of structures (bridges, earthmoving equipment, mining machinery, railway rolling stock, ships, offshore rigs, piping and storage tanks). Removal of cracks and imperfections (blow holes and sand traps in ferrous and non-ferrous forgings and castings). Preparation of plate edges for welding. Removal of surplus metal (riser pads and fins on castings, excess weld bead profiles, temporary backing strips, rivet washing and shaping operations, demolition of welded and unwelded structures) site work. Removal of temporary welded attachments (brackets, strongbacks, lifting lugs and redundant tack welds) during various stages of fabrication and construction work.

Oxy-fuel cutting

The oxyfuel process is the most widely applied industrial thermal cutting process because it can cut 0.5-250mm thicknesses, the equipment is low cost and can be used manually or mechanised. There are several fuel gas and nozzle design options that can significantly enhance performance in terms of cut quality and cutting speed. 15.3.1 Process fundamentals Basically, a mixture of oxygen and the fuel gas preheats the metal to its ignition temperature which, for steel, is 700-900°C (bright red heat) but well below its melting point. A jet of pure oxygen is directed into the preheated area instigating a vigorous exothermic chemical reaction between the oxygen and the metal to form iron oxide or slag. The oxygen jet blows the slag away enabling the jet to pierce through the material and continue to cut through the material.

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There are four basic requirements for oxy-fuel cutting:    

Ignition temperature of the material must be lower than its melting point otherwise the material will melt and flow away before cutting could happen. Oxide melting point must be lower than of the surrounding material so it can be mechanically blown away by the oxygen jet. Oxidation reaction between the oxygen jet and metal must be sufficient to maintain the ignition temperature. Minimal gaseous reaction products should be produced to avoid diluting the cutting oxygen.

As stainless steel, cast iron and non-ferrous metals form refractory oxides, ie the oxide melting point is higher than the material and powder must be injected into the flame to form a low melting point, fluid slag. 15.3.2 Preheating The preheating flame has the following functions in the cutting operation:    

Raises the temperature of the steel to the ignition point. Adds heat energy to the work to maintain the cutting reaction. Provides a protective shield between the cutting oxygen stream and the atmosphere. Dislodges from the upper surface of the steel any rust, scale, paint or other foreign substance that would stop or retard the normal forward progress of the cutting action.

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15.3.3 Selection of fuel gas Factors to be considered when selecting a fuel gas include:     

Time required for preheating when starting cuts. Effect on cutting speed and productivity. Cost and availability. Volume of oxygen required to obtain a neutral flame. Safety in transporting and handling.

Fuel gas characteristics and their applications. Fuel gas

Main characteristics

Applications

Acetylene

Rapid cutting of thin plates Bevel cuts Short, multi-pierce cuts Cutting of thicker sections (100-300mm), long cuts

MAPP Propylene

Highly focused, high temperature flame Rapid preheating and piercing Low oxygen requirement Low temperature flame, high heat content Slow preheating and piercing High oxygen requirement Medium temperature flame Medium temperature flame

Natural gas

Low temperature flame

Cutting of thicker sections

Propane

Cutting underwater Cutting of thicker sections

15.3.4 Cutting quality Generally oxy-fuel cuts are characterised by: Large kerf (<2mm).  Low roughness values (Ra<50µm).  Poor edge squareness (>0.7mm).  Wide HAZ (>1mm). 

The face of a satisfactory cut has a sharp top edge, fine and even drag lines, little oxide and a sharp bottom edge with an underside free of slag.

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A satisfactory cut is shown in the centre. If the cut is too slow (left) the top edge is melted and there are deep grooves in the lower portion of the face. Scaling is heavy and the bottom edge may be rough, with adherent dross. If the cut is too fast (right) the appearance is similar, with an irregular cut edge. Plate thickness 12mm.

With a very fast travel speed the drag lines are coarse and at an angle to the surface with an excessive amount of slag sticking to the bottom edge of the plate, due to the oxygen jet trailing with insufficient oxygen reaching the bottom of the cut.

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A satisfactory cut is shown in the centre. If the preheating flame is too low (left) the most noticeable effect on the cut edge is deep gouges in the lower part of the cut face. If the flame is too high (right) the top edge is melted, cut irregular and there is excess adherent dross. Plate thickness 12mm.

A satisfactory cut is shown in the centre. If the blowpipe nozzle is too high above the work (left) excessive melting of the top edge occurs with a lot of oxide. If the torch travel speed is irregular (right) uneven spacing of the drag lines can be observed and an irregular bottom surface and adherent oxide. Plate thickness 12mm. 15.3.5 Advantages of oxy-fuel cutting       

Steels can generally be cut faster than by most machining methods. Section shapes and thicknesses difficult to produce by mechanical means can be cut economically. Basic equipment costs are low compared with machine tools. Manual equipment is very portable and can be used on site. Cutting direction can be changed rapidly on a small radius. Large plates can be cut rapidly in place by moving the torch rather than the plate. An economical method of plate edge preparation.

15.3.6 Disadvantages of oxy-fuel cutting       

Dimensional tolerances significantly poorer than for machine tools. Process is essentially limited to cutting carbon and low alloy steels. Preheat flame and expelled red hot slag present fire and burn hazards to plant and personnel. Fuel combustion and oxidation of the metal require proper fume control and adequate ventilation. Hardenable steels may require pre- and/or post-heat adjacent to the cut edges to control their metallurgical structures and mechanical properties. Special process modifications are needed for cutting high alloy steels and cast irons (ie iron powder or flux addition). Being a thermal process, expansion and shrinkage of the components during and after cutting must be taken into consideration.

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15.4

Powder cutting Powder cutting is oxygen cutting in which a suitable powder is injected into the cutting oxygen stream to assist the cutting action (from BS 499: Part 1:1991 Section 7 No.72 008). Mild steels readily ignite in a stream of oxygen when they are heated to 700900°C, but for stainless steels, the ignition temperature is over 1500°C. Furthermore, the oxides formed when cutting mild steel have lower melting points than the parent metal and this facilitates a clean cut. With stainless steel the oxide has a higher melting point than the parent metal and hampers cutting. This can be overcome by adding materials to the cutting gas stream which either remove the oxide film or raise the reaction temperature: Flux injection into the cutting gas stream which chemically removes the oxides of chromium. Finely divided iron-rich powder fed separately into the cutting zone in a gaseous medium. Combustion of the iron powder increases the reaction temperature and the fluidity of oxidation products. The iron-rich powder injection technique has also been used for cutting copper, nickel, aluminium and their alloys and cast irons. The quality of the cut surface is, at best, equivalent to flame cut carbon steel; but with many materials, the cut quality is very poor.

15.5

Oxy-fuel gouging

Oxy-fuel or flame gouging offers a quick and efficient method of removing metal, principally ferritic steel. It can be at least four times quicker than cold chipping operations and is particularly attractive because of its low noise, ease of handling and ability to be used in all positions.

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15.5.1 Process description Flame gouging is a variant of conventional oxy-fuel gas cutting. Oxygen and a fuel gas are used to produce a high temperature flame for melting the steel. When gouging, the steel is locally heated to above the ignition temperature (typically 700-900°C) and a jet of oxygen used to melt the metal - a chemical reaction between pure oxygen and hot iron. The jet is also used to blow away molten metal and slag. Compared with oxy-fuel cutting, slag is not blown through the material but remains on the top surface of the work. The gouging nozzle is designed to supply a relatively large volume of oxygen through the gouging jet, as much as 300 l/min through a 6mm orifice. In oxyacetylene gouging, equal quantities of oxygen and acetylene are used to set a near-neutral preheating flame with the oxygen jet flow rate determining the depth and width of the gouge. Typical operating parameters for achieving a range of gouge sizes are: Nozzle orifice diamete r, mm

Gouge dimensions, mm Widt Dept h h

Gas pressure, bar

Gas consumption, l/min

Acetylen e

Oxyge n

Acetylen e

3

6-8

3-9

0.48

4.2

5

8-10

6-12

0.48

6.5

1013

1013

0.55

Travel speed, mm/mi n

15

O2 prehe at 22

O2 goug e 62

5.2

29

31

158

1000

5.5

36

43

276

1200

600

When the preheating flame and oxygen jet are correctly set, the gouge has a uniform profile and its surfaces are smooth and a dull blue. 15.5.2 Operating techniques The depth of the gouge is determined principally by the speed and angle of the torch. To cut a deep groove the angle of the torch is stepped up (increases the impingement angle of the oxygen jet) and gouging speed reduced. To produce a shallow groove, the torch is less steeply angled and speed increased. Wide grooves can be produced by weaving the torch. The contour of the groove is dependent upon the size of the nozzle and the operating parameters. If the cutting oxygen pressure is too low, gouging progresses with a washing action, leaving smooth ripples in the bottom of the groove. If the cutting oxygen pressure is too high, the cut advances ahead of the molten pool which will disrupt the gouging operation especially when making shallow grooves.

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15.6

MMA gouging MMA gouging operates in the same way as the welding process – an arc is formed between the tip of the electrode and the workpiece. As only the arc force ejects metal, it requires special electrodes with thick flux coatings to generate sufficiently strong arc force and gas stream. Unlike MMA welding where a stable weld pool must be maintained, this process forces the molten metal away from the arc zone to leave a clean cut surface. Cutting of thin material can be achieved with these electrodes but it is not very satisfactory, leaving a very ragged edge. The gouging process is characterised by the large amount of gas generated to eject the molten metal. Because the arc/gas stream is not as powerful as a gas or separate air jet, the surface of the gouge is not as smooth as an oxy-fuel or air carbon arc gouge.

DCEN is preferred but an AC constant current power source can be used. MMA gouging is used for localised gouging operations, removal of defects for example and where it is more convenient to switch from a welding electrode to a gouging electrode rather than use specialised equipment. Compared with alternative gouging processes, metal removal rates are low and the quality of the gouged surface inferior. When correctly applied MMA gouging can produce relatively clean gouged surfaces. For general applications welding can be carried out with only light grinding. When gouging stainless steel a thin layer of higher carbon content material will be produced, which should be removed by grinding. The main advantage of MMA gouging is that the same power source can be used for welding, gouging or cutting by changing the type of electrode.

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Grooving electrodes, though based on mild steel core wires, are not just restricted to steels: the same electrode composition may be used for gouging stainless steel and non-ferrous alloys, in which case the cut surface must be ground after the gouging operation has been completed. 15.7

Air carbon arc gouging

15.7.1 Process description An electric arc is generated between the tip of a carbon electrode and the workpiece. The metal becomes molten and a high velocity air jet streams down the electrode to blow it away, leaving a clean groove. The process is simple to apply (the same equipment as MMA welding), has a high metal removal rate and gouge profile can be closely controlled.

As it does not rely on oxidation it can be applied to a wide range of metals. DCEP is normally preferred for steel and stainless steel but AC is more effective for cast iron, copper and nickel alloys. Typical applications include backgouging, removal of surface and internal defects and excess weld metal and preparation of bevel edges for welding. For effective metal removal it is important that the air stream is directed at the arc from behind the electrode and sweeps under the tip of the electrode. The groove width is determined by the diameter of electrode and depth is dictated by the angle of electrode to work piece and rate of travel. Relatively high travel speeds are possible when a low electrode angle is used, producing a shallow groove: a steep angle results in a deep groove and requires slower travel speed. A steeply angled electrode may give rise to carbon contamination.

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Oscillating the electrode in a circular or restricted weave motion during gouging can greatly increase gouging width, useful for removal of a weld or plate imperfection wider than the electrode. The groove surface should be relatively free of oxidised metal and is ready for welding without further preparation but grinding should be carried out if a carbon rich layer has been formed. Dressing may be necessary if working on crack-sensitive material such as HSLA steel. 15.7.2 Advantages  

 

   

Fast - approximately five times faster than chipping. Easily controllable, removes defects with precision. Defects clearly visible and may be followed with ease. Cut depth is easily regulated and slag does not deflect or hamper the cutting action. Low equipment cost - no gas cylinders or regulators are necessary except on site. Economical to operate - no oxygen or fuel gas required. The welder may also do the gouging (no qualification requirements for this operation although adequate training should always be given). Easy to operate - equipment similar to MMA except the torch and air supply hose. Compact - the torch is not much larger than an MMA electrode holder, allowing work in confined areas. Versatile. Can be automated.

15.7.3 Disadvantages     



The air jet causes the molten metal to be ejected over quite a large distance. Because of high currents (up to 2000A) and high air pressures (80-100psi), it can be very noisy. Other cutting processes usually produce a better cut. Requires large volume of compressed air. Increases the carbon content leading to an increase in hardness in the case of cast iron and hardenable metals. In stainless steels can lead to carbide precipitation and sensitisation so grinding of the carburised layer usually follows gouging. Introduces hazards such as fire (due to discharge of sparks and molten metal), fumes, noise and intense light.

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15.8

Plasma arc cutting Plasma arc cutting uses essentially the same torch as for plasma welding. In cutting, the constricted arc issuing from the plasma orifice develops a high velocity jet of ionised gas that blows the melted metal away. Initially a pilot arc is struck between a tungsten electrode and a water-cooled nozzle. In the transferred arc variant, a stronger arc is then developed to the work piece, constricted by the orifice in the nozzle. As plasma gas passes through this arc, it is heated rapidly to in excess of 20,000°C which causes huge expansion of the gas which is accelerated to near the speed of sound as it passes through the constricting orifice towards the work piece. As the arc melts the work piece, the high velocity jet blows away the molten metal. Where materials are electrical insulators, the non-transferred arc method is used where the arc remains within the torch as in the initial, pilot stage of the transferred arc method, only the plasma jet stream travels toward the work piece.

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Transferred arc.

Non-transferred arc.

Plasma arc cutting is seen as an alternative to the oxy-fuel process, however the important difference between the two is that while the oxy-fuel process oxidises the metal and the heat from the exothermic reaction melts the metal, the plasma process uses the heat from the arc to melt the metal. The ability to melt metal without oxidation is essential when cutting metals, such as stainless steel, which form high temperature oxides. The plasma process was therefore first introduced for cutting stainless steel and aluminium alloys. The first plasma torches gave poor quality cuts and the process suffered from excessive noise and fume, especially when cutting thicker material. Over the last thirty years, the process has been highly refined and is now capable of producing high quality cuts, at increased speeds, in a wide range of material thicknesses. 15.8.1 Power source The power source for the plasma arc process must have a drooping characteristic and a high voltage. Although the operating voltage to sustain the plasma is typically 50-60V, the OCV to initiate the arc can be up to 400V DC. On initiation, a pilot arc is formed within the body of the torch between the electrode and nozzle. For cutting metals the arc should be transferred to the work piece in the so-called transferred arc mode. The electrode is negative and the work piece positive so that the majority of the arc energy (approximately ⅔ ) is used for cutting.

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15.8.2 Gas composition In the conventional system using a tungsten electrode, the plasma is inert, formed using Ar, Ar-H2 or N2. However, as described in Process Variants, oxidising gases, such as air or O2, can be used but the electrode must be copper with a hafnium tip. The plasma gas flow is critical and must be set according to the current level and the nozzle bore diameter. If too low for the current level or the current level too high for the nozzle bore diameter, the arc will break down forming two arcs in series, electrode to nozzle and nozzle to work piece. The effect of double arcing is usually catastrophic with the nozzle melting. 15.8.3 Cut quality Plasma cut quality is similar to with the oxy-fuel process, but as the plasma process cuts by melting, a characteristic feature is the greater degree of melting towards the top of the metal resulting in top edge rounding, poor edge squareness or a bevel on the cut edge. As these limitations are associated with the degree of constriction of the arc, several torch designs are available to improve arc constriction to produce more uniform heating at the top and bottom of the cut. 15.8.4 Air plasma The inert plasma forming gas (Ar or N2) can be replaced with air but this requires a special electrode of hafnium or zirconium mounted in a copper holder. Air can replace water for cooling the torch and the use of compressed air rather than more expensive cylinder gas, makes this process highly competitive with the oxy-fuel process. A variant of the air plasma process is the monogas torch in which air is used for both the plasma and the cooling gas. Air plasma is more widely applied in light engineering industries, eg cutting sheet steel of 1-20mm and is most often used on C-Mn and stainless steels but will also cut SG cast iron and non-ferrous materials. For thin section material of a few millimetres, the process is much faster than oxy-fuel, but at thicknesses approaching 30-40mm, air plasma becomes relatively slow.

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The obvious cost advantages of using air in preference to expensive gases (for the plasma and oxy-fuel processes) may be offset when other operating costs have been taken into account. For example, the air must be fed at a relatively high pressure (typically 150 l/min at 5bar) and clean which will require a sizeable compressor with suitable filters for dust particles and oil. The hafnium or zirconium electrodes are expensive and their operating life can be severely shortened if there are frequent stop and starts. Low current air plasma torches, typically less than 40A, are particularly attractive for cutting thin sheet material, in that compressed air is used for both the plasma forming gas and cooling the torch. As N2 and O2 suppress the formation of a series arc, compared with Ar, contact cutting can be practised with the air plasma system. The process is becoming more widely used for manual cutting of thin sheet components in both C-Mn and stainless steel, where contact cutting greatly deskills the operation. 15.8.5 Advantages 

       

Not limited to materials which are electrical conductors so is widely used for cutting all types of stainless steels, non-ferrous materials and nonconductive materials. Operates at a much higher energy level compared with oxy-fuel cutting resulting in faster cutting speed. Instant start-up is particularly advantageous for interrupted cutting and allows cutting without preheat. Can be used with a wide range of materials, including stainless steel and aluminium. High quality cut edges can be achieved, eg the HTPAC process. Narrow HAZ formed. Low gas consumable (air) costs. Ideal for thin sheet material. Low fume (underwater) process.

15.8.6 Disadvantages  

       

Dimensional tolerances are significantly poorer than machine tool capabilities. The process introduces hazards such as fire, electric shock (due to the high OCV), intense light, fumes, gases and noise levels that may not be present with other processes. In underwater cutting fumes, UV radiation and noise are reduced to a low level. Compared with oxy-fuel cutting, plasma arc cutting equipment tends to be more expensive and requires a fairly large amount of electric power. Being a thermal process, expansion and shrinkage of the components during and after cutting must be taken into consideration. Cut edges slightly tapered. Air plasma limited to 50mm thickness plate. High noise especially when cutting thick sections in air. High fume generation when cutting in air. Protection required from the arc glare. High consumable costs (electrodes and nozzles).

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15.9

Plasma arc gouging The use of plasma arc as a gouging tool dates from the 1960s when the process was developed for welding. Compared with the alternative oxy-fuel and MMA gouging techniques, plasma arc has a needle-like jet that can produce a very precise groove, suitable for application on almost all ferrous and non- ferrous materials.

15.9.1 Process description

Plasma arc gouging is a variant of the plasma arc cutting process. The temperature and force of the constricted plasma arc is determined by the current level and plasma gas flow rate so the plasma can be varied to produce a hot gas stream or a high power, deeply penetrating jet. This ability to control quite precisely the size and shape of a groove is very useful for removing unwanted defects from a work piece surface. 15.10

Laser cutting

15.10.1 Introduction to laser cutting Laser cutting is used extensively for producing profiled flat plate and sheet for many and diverse applications in engineering industry. For three-dimensional components, multi-axis gantry laser beam manipulators have extended laser cutting to the automotive sector with this type of equipment used for the trimming of pre-production body panels at all leading car manufacturers. More recently, laser cutting has also found its way, very successfully, into other industry sectors such as shipbuilding, traditionally known to be fairly slow in the adoption of high technology processes. The CO2 gas laser dominates cutting applications, being used on steels and nonmetallic materials, including man-made fabrics. The Nd:YAG solid state laser is also used as its wavelength is readily absorbed by aluminium and copper.

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Almost all cutting operations with the above lasers use some sort of gas to assist the process. The degree of assistance can be from simply providing protection to the beam focusing lens or via production of an exothermic reaction with a gas such as O2, to increase significantly achievable cutting speeds. This has led to the term gas assisted laser cutting which is often used synonymously in the industry with the term laser cutting. 15.10.2 Advantages     

Very fast speed. No delay for preheating necessary. Readily automated and can follow three dimensional tracks. Can cut polymers and other non-metallic materials. Good quality square-edged kerf.

15.10.3 Disadvantages  

High equipment cost. Need to isolate personnel from laser beam.

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IWS Revision questions 1

Describe the four basic requirements for successful oxy-fuel gas cutting and what happens if each is not met.

2

What are the functions of the preheating flame prior to injection of the cutting oxygen stream?

3

How does MMA gouging work?

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Section 16 Surfacing and Spraying

16

Surfacing and Spraying

16.1

Background Surfacing may be required for a number of reasons including: Repair build-up Replacing worn or damaged surfaces by building up the surface with a weld metal which approximately matches the composition and/or mechanical properties of the parent metal. Hardfacing Giving a softer material a wear, abrasion or erosion resistant surface. Cladding Providing a corrosion or oxidation resistant surface on a less corrosion resistant material, eg deposition of a stainless steel or nickel-based layer on a carbon steel base. One advantage of this technique is cost-saving when surfacing a relatively inexpensive metal, such as a carbon steel, with a more expensive but corrosion resistant layer of stainless steel. Material and weight savings may be gained when a clad, high strength, quenched and tempered steel is used in a corrosive environment. Buttering Depositing a layer of weld metal on to the face of a weld preparation or surface which will then form part of a welded joint, eg buttering an alloy steel weld preparation with a nickel-based weld metal and post-weld heat treating this part before making the joining weld between the buttering and a steel, which would be degraded by heat treatment. Surfaces of a different material may be achieved by a variety of methods: Solid-state bonding Joining the surface layer to the substrate by pressure or combination of pressure and heat. Clad plate may be made by rolling a sheet of the surfacing material and the substrate together or by explosively forcing the surface sheet, set up as a flier plate into intimate contact with the parent plate. Friction may be used to rub a new material on to the surface of the base plate. For small components, diffusion bonding may be used where two sheets are held under pressure and heated under vacuum to close to the melting point of the lower melting material for an extended time. Electrically melted Arc welding is the obvious technique with virtually all processes applicable, but other techniques such as electroslag strip cladding and electric discharge surfacing also possible. Spraying Usually involving a heat source used in welding – oxy-fuel, plasma, laser – but also possible as cold spraying by forcing the powder on to a surface with sufficient force to cause it to adhere.

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Surfacing techniques have been used in a variety of applications but only since the 1940s has arc welding. Since then all arc welding processes have been used. Every sector of industry - oil and gas, automotive, aerospace, power generation, yellow goods, etc - uses arc surfacing techniques for repair, recovery and to improve service performance. 16.2

Friction surfacing

A solid consumable bar rotated with one of its ends pressed hard against a substrate material. Heat is generated at the consumable tip, producing a plasticised layer. Lateral movement of the substrate, relative to the rotating consumable, deposits this plasticised material on to the substrate (see figure). There is no melting of the substrate material so no dilution of the substrate into the deposit. The composition of the deposit is the same as that of the consumable. 16.3

Surfacing by arc welding All arc welding processes can be used for overlay as well as joining applications. The choice of process depends on application, component size and geometry. Overlays are typically in excess of 2mm thickness and can be considerably thicker. Good adhesion is secured to the substrate through a metallurgical bond but weld surfacing involves fusion of the substrate to a certain extent and at the same time dilution of the overlay by the substrate material. Weld surfacing can be used to overlay new components with wear or resistant coatings or to restore worn components to their original dimensions. The most commonly used arc welding processes are MMA, MIG/MAG and SAW, the last using a wire or flat strip consumable. FCAW is being used increasingly because of the ease of tailoring the composition of the consumable to the application. Drawing a hardfacing wire down to a small diameter for MIG/MAG or SAW is, in many instances, impossible so cored wires are normally used. For specialised operations such as high alloy cladding of offshore oil and gas equipment, the hot wire TIG process is sometimes used. The main consideration with the surfacing process is achieving correct composition of the surfacing material. Selection of the most appropriate alloy is paramount but the amount of parent metal melted and mixed in with the filler metal (degree of dilution) is also of crucial importance, generally expressed as a percentage dilution of parent metal in the surfacing. Dilution varies from process to process and is influenced by welding parameters, in particular electrode polarity, welding current and travel speed. These need to be closely controlled to achieve consistency.

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Examples of common applications and typical alloy selection:

       

Repair of cast iron tooling (nickel alloys). Repair of injection moulding tools (martensitic steels). Repair of hot work tool steels (high speed steels). Engine exhaust valves (cobalt alloys). Wear plate for earthmoving, mineral extraction and transportation, concast rolls (ferro-chromium chromium carbide). Screw press flights (martensitic steels, cobalt alloys). Gate valves (cobalt alloys). Steelworks rolls (chrome alloys).

Excavator bucket fabricated from wear plate manufactured by open arc welding. 16.4

Thermal spraying A generic category of coating processes that apply a powder or wire consumable as a spray of finely divided molten or semi-molten droplets to produce a coating. Heat may be generated by oxy-fuel combustion (flame and HVOF) or electrically (arc and plasma). Thermal spraying processes have been widely used for many years throughout all the major engineering industry sectors for component protection and reclamation.

16.4.1 Lower energy processes The lower energy or metallising processes are arc and flame spraying and are widely used for reclamation of worn or damaged components and for depositing coatings of metals such as aluminium and zinc alloys to protect steel structures from corrosion. Coatings prepared with lower energy processes are quite porous and adhesion is lower than achieved with the higher energy techniques and the pores are often impregnated with a sealant or lubricant to improve coating performance. Sealants are widely used in applications where the surface must be resistant to corrosive environments. With the lower energy processes of flame and arc spraying, adhesion to the substrate is considered largely mechanical and is dependent on the substrate surface being very clean and suitably rough. Roughening is carried out by grit blasting and, occasionally, machining.

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16.4.2 Higher energy processes The higher energy processes of plasma, high velocity oxy-fuel and detonation spraying have been developed to produce coatings with much lower porosity and oxide levels, together with greater adhesion to the substrate, partly by spray particles having higher impact velocities. Surface preparation by cleaning and grit blasting is still extremely important. The range of coating types that can be deposited by higher energy processes is wider and increases the range of applications to include protective coatings for severe wear, high temperature oxidation and gaseous corrosion. The characteristics and properties of thermal spray coating material can vary significantly with process. Typical process characteristics and coating properties that can be obtained with the most widely used thermal spray processes are compared below. Attribute Typical flame temp., °C Typical particle velocity, m/s Gas flow, l/min Gas types Power, kW Powder particle size, µm Typical feed rate, kg/hr Typical materials Coating density, % Porosity, % Oxides, % Upper bond strength, MPa Typical deposit thickness, mm

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Flame spray >3000

Wire arc

Air plasma

HVOF

>3000

>5000

~3000

50-100

50-150

100-400

400-800

100-200

500-3000

100-200

400-1100

O2, C2H2

Air, N2, Ar

N2, Ar, H2, He

20 5-100

40-200 5-100

2-10

2.5 Wire size 1.2–4.8mm diameter 3-18

CH4, C2H2, H2, C3H6, O2 150-300 5-45

3-6

1-4

Metals, ceramics 85-90

Metals, cermets (cored wire) 80-95

Ceramics, metals 90-95

Ceramics, metals, cermets > 95

10-15 10-20 50

5-10 10-20 50

5-10 1-3 > 80

1-2 1-2 > 80

0.2-10

0.2-10

0.2-2

0.2-2

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Benefits:   

   

Comprehensive choice of coating materials: metals, alloys, ceramics, cermets and carbides. Thick coatings can be applied at high deposition rates. Coatings are mechanically bonded to the substrate, can often spray coating materials which are metallurgically incompatible to the substrate, eg materials with a higher melting point than the substrate. Components can be sprayed with little or no pre- or post-heat treatment and component distortion is minimal. Parts can be rebuilt quickly and at low cost, usually at a fraction of the price of a replacement. By using a premium material for the thermal spray coating, coated components can outlive new parts. Thermal spray coatings may be applied manually and automatically.

16.4.3 Applications          

Protective coatings for corrosion resistance. Protective coatings for abrasive and adhesive wear and erosion resistance. Coatings for composite materials. Functional coatings for electronic applications. Functional coatings for medical applications. Repair and maintenance. Spray form bearings. MCrAlY coatings. Thermal barrier coatings High temperature applications.

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IWS Revision questions 1

What are the likely reasons for surfacing one material with another?

2

What are the advantages of thermal spraying over arc surfacing?

3

What are the key features of solid state surfacing? Give an example.

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APPENDIX 1

Appendix 1 WPE 1 multiple choice questions day 1 Training only 1

What is a homogenous welded joint?

a b c d

The filler material is of a different composition. There is no filler material used. The filler material is of a similar composition. There is no heating of the joint.

2

In brazing, the melting point of the filler material is?

a b c d

Above that of the parent material. Below 450ºC. Above 450ºC. About the same as the material.

3

What is capillary action?

a b c d

The ability to weld below the material’s melting point. The movement of liquids against the force of gravity. A magnetic force that causes fusion. Fusion between different grades of material.

4

A solid state welding process is where?

a b c d

Only one material is welded. Both materials are melted. There is heating but no melting. The material becomes solid immediately.

5

A fillet weld with a 12mm leg length has an actual throat thickness of 10mm. What is the amount of excess metal?

a b c d

1.6mm. 8.4mm. 2.5mm. 3.5mm.

6

Which standard is used to demonstrate a welder’s skill without working to a procedure?

a b c d

EN EN BS BS

287. 15614. 5500. 4872.

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7

If the weld symbol to EN 22553 is on the indication line, where does the weld go?

a b c d 8

Arrow side. Opposite arrow side. Other side. Near side. The letter ‘a’ represents what to EN 22553?

a b c d

Leg length. Penetration depth. Design throat. Actual throat.

9

In general terms, when welding two different thicknesses in a fillet weld configuration, the leg length is determined by?

a b c d

Thickest material. Smallest material. Average of the two. It does not matter.

10 Where does the indication line go on an EN 22553 weld symbol? a b c d

Above reference line. Below reference line. It does not matter. It depends on joint type.

11 An inert gas is one of the following? a b c d

Does not react with other substances. Does react with other substances. Is explosive. Has a distinct smell.

12 What does OCV mean? a b c d

On current voltage. Over current voltage. Open circuit voltage. Often creates voltage.

13 The term OEL means? a b c d

Other elements limited. Occupational exposure limits. Occupational employment life. On extreme limits.

Rev 4 January 2013 Appendix 1 Practice Exams Day 1

A1-2

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14 Which fuel gas produces the most heat? a b c d

Acetylene. MAPP. Propane. Natural gas.

15 Which type of flame is used for gas welding? a b c d

Neutral. Oxidizing. Cabourizing. High pressure.

16 When turning off gas welding equipment, which gas is turned off first? a b c d

Oxygen. Acetylene. All at the same time. It does not matter.

17 At what pressure is oxygen stored in the cylinder? a b c d

200bar. 300bar. 400bar. 50bar.

18 The left ward welding technique used for gas welding, is typically used for what? a b c d

5mm and below. 5mm and above. Butt welds. Fillet welds.

19 On most welding equipment the typical OCV is? a b c d

60-100v. 222-240v. 23-30v. 30-50v.

20 At what decibel level should an employer provide hearing protection? a b c d

Above 95db. Above 105db. Between 80-85db. Between 85-90db.

Rev 4 January 2013 Appendix 1 Practice Exams Day 1

A1-3

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WPE 1 multiple choice questions day 2 Training Only 1

What material is the best conductor of electricity?

a b c d

Copper. Aluminium. Silver. Tin.

2

Electrons move at the speed of light, how fast is this?

a b c d

!0,000 miles per second. 86,000 miles per second. 186,000 miles per second. 286,000 miles per second.

3

The term EMF means

a b c d

Electron movement ferocity. Electro motive force. Electric motive force. Electricity moves fast.

4

What is the purpose of a transformer on welding equipment?

a b c d

Smooth the welding current. Step down voltage and step up amperage. Step down amperage and step up voltage. Improve arc initiation.

5

Which statement is true, regarding the welding arc?

a b c d

Electrons are negatively charged, Ions are positively charged. Electrons are positively charged and Ions are negatively charged. Electrons and Ions change their polarity depending on Welding current. They alternate between negative and positive.

6

Inductance in a current does what?

a b c d

Controls the arc gap. Changes the rate at which current rises. Sets up a resistance so filler material heats quicker. Ensures that arc initiation is smoother.

Rev 4 January 2013 Practice Exams Day 2

A1-4

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7

Electrical power consumption is measure using?

a b c d

A=W x V. W= I x V. V= I x R. R= W x I.

8

Which of the following statements is true?

a b c d

The Anode is positive the Cathode is negative. The Cathode is positive the Anode is negative. They alternate during welding on DC. All of the above.

9

Ohms law is?

a b c d

W= I x R. V= I x R. I=R x W. R=W x I.

10 The term CPS means? a b c d

Current positive system. Cycles per second. Cycles positive sometimes. Current permanently smooth.

11 What is the purpose of a slope out device? a b c d

Prevent over penetration. Give greater penetration. Prevent crater cracking. Prevent arc blow.

12 Why are pure tungsten electrodes not used extensively? a b c d

Too expensive. They melt at high temperatures. They have an unstable arc. Difficult to prepare.

13 Which shielding gas gives the highest penetration? a Argon. b CO2. c Argon plus CO2. Helium. 14 Why is A.C. used predominantly to weld aluminium? a b c d

Gives a smoother arc. Produces a cathodic cleaning action. Improves penetration. Reduces cracking.

Rev 4 January 2013 Practice Exams Day 2

A1-5

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15 What is the purpose of the gas delay function? a b c d

Helps purge the gas line. Cools the weld. Prevents tungsten contamination. Improves fusion.

16 What is the purpose of an A.C. balance control function? a b c d

Gives good arc stability. Allows greater control over amperage. Gives control over penetration. Allows switching from DC to AC.

17 What determines arc energy? a b c d

Filler size, gas flow, tungsten diameter. Amps, volts and travel speed. Gas type, polarity and ceramic size. Tungsten type, electrode extension and angle of electrode.

18 What does the high frequency do? a b c d

Allows arc striking without electrode contact. Pulses the welding current. Smoothes the welding current. Increase tungsten life.

19 Which tungsten type is used for welding aluminium? a b c d

Zirconiated. Thoriated. Chrominium. Clad.

20 What is meant by the term autogenous? a b c d

High welding speeds. Low welding speeds. Welding without filler. Positional welding.

21 What’s the purpose of the slope up device on a TIG welding set? a b c d

Helps prevent tungsten inclusions. Gives greater penetration. Improves positional welding. Improves arc initiation

22 Which shielding gas would normally be used for welding aluminium? a b c d

CO2. Argon. Argon plus CO2. Nitrogen.

Rev 4 January 2013 Practice Exams Day 2

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23 What characteristics, best describes aluminium a b c d

Low thermal conductivity. High thermal conductivity. Hard and heavy. Good abrasive qualities.

24 What is the correct polarity for welding stainless steel with the TIG process? a b c d

AC. DC-. DC+. Does not matter.

25 What would be the typical gas flow rate for welding aluminium with the TIG process? a b c d

8-12 litres per minute. 4-6 litres per minute. 16-20 litres per minute. 20-24 litres per minute.

26 What would happen if the current range was exceeded for a tungsten electrode? a b c d

Greater penetration. Tungsten inclusions. Porosity. Poor weld profile.

27 Why are stainless steel root runs purged using the TIG process? a b c d

To To To To

give greater penetration. prevent oxidization. give less penetration. prevent suck back.

28 What are the characteristics of stainless steel? a b c d

High thermal conductivity and low distortion. Low thermal conductivity and high distortion. High distortion and high thermal conductivity. Low distortion and low thermal conductivity.

29 What is the typical amperage range for a 1.6mm thoriated tungsten electrode? a b c d

90-150amps. 30-100amps. 250-450amps. 200-300amps.

30 What are the main features of an inert shielding gas? a b c d

Can be smelt and lighter than air. Cannot be smelt and heavier than air. Can be seen and smelt. Can be smelt and combines with other elements.

Rev 4 January 2013 Practice Exams Day 2

A1-7

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WPE 1 Multiple choice questions day 3 Training Only 1

In MIG/MAG welding, if stick out length was increased what would be the affect?

a b c d

Voltage would increase. Amperage would increase. Amperage would decrease. Voltage would decrease.

2

The inductance in MIG/MAG equipment controls?

a b c d

Rate of current rise. Wire speed rate. Arc length. Burn back.

3

Solid wire MIG/MAG uses which polarity?

a b c d

AC.. DC-. DC+. All of the above.

4

Solid wire spray transfer has the following feature?

a b c d

Low deposition rate. High deposition rate. Good positionally. High solidification rate.

5

A typical shielding gas for welding aluminium

a b c d

95% Ar 5% CO2. 100% Ar. 100% CO2. 80% Ar 20% CO2.

6

Increasing voltage in MIG/MAG would have what affect?

a b c d

Increase penetration. Decrease penetration. Increase excess weld metal. Increase welding speed.

Rev 4 January 2013 Practice Exams Day 3

A1-8

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7

The current in MIG/MAG welding is controlled by which parameter?

a b c d

Voltage. Wire feed speed. Inductance. All of the above.

8

In MIG/MAG welding which mode of metal transfer can be used for the widest range?

a b c d

Spray. Dip. Globular. Free flight transfer.

9

How would spatter be controlled using Dip transfer?

a b c d

Use Use Use Use

pure CO2 DC negative polarity. the inductance. pure argon.

10 Which electrical characteristic is associated with MIG/MAG equipment? a b c d

Constant Constant Constant Constant

current. voltage. amperage. output.

11 In MIG/MAG welding, which mode of metal transfer can suffer from lack of fusion? a b c d

Pulse. Dip. Spray. Free flight.

12 In MIG/MAG welding, the spray mode of metal transfer can be characterised by? a b c d

An open and closed arc cycle. A long open arc. The droplet being pulsed across the arc. All of the above.

13 Which of these statements is true concerning spray transfer? a b c d

Can be used positionally. Can be used positional with FCAW. Produces a relatively low heat input. Cannot be used for aluminium.

14 Which of these processes cannot be used for welding steel? a b c d

MMA. MIG. TIG. SAW.

15 What does the term duty cycle refer to?

Rev 4 January 2013 Practice Exams Day 3

A1-9

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a b c d

Time the welder spends welding. Amount of time an electrode is being used. Amount of time a welding machine is being used. Amount of electricity being consumed.

16 In MIG/MAG welding, which mode of metal transfer uses inductance to control welding conditions? a b c d

Spray. Dip. Globular. Pulse.

17 Select a typical range for spray transfer? a b c d

21V 26V 24V 20V

200A. 230A. 190A. 150A.

18 In MMA welding, which electrode would give the highest level of penetration? a b c d

Rutile. Basic. Cellulostic. Iron powder

19 In MMA welding, which electrode is not designed for positional welding? a b c d 20

Rutile. Basic Cellulostic. Iron powder. Which type of electrical output characteristic is associated with MMA?

a b c d

Constant voltage. Flat characteristic. Constant current. All of the above.

21 With MMA electrode classifications, what does the letter E represents? a b c d

Extruded. Electrode. Covered electrode. Extended.

22 With MMA electrode classifications, what does the first two numbers represent? a b c d

Charpy value. Welding position. Recovery rate. Tensile strength.

23 Which electrode would give the highest hydrogen content? a b

Rutile. Iron powder.

Rev 4 January 2013 Practice Exams Day 3

A1-10

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c d

Basic. Cellulostic.

24 In MMA welding, which polarity would give the highest level of penetration? a b c d

AC. DC-. DC+. About the same.

25 With MMA welding, if the arc length is increased, what is the most likely outcome? a b c d

Higher penetration. Decrease in amperage. Decrease in voltage. All of the above.

26 In MMA welding, what is known as recovery rate? a b c d

How How How How

much flux is recovered as weld. long it takes for the welding equipment to recover from welding. much of the electrode is recovered as weld metal. much of the core wire is recovered as weld metal.

27 What is the purpose of a rectifier on welding equipment? a b c d

Changes AC to DC. Steps up amperage, steps down voltage. Steps up voltage and steps down amperage. All of the above.

28 ROL means? a b c d 29

Regulating open latitude. Rolling over length. Roll out length. Roll out limit. Which MMA electrode would give the highest recovery rate?

a b c d

Rutile. Iron powder. Cellulostic. Basic.

30 Which MMA electrode is to an EN classification? a b c d

E6011. E 35 3 B. E 45 35 B. E 7013

Rev 4 January 2013 Practice Exams Day 3

A1-11

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WPE 1 Multiple choice questions day 4 Training only 1

In the SAW process, which polarity is often used to prevent arc blow?

a b c d

AC. DC+. DC-. All of the above.

2

Which type of Saw flux is prone to picking up moisture?

a b c d

Fused. Agglomerated. Acidic. Neutral.

3

In the SAW process, what is the main effect of increasing the voltage?

a b c d

Wider weld. Narrower weld. Greater penetration. All of the above.

4

The SAW process used below 1000amps would be classified as having a?

a b c d

Flat characteristic. Drooping characteristic. Constant current. High output characteristic.

5

Twin wires are often used in the SAW process to improve deposition rates. To prevent arc blow the polarity combination is?

a b c d

DC leading and AC following. AC leading and DC following. DC+ leading DC- following. DC- leading and DC+ following.

6

In the Saw process welding above 1000 amps the static electrical characteristic is?

a b c d

Constant current. Constant voltage. Flat characteristic. DC+.

Rev 4 January 2013 Practice Exams Day 4

A-12

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7

Which SAW flux can break down into fine particles?

a b c d

Fused. Agglomerated. Rutile. All of the above.

8

In the Saw process, what is the typical depth of flux whilst welding?

a b c d

10-15mm. 25-30mm. 40-50mm. 50-60mm.

9

What is the typical maximum thickness for the ESW process?

a b c d

100mm. 200mm. 300mm. 400mm.

10 Which of the following is a major advantage of the ESW process? a b c d

Defect free. All positional. Good toughness values. Very versatile.

11 What is the typical thickness range for the oxy fuel cutting process? a b c d

5-100mm. 5-150mm. 3-150mm. 0.5-250mm.

12 Using the oxy fuel cutting process on steel, what is the typical ignition temperature? a b c d

700-900ºC. 1200-1400ºC. 500-600ºC. 1500-1600ºC.

13 Why cannot aluminium be cut using the oxy fuel process? a b c d

Melting point too low. High thermal conductivity. Oxide coating. High distortion rate.

14 If using the oxy fuel process to cut steel, which one of these statements is true? a b c d

It is cut below its melting point. It is cut at its melting point. It is cut above its melting point. The temperature is not important.

Rev 4 January 2013 Practice Exams Day 4

A-13

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15 What is the typical temperature of a plasma cutting arc stream? a b c d

6000ºC. 10,000ºC. 15,000ºC. 20,000ºC.

16 A plasma cutting power source has what type of static output characteristic? a b c d

Constant voltage. Constant current. Flat characteristic. Variable.

17 In the plasma arc cutting process, what type of polarity is used? a b c d

DC-. DC+. AC. All of the above.

18 Which of these cutting processes does not melt the material? a b c d

Oxy fuel. MMA gouging. Plasma. Arc air.

19 What material can oxy fuel cut successfully? a b c d

Aluminium. Stainless steel. Carbon steel. Copper.

20 What’s the main reason why oxy fuel gas cutting cannot cut stainless steel? a b c d

Higher melting point. Re factory oxides. Low thermal conductivity. It can be cut successfully.

Rev 4 January 2013 Practice Exams Day 4

A-14

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Welcome

Welding Processes and Equipment IIW/EWF Diploma in Welding Objectives Welcome - What this module is about

Welcome to the Welding Processes and Equipment module of TWI’s Diploma course approved by the International Institute of Welding (IIW) and European Welding Federation (EWF)

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What Does This Module Cover?      

Absolute basics – defining what a weld is. Detailed principles – how plasma is formed. Electricity – how it is used in welding. Processes – common and more specialised. Standards – briefly, those on fabrication. Symbols – how to show welds on drawings.

What Can I Expect?  Working to international syllabus. 

IAB-252r8-07 (short version on IIW website www.iiw-iis.org )

 This is one of four modules each examined separately.  Qualification towards TWI Diploma.  Qualification towards IIW/EWF Diploma. 

Requires entrance criteria to be met

 Greater understanding of important aspects of welding.

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What Learning Methods Are Used?     

Example – Self-Adjusting Arc

Binder has notes and powerpoints. Lectures given in classroom style. Extra study encouraged – necessary really. Interaction – especially for engineer. Tuition and counselling – talk to us.

Feed speed = burn off

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V up, i down, burn off down. Feed speed > burn off

Wire advances, i increases until: Feed speed = burn off Copyright © TWI Ltd 2015

0-1

Example – Laser Deposition

Why Is This Module Important To Me?  Welding Engineer, Technologist, Specialist must know fundamentals of processes.  Regarded as company specialist.  Choose best process for job.  Make decisions on best use of processes.

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My Company Has Fixed Ideas  WL Bateman: "If you keep on doing what you've always done, you'll keep on getting what you've always got."  Everyone wants cost efficiency.  Today’s equipment and control make even a few years-old gear obsolete.  Future developments always seek to improve.  Your company will want you input.

I Just Need To Sign The Paperwork  Short-term objective gaining Welding Co-ordinator status is excellent.  Co-ordinator does not just sign paperwork.  Contracts need co-ordinator.  Future contracts need to be at required quality and profitable.  Co-ordinator can advise best practice and save company money.

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What Will I Do That I Don’t Now?  Tricky – all individuals coming with different backgrounds.  Depth of understanding can sort problems.  New perspectives on traditional processes – experience from another viewpoint helps.  New processes detailed – could be applicable now or in future.

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What’s In It For Me?     

Knowledge – better performance at job. Where to find reference material when needed. Ability to respond to changing needs. Possibility of Professional Qualification. More assured future with wider prospects.

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0-2

Joining

General Introduction to Welding TWI Training & Examination Services

       

Welding. Brazing. Soldering. Adhesive bonding. Diffusion bonding. Riveting. Clinching. Sewing, stapling, etc.

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Welding

An operation in which two or more parts are united by means of heat or pressure or both, in such a way that there is continuity in the nature of the metal between these parts.

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Parts To Be Joined  Parent material, base material. 

   

Metals. Plastics. Ceramics. Composites.

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Brazing  A process of joining in which, during or after heating, molten filler metal is drawn into or retained in the space between closely adjacent surfaces of the parts to be joined by capillary attraction.

Plate, pipe, section

 Filler, consumable. 

Weldable/Un-weldable

Electrode, wire, powder

Completed item may be called:  Joint.  Weld.  Weldment.

 In general, the melting point of the filler metal is above 450°C but always below the melting temperature of the parent material.

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1-1

Soldering  A similar process to brazing, relying on capillary attraction to draw molten filler into a gap between parts that remain solid throughout. Solders melt at low temperatures, less than 450ºC.

Welding  Fusion. 

Melting of parent, filler, or usually both

 Solid state. 

May or may not be heated, but no melting

 For steel and copper, solders are usually alloys of tin.

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Fusion Welding          

Oxy-fuel gas (OFW). Manual metal(lic) arc (MMA). Metal inert/active gas (MIG/MAG). Flux cored arc (FCAW). Submerged arc (SAW). Electroslag (ESW). Electron beam (EBW). Laser. Resistance. Magnetically impelled arc butt (MIAB).

Solid State Welding  Forge or blacksmith.  Friction – many variations, including friction stir.  Explosive.  Cold pressure.  Ultrasonic.

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Joint Terminology

Edge

Open and Closed Corner

Butt Preparations

Lap

Square edge closed butt Cruciform

Tee

Square edge open butt

Butt

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1-2

Single Sided Butt Preparations Single sided preparations are normally made on thinner materials, or when access form both sides is restricted.

Single Bevel

Single Vee

Single-J

Single-U

Double Sided Butt Preparations Double sided preparations are normally made on thicker materials, or when access form both sides is unrestricted

Double -Bevel

Double - J

Double -Vee

Double - U

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Joint Preparation Terminology Included angle

Included angle

Joint Preparation Terminology Angle of bevel

Angle of bevel

Angle of bevel Root Radius

Root Radius

Root Face

Root Face

Root Gap

Root Gap

Single-V Butt

Single-U Butt

Root Face Root Gap

Single Bevel Butt

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Fillet weld

Single-J Butt

Penetration

Spot weld

Full penetration

Edge weld

Root Face Land

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Weld Terminology

Butt weld

Root Gap

Partial penetration

Plug weld Compound weld

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1-3

Sides

Single sided

Runs

Single run

Double sided

Multirun

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Stringer or Weave

Welding Positions

Flat - PA

Stringer bead

Horizontal-Vertical PB

Horizontal - PC

Weave

Overhead - PD

Horizontal-overhead - PE

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Vertical-up - PF Vertical-down - PG

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Slope and Rotation

Weld Zone Terminology

Weld slope  The angle between root line and the positive X axis of the horizontal reference plane, measured in mathematically positive direction (ie counter-clockwise).

Face

A

B

Weld metal

Weld rotation  The angle between the centreline of the weld and the positive Z axis or a line parallel to the Y axis, measured in the mathematically positive direction (ie counter-clockwise) in the plane of the transverse cross section of the weld in question.

Heat affected zone

Weld boundary

C D Root A, B, C and D = Weld Toes Copyright © TWI Ltd 2015

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1-4

Weld Zone Terminology Excess Cap height

Weld Zone Terminology

Weld width

Excess root penetration

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Toe Blend  The higher the toe blend angle the greater the amount of stress concentration.

Features to Consider Fillet welds - toe blend

 The toe blend angle ideally should be between 20o-30o.

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Fillet Weld Profiles

Fillet Weld Profiles

Fillet welds - shape Excess Weld Metal

Vertical Leg Length

Mitre fillet

Convex fillet

Design Throat Horizontal leg Length

Concave fillet

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1-5

Fillet Weld Throat Thickness

Fillet Weld Throat Thickness

a

b

a = Design throat thickness

b = Actual throat thickness

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Leg and Throat Relationship

Throat, a = 0.7 x Leg, z Leg, z = 1.4 x Throat, a a = z/√2

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1-6

Types of Standard  Application and design.  Specification and approval of welding procedures.

Fabrication Standards

 Approval of welders.

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Levels of Standards  Company or industry specific standards.  National BS (British Standard).  European BS EN (British Standard European Standard).  US AWS (American Welding Society) and ASME (American Society of Mechanical Engineers).  International ISO (International Standards Organisation).

Welding Procedure Approval Test  Carried out by a competent welder.  Quality of the weld is assessed using NDT and mechanical testing techniques.  Demonstrate proposed welding procedure gives welded joint to specified weld quality and mechanical properties.

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Process Terminology – BS EN ISO 4063

Welder Approval Test  Examines welder's skill and ability to make satisfactory test weld.  Test may be performed with or without a qualified welding procedure.  BS EN 287, BS ISO EN 9606 and ASME Section IX for quality work.  BS 4872 shows an adequate level of skill from general work.

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        

1 2 3 4 5 6 7 8 9

– – – – – – – – –

Arc welding. Resistance welding. Gas welding. Welding with pressure. Beam welding. Not used. Other welding processes. Cutting and gouging. Brazing, soldering and braze welding.

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2-1

Process Terminology – BS EN ISO 4063 Actual processes depicted by three digits, eg:    

111 114 121 125

– – – –

 131 –  135 –  136 –  141 –

Manual metal arc welding Self-shielded tubular-cored arc welding Submerged arc welding with one wire electrode Submerged arc welding with tubular cored electrode Metal inert gas welding (MIG welding) Metal active gas welding (MAG welding) Tubular cored metal arc welding with active gas shield Tungsten inert gas arc welding (TIG welding)

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2-2

Why Are Symbols Needed?    

To To To To

avoid excessive wording on drawing. give universally accepted description. ensure everyone has same understanding. achieve design requirement on shop floor.

Weld Symbols TWI Training & Examination Services

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Basic Design of Symbols

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Supplementary Symbols

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Basic Symbols for Edge Preparation

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Complementary Symbols

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3-1

Dimensioning Fillet Welds

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Symbols for Intermittent Welding

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Summary of Weld Symbols

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3-2

Creation and Protection of Weld Pool

Introduction to Fusion Welding TWI Training & Examination Services

Fusion welding:  Heat to melt parent plate and filler.  Protection of melt from atmosphere. Heat:  Flame.  Electric arc.  Electrical resistance.  Power beam. Protection:  Vacuum or controlled atmosphere.  Shielding gas and/or flux.

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Protection

Gas Shielding Inert gas.  Argon – Ar.  Helium – He.  Ar-He.  Nitrogen – N2 (inert for copper, but not others). Active gas.  CO2.  Ar-CO2.  Ar-O2.  Ar-H2.

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Flux Shielding

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Leftward and Rightward Directions

 Flux may create gas to shield arc.  Flux may have ingredients that react with oxygen or nitrogen.  Flux melts and solidifies to slag that covers hot metal and excludes air.

Leftward technique

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Rightward technique

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4-1

Creation of a Molten Pool    

Flame. Arc. Resistance. Power beam.

Flame  Burning fuel gas with oxygen creates flame temperature around 3000°C.  Cannot melt refractory metals – Nb. Mo, W.  Heat transfer by conduction and small amount radiation.  Parent material and filler, if used, melt and mix in pool.

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Arc  Electrical potential ionises gas to give conductive path between electrode and work.  Arc generates plasma of ionised gas.  Temperature very high – ca 10,000°C.  Heat transfer by conduction and radiation.  Will melt all metals.

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Resistance  Two sheets of metal pressed together by electrodes of Cu-Cr alloy.  Current passed between electrodes has to cross boundary between sheets.  High resistance at boundary generates heat that melts the interface.  Pressure applied to compact the molten area into a nugget.

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4-2

Compliance  Government legislation – The Health & Safety at Work Act.  Health & Safety Executive – COSHH Regulations, Statutory instruments.  British Standards – OHSAS 18001.  Company Health and Safety Management Systems.  Work instructions – permits to work, risk assessment documents etc.  Local Authority requirements.

ARC Welding Safety TWI Training & Examination Services

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Must Consider       

Electric Shock  Primary 240 or 460V mains.  Do not open welding equipment.  Only qualified electrician to wire or repair machine.  Secondary 60-100V high current.  Don’t touch metal parts of torch or electrode holder – certainly not when touching an earth.  Don’t work with worn cables.  Cables must have capacity for max current.

Electric shock. Heat and light. Fumes and gases. Noise. Gas cylinder handling and storage. Working at height or in restricted access. Mechanical hazards: trips, falls, cuts, impact from heavy objects.

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Copyright © TWI Ltd 2015

Electric Shock Assistance

Heat         

 Don't touch the person.  Keep others from being harmed.  Switch off power.  Use non-conductive pole to free the person.  Check obvious injury.  Move victim only when power off and no neck or spine injuries.

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Burns can be severe. Assume all metal around welding is hot. Don’t use hand pat to check. Use indicator stick. Sparks ignite flammable material – remove. Hot metal spatter gives very serious burns. Don’t tuck trousers in boots. Don’t wear turn-ups. Ventilate and cool welder in confined space.

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5-1

Light  Different hazards according to type.  Type depends on wavelength.  Welding creates all three types.

Type

Wavelength, nm

Infra-red (heat)

>700

Visible light

400-700

Ultra-violet radiation

<400 Copyright © TWI Ltd 2015

Visible  Intense visible light from arc can dazzle and damage network of nerves on the retina.  Effects depend on the duration and intensity of exposure.  Natural reflex to close eyes.  Normally this dazzling does not have longterm effect.

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Ultra-Violet Effect on Skin  UV from arc processes does not produce attractive browning effect of suntan.  Gives acute reddening and irritation caused by changes in minute surface blood vessels.  Skin can be severely burned and blister.  Reddened skin may die and flake off later.  Intense, prolonged or frequent exposure, can give skin cancer.

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Infra-Red  Years of exposing eyes to IR causes gradual but irreversible opacity of the lens.  IR emitted by welding arc causes damage only short distance from the arc.  Burning sensation in the skin surrounding eyes exposed to arc heat. Natural reaction to move or cover up.  Rest of skin absorbs heat so cools the welder – Do not remove clothing to cool.

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Ultra-Violet Effect on Eye Cornea, conjunctiva inflammation – Arc eye.  Arc eye caused by UV damaging layer of cells in cornea.  Damaged cells die and fall off cornea exposing highly sensitive nerves.  Rubbing of eyelid causes intense pain, usually described as sand in the eye.  Pain becomes even more acute if eye is exposed to bright light after damage.  Arc eye develops some hours after exposure.

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Fume  Fume is from vaporisation, condensation and oxidation of substances by arc.  Particles very small remain in air for long time so may be breathed.  Small particles are respirable penetrate the innermost regions of the lung where they have the most potential to do harm.  Welding fume may be hazardous to health must be controlled to regulation limits.

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5-2

Is Fume Hazardous?  Degree of risk depends on:   

Composition. Concentration. Length of time of exposure.

 Need to know parent plate, any coating, filler and composition of fume generated.  Different fume components vary in toxicity.  Limits given in guidance note EH40 Workplace Exposure Limits available from the Health and Safety Executive (HSE).

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Gases Toxic gases can appear in welding and cutting:  Fuel gases when burnt form CO2 and CO.  Shielding gases Ar, He, CO2.  CO2 and CO from welding flux or slag.  NO, NO2, O3 from heat or UV on atmosphere surrounding the welding arc.  Gases from the degradation of solvent vapours or surface contaminants on the metal.

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Local Fume and Gas Extraction

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Is Fume Hazardous?  Fe3O4, CaCO3, TiO2 have WEL of 4 or 5mg/m3.  Similar to any dust – no specific health issue but needs control for proper lung function.  Mn, Cr3+, soluble Ba set at 0.5mg/m3.  Cu is 0.2mg/m3.  Cr6+, NiO potential carcinogens so:  

Soluble Ni WEL of 0.5mg/m3. Cr6+ only 0.05mg/m3.

 Exposure over time-weighted average 8hours.

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Why Are Gases a Problem?  Ar, He, CO2 all asphyxiants – can’t see or smell them in confined space.  Breathing <18% O2 can pass out in seconds.  CO is toxic, WEL 30ppm – can be formed in OFW, MMA, MIG, SAW.  NO and NO2 ‘NOx’ formed by plasma cutting.  O3, WEL 0.2ppm, formed in TIG and MIG, especially on Al, at a distance from arc.

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Portable Fume Extraction Equipment

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5-3

Noise  Welding not excessively noisy but:   

Gas Handling and Storage  Gas cylinders can be pressurised to 300bar.  Sudden release creates 100kg missile:

Air-arc gouging. Grinding. Metalworking.

 

Can all give excessive noise levels >85dB hearing protection MUST be worn 80-85dB protection must be available and given if operator requests it.

   

http://www.youtube.com/watch?v=ejEJGNLTo84 http://www.youtube.com/watch?v=CHDAbM09Y1o

Must keep in secure cradle or trolley. Should not be lifted by single person. Fit correct pressure regulator. Check for leaks in hoses and equipment.

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Working at Height All standard precautions:  Correctly erected scaffolding  Ladders tied in  Handrails, safety cages on any lifts  Running boards and kickboards fitted and tied Risk assessment for welding:  Can you lift gas cylinders?  Is welder protected from fall if electric shocked?  Are others protected from falling hot metal?

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Working in Confined Space All standard precautions:  Permit to work.  Risk assessment.  Emergency evacuation procedure. Additional risks when welding or cutting:  Gas accumulation – asphyxiation, explosion.  Toxic gas, eg CO, if poorly set-up.  Use externally-fed helmets.  Operate buddy scheme.

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Copyright © TWI Ltd 2015

Mechanical Hazards      

Large heavy metal pieces need manipulation. Thin metal has sharp edges. Welding cables can be trip hazard. Spark and spatter ejection risk to others. Mechanised welding needs guards. Vibration White Finger can result from 30min of chipping hammer per day.

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Mechanical Hazards Vibration white finger from:  Grinders.  Pneumatic burr tools.  Chipping hammers.  Needle guns.

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5-4

Acetylene     

Oxy-Fuel Gas Welding

Highest temperature. Highest heat energy in inner flame. Lowest ratio of O2. Ideal for welding higher MPt metals, eg steel. Good for cutting.

TWI Training & Examination Services

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Copyright © TWI Ltd 2015

Propane  Highest heat energy in outer flame.  Ideal for preheating.  Can preheat steel prior to oxygen injection so can be used for cutting.

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Flame Type  Neutral – equal C2H2 and O2.

 Oxidising – excess O2.

 Reducing – excess C2H2.

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MAPP    

Methylacetylene and propadiene. Can be readily compressed. Useful for underwater work. Cutting and welding possible.

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Neutral Flame  Inner cone rounded and distinct – white – C2H2 and O2 burn to CO and H2.  Surrounded by colourless tongue where CO and H2 will reduce any metal oxides.  Outer zone – slightly blue – CO and H2 burn with O2 from air to give CO2 and H2O.  Fizzling sound.  Used for welding ferritic steel, stainless steel, copper alloys, brazing, braze welding.

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6-1

Oxidising Flame    

Very small pointed inner cone. Bright blue, almost violet, outer zone. Excess O2 means oxide will form. Used for welding zinc to avoid vapourisation.

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Gas Welding Equipment

Reducing Flame  Long white inner cone.  Excess C2H2 burns at edge of outer zone with O2 from air, ragged edge.  Luminous, slightly yellow.  No sound.  Carburising so used for hardfacing.  Used for Al alloys to avoid oxide build up.

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Gas Welding Equipment  Oxygen supply – white top to cylinder.  Acetylene supply – maroon top to cylinder.  Regulators, specific for gas, to reduce pressure.  Flashback arrestors.  Hoses colour coded for gas.  Non-return valves.  Blowpipe.  Nozzles to suit application.

*

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Safety Checks Before Welding  Flashback arrestors and non-return valves.  Hoses, blue for oxygen and red for acetylene, have no sign of wear.  Regulators are correct type for the gas.  Cylinder key in each cylinder.  All connections are tight, no leaks.  No oil or grease near any part of oxygen line or cylinder.  No copper containing material in direct contact with acetylene.

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Copyright © TWI Ltd 2015

Gas Welding Mode of operation  Fuel gas and oxygen mixed in body of blowpipe then fed through nozzle and burnt.  Welder manipulates blowpipe to melt edges of workpiece and so form weld pool.  Filler metal (rod) is added as required.  Weld pool protected from atmospheric contamination by the burnt gas products and can be made mildly oxidising or reducing.

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6-2

Gas Welding Parameters      

Nozzle size. Gas pressure. Gas flow rate. Tip to work distance. Travel speed. Leftward or rightward technique.

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6-3

Conductors and Non-Conductors

Electricity as Applicable to Welding TWI Training & Examination Services

Conductors.  Metals.  Graphite.  Salt solutions.  Plasma (ionised gas). Non-conductors.  Most non-metallic materials, eg rubber, O2 gas.  Most organic material, eg wood, cotton.  Most minerals, eg limestone, clay, rocks.

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Copyright © TWI Ltd 2015

How a Conductor Works  Electrons on outside of atom loosely bound.  Can be stripped from atom by electrical potential (+/- voltage).  Electrons are negatively charged so flow towards positive.  Rest of atom positively charged, called ion, flows toward negative.  Metals have loose electrons helping to bind atoms together, even without electricity, so very good conductors when potential applied.

Magnetism and Electricity  Magnetism is naturally occurring, earth has a magnetic field.  Concept of North and South poles for earth and for magnets.  Magnets apply force on charged particles.  

North is +ve – will attract electrons South is –ve – will attract positive ions

 Loosely bound electrons in metal move in a magnetic field.

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Copyright © TWI Ltd 2015

Dynamo Principle  Move metal wire through magnetic field and electrons move along it to try to stay close to positive North pole.  Use many wires and keep moving, many electrons flow along wires.  Collect electrons from wires – flow of electricity.  Can have annular magnets and spin wire bundle in centre or make wire bundle annular and spin magnet in centre – dynamo. Copyright © TWI Ltd 2015

Dynamos

Principle of bicycle dynamo

Gramme Dynamo 1870

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7-1

Electrical Terms Potential or voltage.  Creates drive - size of difference between + and  Termed V, measured in volts. Electromotive force, EMF.  Drive created by electrical potential.  Termed ε, measured in volts. Current.  Flow of electrons and ions.  Termed i, measured in amperes (Amps).

Power Available power depends on both i and V:  240V indicator lamp on equipment – dim.  12V battery lamp – very bright. Product of i and V is power consumption, W, measured in watts: W=ixV Available power measured in same way, eg 240V mains on 13A fused circuit has: W = 13 x 240 = 3120 = 3.12kW.

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Copyright © TWI Ltd 2015

Resistance      

House Cu wire – 3kW no noticeable effect. Electric fire wires glow red and give out heat. Cu low resistance, passes current very easily. Ni-Cr high resistance, current flow difficult. Resistance, R, measured in Ohms, Ω. Ohms Law: V=ixR

Heating Effect  Difficulty of flow in Ni-Cr wire gives energy loss as heat.  Happens in all conductors, even Cu house cables can heat up.  Heating effect proportional to resistance of wire and square of current carried: i2R Effect

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Copyright © TWI Ltd 2015

Direct and Alternating Current  Dynamo, and modern generator, gives current all in same direction – direct current (DC).  National Grid supplies current that changes direction – alternating current (AC).

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Frequency    

Number of cycles per second can vary. One cps is called 1 Hertz, 1HZ. European grid supply is 50 cps, 50Hz. US grid supply is 60Hz.

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7-2

Transformation  To minimise loss, grids have very high voltage – 400,000V.  Reduce for domestic and industrial use.  Link between electricity and magnetism used.  Current at high voltage passed through coil with iron core – gives magnetic flux in iron.  Core is loop and passes through second coil of wire – induces current in this coil.

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Welding Current and Voltage  Welding needs high current but low voltage.  At 80V (typical starting voltage for arc) mains 15A ring main supply transforms to 45A.  Even cooker supply gives only 90A. Can be used for small hobby jobs as arc runs at around 30V after start.  Industrial jobs need industrial supply.  415V, 125A transforms to 650A at 80V or 1700A at 30V. Copyright © TWI Ltd 2015

Full-Wave Rectification Use four diode bridge.

Turns negative half-cycle to positive.

The Transformer  Voltage in 2nd coil depends on turns. V1/V2 = n1/n2  High V, more turns.  Low V , few turns.  Energy preserved so:  High V, low i.  Low V, high i.

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Rectification Half-wave:  Pass AC through diode, only allows one way flow:

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Series and Parallel  Daisy chain resistors in series: R = R1 + R2 + R3 + ...

A

 Link resistors piggy back in parallel:

1/R = 1/R1 + 1/R2 + 1/R3 + ...

B

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Copyright © TWI Ltd 2015

7-3

Inductance  Current in wire generates magnetic field.  Magnetic flux proportional to current.  So, if current changes, magnetic field intensity also varies.  Faraday Law: changing field of magnetic flux induces an EMF in wire that acts to oppose the increase in current.  Phenomenon is known as inductance.  Useful in welding. Rapid changes in current can give instability. Inductance slows change. Copyright © TWI Ltd 2015

Inductors  Purpose-built inductors wound as coils to maximise magnetic effect.  An inductor may have ferromagnetic core to amplify effect.  Some cores may move to vary inductance.  Symbols for inductors.

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Current Rise and Inductance

No inductance

With inductance

Current

Time

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Inverters  Inverter electronically switches DC to give negative cycle.  Speed of switching can be varied and can be very high – 100kHz.  HF transformer can be very much smaller.  Transformer in inverter power source is very small yet handles high current without overheating.

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Size Comparison

Conventional MMA

Inverter MMA

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7-4

Generators  Use dynamo principle rotating wires through magnetic fields to produce DC electricity.  Petrol or diesel driven engines, generators require no electricity so very portable.  Used for site work.  Not popular for shop work as noisy.

Power Sources TWI Training & Examination Services

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Copyright © TWI Ltd 2014

Transformers  In simplest form, step-down transformer to take 415V mains to 80-100V.  Output current adjusted by adding inductance and capacitance (reactance).  This is called choking and the adjustment control often called the choke.  Can tap at different points of output coil of transformer or use moving iron core.

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Rectifier

Transformers Tapped transformer

Moving core transformer

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Inverters

HF AC transformation gives very small size Transformer coupled with rectifier gives DC

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Copyright © TWI Ltd 2014

8-1

Current/Voltage Relationship

Drooping Characteristic 100

 415V drawing 20A has power input 8.3kW.  Allow for loss, say 7.5kW.  Theoretically: 

75A at 100V. 375 A at 20V.

80 70

Voltage



 Straight line graph. V  Not so in practice.

O.C.V. Striking voltage (typical) for arc initiation

90

60 50 40

Normal Operating Voltage Range

30 20 10 20

40

60

i

80

100

120

130

140

160

180

200

Amperage Copyright © TWI Ltd 2014

Copyright © TWI Ltd 2014

Drooping Characteristic Known as constant current (CC). No current – open circuit voltage (OCV). For MMA OCV helps strike arc. Used on steep slope where large change to voltage makes small change to current.  Manual welding difficult to hold electrode at exactly same height, so voltage varies.  Very little effect on current so penetration stays the same.  Ideal for MMA and TIG.    

Flat Characteristic Small change in voltage = large change in amperage

V

i Copyright © TWI Ltd 2014

Copyright © TWI Ltd 2014

Flat Characteristic

Self-Adjusting Arc

 Known as constant voltage (CV) or constant potential (CP).  Very shallow, almost straight line graph.  Large effect on current changes burn off rate of a wire electrode.  Used for MIG/MAG and SAW.  Self-adjusting arc.

Feed speed = burn off

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V up, i down, burn off down. Feed speed > burn off

Wire advances, i increases until: Feed speed = burn off Copyright © TWI Ltd 2014

8-2

Self-Adjusting Arc

Multi-Process Power Sources    

Solid state control. Inverter small size. Circuitry to adjust between CC and CV. Machines do all:     

Feed speed = burn off

V down, i up, burn off up. Feed speed < burn off



Wire retracts, i decreases until: Feed speed = burn off

MMA. TIG. MIG. Pulsed MIG. FCAW. Carbon arc gouging.

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Copyright © TWI Ltd 2014

Pulsed Power  Switching off or reversing polarity in programmed manner.  Useful for heat input and weld pool control.  Makes positional welding easier, eg MIG with spray transfer during peak current pulse.  Balancing melting and cleaning when AC TIG welding aluminium alloys.

Pulsing by Wave Chopping

i

High current

t

i

Low current

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Synergic Control MIG

t

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One-Knob Control

 Can adjust pulse parameters – height, duration, frequency – to melt and detach one drop per pulse.  Different for each filler and each wire size.  Can programme machine with most common combinations.  Select via buttons or knob.  One-knob control.  Select material/wire/gas combination on knob in wire feeder compartment.  Adjust voltage on front panel for thickness. Copyright © TWI Ltd 2014

Copyright © TWI Ltd 2014

8-3

Slope Control TIG        

Starts can have porosity and tungsten defects. Worse if started at full current. Start at very low current then build up. Slope-in or slope-up. Stops can have crater cracking. Step down to low current before switch off. Slope-out, slope-down or crater-fill. Gas pre- and post-purge also help minimise defects.

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Duty Cycle     

Heat generated by current through wires. May degenerate insulation, electrical safety. Fire hazard. After use require a cooling period. Length of time in use in ten minute cycles with the rest for cooling to remain within temperature limit. Duty Cycle

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BS EN 60974 Label for Duty Cycle

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8-4

TIG Basics

TIG Welding TWI Training & Examination Services

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Equipment for TIG Power control panel

Power return cable Torch assemblies Tungsten electrodes

Transformer / Rectifier

Inverter power source Power control panel Power cable Flow-meter Copyright © TWI Ltd 2015

Polarity DCEN:  Most used.  Tungsten cooled by electron emission.  Workpiece receives more heat. DCEP:  Will clean oxide from Al and Mg.  Heat tends to melt tungsten.  Can be done with water cooled torch. AC:  Usual way to weld Al and Mg to get cleaning.

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Copyright © TWI Ltd 2015

Arc Starting Scratch start:  Tungsten touched on workpiece.  Short-circuit starts current.  Arc established as torch lifted.  Can leave tungsten inclusions. Lift arc:  Electronic control very low short-circuit current.  Builds to operational current as torch lifted. HF:  Superimposition of HF high voltage spark. Copyright © TWI Ltd 2015

Tungsten Types Pure W – green band:  Cheap, but short life. Poor arc start. W +ThO2 – yellow (1%), red (2%):  High current carrying but slightly radioactive. W + CeO2 – grey (Europe), orange (US):  Good for low current DC work. W + La2O3 – black:  Increasing use to replace thoriated. W + ZrO2 – white (Europe), brown (US):  Used for AC.

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9-1

GTAW Torch Torch types:

GTAW Torch Tungsten electrode Torch cap/tungsten housing

Collet holder

Electrode collet

Torch body

 Gas cooled: cheap, simple, large size, short life for component parts.  Water cooled: recommended over 150A, expensive, complex, small size, longer life of parts.

Ceramic nozzle On/off switch

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Copyright © TWI Ltd 2015

Correct Gas Flow

Pre- and Post-Flow

 Too low and air can reach pool from sides.

 Gas flow is started before and continues after, welding current.

 Too high and eddies draw in air.

 Better protection against oxidation.

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Copyright © TWI Ltd 2015

Electrode Tip for DCEN

Electrode Tip for AC

2-2.5 times electrode diameter

Penetration increase

Electrode tip for low current welding

Increase Vertex angle Decrease Bead width increase

Electrode tip for high current welding Copyright © TWI Ltd 2015

Electrode tip ground

Electrode tip ground and then conditioned Copyright © TWI Ltd 2015

9-2

Grinding Tungstens        

Reserve grinder for tungsten only. Use diamond or boron nitride wheels. Grind longitudinally and concentrically. Never use belt sander or sides of wheels. Do not breath grinding dust. Use exhaust system for thoriated tungsten. Tungsten splinters. Wear gloves and glasses. Use grinding wand. Electrodes get hot.

Potential Defects Tungsten inclusions:  Thermal shock Tungsten splinters can.  Touch start fuses spots to workpiece.  Overheating can project tungsten fragments into the weld pool.  Very visible on radiograph but not critical defect. Solidification cracking:  Some compositions inherently crack sensitive.  Impurities often make eutectics.  Fillers designed with elements to react with impurities, eg Mn used to give high MPt MnS.

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Copyright © TWI Ltd 2015

Potential Defects Oxide inclusions:  Oxides contribute to lack of fusion.  No fluxing to absorb oxides.  Need to keep good gas cover to avoid oxidation of reactive metals. Diffraction mottling:  Not real defect.  Black and white parallel lines on radiograph.  Can obscure real lack of fusion defect.

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Advantages of TIG       

No spatter, high cleanliness. Good welder easily produces quality welds. Good for penetration beads in all positions. Wide range metals, including dissimilar. Good protection for reactive. Very good for joining thin materials. Very low levels of diffusible hydrogen.

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Disadvantages of TIG      

Low deposition rates. Higher dexterity and co-ordination. Less economical for thicker sections. Not good in draughty conditions. Low tolerance of contaminants. Tungsten inclusions can occur.

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9-3

MIG/MAG Welding Also known as gas metal arc welding. Uses continuous wire electrode. Weld pool protected by shielding gas. Classified as semi-automatic – may be fully automated.  Wire can be bare or coated solid wire, flux or metal cored hollow wire.    

MIG/MAG FCAW Welding TWI Training & Examination Services

Copyright © TWI Ltd 2015

Copyright © TWI Ltd 2015

MIG/MAG - Principle of Operation

Process Characteristics  DCEP from CV power source.  Wire 0.6-1.6mm diameter. Gas shielded.  Wire fed through conduit. Melt rate maintains constant arc length/arc voltage.  WFS directly related to burn-off rate.  Burn-off rate directly related to current.  Semi-automatic – set controls arc length.  Can be mechanised and automated.

Copyright © TWI Ltd 2015

Copyright © TWI Ltd 2015

MIG/MAG Equipment External wire feed unit Internal wire feed system

Power control panel 15kg wire spool Power return cable

Wire Feeding

Transformer / Rectifier

Power cable & hose assembly

Liner for wire Welding gun assembly

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Separate feeder

Feeder in set

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10-1

Feeder Drive Rolls Internal wire drive system

Types of Wire Drive System

Plain top roller

Two roll Half grooved bottom roller

Four roll

Wire guide Copyright © TWI Ltd 2015

Copyright © TWI Ltd 2015

Roll Grooves      

Push-pull

Liners for MIG/MAG

Often have plain top roll. Bottom and sometimes top, roll grooved. V shape for steel. U shape for softer wire, eg Al. Knurled for positive feed. Care needed on tightness of rolls.  

Too light – rolls skid, wire stalls Too tight – rolls deform wire, wire can jam

 If wire stops arc burns back to contact tube.

Close wound stainless steel wire

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Copyright © TWI Ltd 2015

Welding Gun Types

Torch Components Welding gun assembly (less nozzle)

Swan neck

Teflon liner

Welding gun body On/Off switch

Spatter protection

Push-pull

Hose port

Spot welding spacer

Nozzles or shrouds Gas diffuser

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Contact tips

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10-2

Push-Pull Torch Assembly Gas diffuser

Contact tip

Power Source Characteristic Small change in voltage = large change in amperage

Union nut

V WFS remote control potentiometer

Trigger Gas nozzle

i Copyright © TWI Ltd 2015

Copyright © TWI Ltd 2015

Self-Adjusting Arc

Feed speed = burn off

V up, i down, burn off down. Feed speed > burn off

Wire advances, i increases until: Feed speed = burn off

Self-Adjusting Arc

Feed speed = burn off

V down, i up, burn off up. Feed speed < burn off

Wire retracts, i decreases until: Feed speed = burn off

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Wire feed speed:  Increasing wfs automatically gives more current. Voltage:  In spray, controls arc length and bead width. Current:  Not separately set. Mainly affects penetration. Inductance:  In dip, controls rise in current. Lowers spatter. Gives hotter or colder welding. More info on several websites, eg. 

www.millerwelds.com/resources/articles/MIG-GMAW-weldingbasics. Copyright © TWI Ltd 2015

Wire Feed Speed/Current Relationship 500 450 400

Welding Current, A

Welding Parameters

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350 300 0.8

250

0.9 1.2

200

1.6 150 100 50 0 2.5

5

7.5

10

Wire Feed Speed, m/min

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10-3

Process Variables

Process Variables Electrode orientation

Arc voltage Increasing Voltage Reduced penetration, increased width Excessive voltage can cause porosity, spatter and undercut

Electrode extension

Penetration

Deep

Moderate

Excess weld metal

Max

Moderate

Undercut

Severe Moderate

Shallow Min Minimum

Travel speed Increasing travel speed Reduced penetration and width, undercut

Increased extension

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Copyright © TWI Ltd 2015

Shielding Gas Argon:  OK for all metals weldable by MIG.  Supports spray transfer, not good for dip.  Low penetration. Carbon dioxide:  Use on ferritic steel.  Supports dip and globular, not spray. Ar based mixtures:  Add He, O2, CO2 to increase penetration.  >20Ar + He, >80Ar + O2, CO2 can spray and dip. Copyright © TWI Ltd 2015

MIG and MAG Shielding Gases Metal inert gas (MIG):  Usually Ar shielding.  Can be Ar + He mixture – gives hotter action.  Used for non-ferrous alloys, eg Al, Ni. Metal active gas (MAG):  Has oxidising gas shield.  Can be 100% CO2 for ferritic steels.  Often Ar + 12-20% CO2 for both dip and spray.  Ar + O2 for stainless steel.

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Transition Current Dip to Spray Material

C-Steel

Shielding Gas Ar + 10%CO2 Ar +2%O2

Stainless steel

Ar +2%O2

Wire Dia, mm 0.8 0.9 1.2 1.6 0.9 1.2 1.6 0.8 0.9 1.2 1.6

Transition Current, A 155-165 175-185 215-225 280-290 130-140 205-215 265-275 120-130 140-150 185-195 250-260

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Metal Transfer Modes Depending on shielding gas and voltage, metal crosses from wire to work in:  Spray mode – wire tapers to a point and very fine droplets stream across from the tip.  Globular mode – large droplets form and drop under action of gravity and arc force.  Short-circuiting (dip) mode – wire touches pool surface before arc re-ignition.  Pulsed mode – current and voltage cycled between no transfer and spray mode.

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10-4

Use of Transfer Modes

Dip Transfer

 Spray transfer: V > 27; i > 220: 

Thicker material, flat welding, high deposition

 Globular transfer: between dip and spray. 

Mechanised MAG process using CO2

 Dip transfer: V < 22; i < 200. 

 Droplet stays attached and touches pool causing shortcircuit.  Current rises very quickly giving energy to ‘pinch-off’ droplet violently .  Akin to ‘blowing a fuse’ – causes spatter .  Droplet detaches, arc re-establishes and current falls.  Cycle occurs up to 200 times per second.

Thin material positional welding

 Pulse transfer: spray plus no transfer cycle.  

Frequency range 50-300 pulses/second Positional welding and root runs

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Copyright © TWI Ltd 2015

Dip Transfer Attributes

Globular Transfer

Advantages:

 Transfer by gravity or short circuit.  Requires CO2 shielding  Drops larger than electrode hence severe spatter.  Can use low voltage and bury arc to reduce spatter.  High current and voltage, so high distortion.

 Low energy allows welding in all positions.  Good for root runs in single-sided welds.  Good for welding thin material.

Disadvantages:   

Prone to lack of fusion. May not be allowed for high-integrity applications. Tends to give spatter.

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Copyright © TWI Ltd 2015

Spray Transfer

Pulsed Transfer Simplest form uses mains frequency and chops to control current.

 Tapered tip as anode climbs wire.  Small droplets with free flight from pinch effect.  Requires Ar-rich gas.  High current and voltage, high distortion.  Large pool, not positional.  Used for thick material and flat/horizontal welds.

i

t

i

t

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Copyright © TWI Ltd 2015

10-5

Electronic Generation  With synthesised pulse height, duration and frequency can be controled.  Droplets spray during peak current across the arc.  No transfer during background – current too low for dip.  Can select conditions to give single drop transfer each pulse – synergic MIG.

Pulsed Transfer Attributes Advantages:  Good fusion.  Small weld pool allows all-position welding. Disadvantages:  More complex and expensive power source.  Difficult to set parameters.  But synergic easy to set, manufacturer provides programmes to suit wire type, diameter and type of gas.

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Copyright © TWI Ltd 2015

Potential Defects  Most defects caused by lack of welder skill, or incorrect settings of equipment.  Worn contact tip causes poor power pick up and this causes wire to stub into work.  Silica inclusions can build up with poor interun cleaning.  Lack of fusion (primarily with dip transfer).  Porosity (from loss of gas shield on site etc)  Cracking, centerline pipes, crater pipes on deep narrow welds.

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MIG/MAG Attributes Advantages:  High productivity.  Easily automated.  All positional (dip and pulse).  Material thickness range.  Continuous electrode.

Disadvantages:  Lack of fusion (dip).  Small range of consumables.  Protection on site.  Complex equipment  Not so portable.

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Gas Shielded Principle of Operation

Flux Core Arc Welding

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10-6

Shielded Principle of Operation

Benefit of Flux  Flux assists in producing gas cover, more tolerant to draughts than solid wire.  Flux creates slag that protects hot metal.  Slag holds bead when positional welding.  Flux alloying can improve weld metal properties.  Reduced cross-section carrying current gives increased burn-off at any current.

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Copyright © TWI Ltd 2015

FCAW - Differences from MIG/MAG  Usually operate DCEP but some self-shielded wires run DCEN.  Some hardfacing wires are larger diameter – need big power source.  Don't work in dip.  Need knurled feed rolls.  Self-shielded wires use a different torch.

Self-Shielded Welding Gun

Close wound stainless steel spring wire liner (inside welding gun cable)

Handle

24V insulated switch lead

Conductor tube

Trigger

Thread protector

Welding gun cable

Hand shield

Contact tip

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Travel Angle

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Backhand (Drag) Technique Advantages:  Preferred for flat or horizontal with FCAW.  Slower travel.  Deeper penetration.

75 °

90 °

75 °

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Disadvantages  Produces higher weld profile.  Difficult to follow weld joint.  Can lead to burnthrough on thin sheet.

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10-7

Forehand (Push) Technique Advantages:  Preferred method for vertical up or overhead with FCAW.  Arc gives preheat effect.  Easy to follow weld joint and control penetration.

Disadvantages:  Produces low weld profile, with coarser ripples.  Fast travel gives low penetration.  Amount of spatter can increase.

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Deposition Rate for C-Steel

FCAW Advantages Less sensitive to lack of fusion. Smaller included angle compared to MMA. High productivity. All positional. Smooth bead surface, less danger of undercut. Basic types produce excellent toughness. Good control of weld pool in positional welding especially with rutile wires.  Ease of varying alloying constituents gives wide range of consumables.  Some can run without shielding gas.       

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FCAW Disadvantages  Limited to steels and Ni-base alloys.  Slag covering must be removed.  FCAW wire is more expensive per kg than solid wires (except some high alloy steels) but note may be more cost effective.  Gas shielded wires may be affected by winds and draughts like MIG.  More fume than MIG/MAG.

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Copyright © TWI Ltd 2015

10-8

Early History

Manual Metal Arc (MMA) Welding TWI Training & Examination Services

 Bernados and Olszewaski often cited as inventors from 1885 British patent but this was carbon arc welding with two electrodes.  Coffin in 1890 gained US patent for replacing one carbon with metal rod. First instance of metal transfer through an arc.  Slavianoff also suggested using metal rods.  In 1908 Kjellberg patented coated electrode dipped in CaCO3, clay and silicate.  In 1909 Strohmenger patented asbestos wound rods, stable on AC.

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Copyright © TWI Ltd 2015

Developments  In WW1 USA short of asbestos rods. Smith tried making the first cellulosic rod.  Extruded electrodes appeared in the 1920s. AO Smith selling heavy coated rods in 1926.  Rutile tried in 1930s, for flat and horizontal welding.  Roberts made rutile Vodex (Vertical, Overhead, Downhand for MurEX) in 1936.  MMA dominated welding 1940s to 1980s.  Also known as shielded metal arc welding (SMAW).

MMA - Principle of Operation Electrode angle 75‐ 80o to the horizontal Filler metal  core Flux coating

Solidified  slag

Weld metal

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Copyright © TWI Ltd 2015

Direction of electrode  travel Gaseous  shield Molten weld  pool Parent  metal

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MMA Welding Main features:  Shielding provided by decomposition of flux.  Consumable electrode.  Manual process. Welder controls:  Arc length.  Angle of electrode.  Speed of travel.  Current setting.

Arc

Consumable  electrode

MMA Basic Equipment

Control panel (amps, volts) Electrode oven Electrodes

Power source Holding oven Inverter power source

Return lead Electrode holder Welding visor filter glass

Power cables

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11-1

Constant Current Power Source 100

MMA Electrode Holder

O.C.V. Striking voltage (typical) for arc initiation

90 80

Voltage

70 60 50 40

Normal Operating Voltage Range

30 20 10 20

40

60

80

100

120

130

140

160

180

Collet or twist type

200

Amperage Copyright © TWI Ltd 2015

Copyright © TWI Ltd 2015

Process Characteristics  Straight lengths of coated electrode 250450mm long and 1.6-6.0mm diameter.  DCEP, DCEN and AC all possible.  Coatings grouped:     

Cellulosic. Iron oxide. Rutile. Basic. With or without iron powder.

Cellulosic Electrodes  Use industrially extracted cellulose powder, or wood flour in the formula.  Characteristic smell when welding.  Slag remains thin and friable, although the high arc force can create undercut and/or excessive ripple which may anchor the slag, thus requiring grinder inter-run cleaning.  Strong arc action and deep penetration.  AWS E6010 types DC; E6011 run on AC.  Gas shield principally hydrogen.  Only used on C- and C-Mn steels.  High arc force allows V-D stovepiping.

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Copyright © TWI Ltd 2015

Rutile Electrodes         

High amount of TiO2, (rutile sand or ilmenite). Coatings often coloured. AWS type E6012 are DC; E6013 run on AC. Many designed for flat position. Fluid slag, smooth bead, easy slag removal. Need some moisture to give gas shield. Not low hydrogen. Available for ferritic and austenitic steels. Fair mechanical properties.

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Tongs type with spring-loaded jaws

Rutile High Recovery Electrodes High amount Fe powder added. More weld metal laid at the same current. Coating much thicker, forms deep cup. End of coating can rest on workpiece. Slag easy release, sometimes self-releasing. Only for flat position. These AWS E7024 have recovery between 150-180%.  Recovery = Weld metal wt x100/core wire wt.       

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11-2

Basic Electrodes CaCO3 and CaF2 main ingredients. AWS E7015 first modern basic rods. Ran DC. Superseded by E7016 or E7018 – AC and DC. E7018 has Fe powder to help stabilise arc. E7016 good rooting and all-positional. Both can give good mechanical properties. Often hybrid; small diameter no Fe powder, larger dia. increasing amounts.  Used for ferritic, stainless steels, Ni and Cu.

      

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Setting Up for MMA Welding  Slag will help clean but rust and scale must be removed. For stainless and Ni wire brush.  Edge preparation usually needed:    

60° for ferritic – deep penetration rods available 70-80° for stainless and Cu – less forceful rods Up to 90° for Ni alloys – sluggish, viscous pool Root gap 1-3mm for most applications

Other MMA Coatings AWS E7028:  Basic with high levels of Fe powder added.  Flat and horizontal only.  Good mechanical properties. AWS E6020:  High levels of iron oxide.  Rare now, used for painted steel.  High arc force, relatively poor properties. Asbestos wound:  No longer permitted. Copyright © TWI Ltd 2015

Process Characteristics  Arc melts both electrode and parent plate.  Flux forms gas to protect and form a plasma and slag to protect hot metal.  Short runs as finite length electrode.  Must de-slag before next run.

 Good earth connection. Weld towards it on DC to minimise arc blow (or use AC).

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MMA Welding Variables Open circuit voltage (OCV)  Value of potential difference delivered by set with no load. Must be enough for specific electrode.  Electrodes labelled with min OCV, usually. ~80V. Voltage  Measure arc voltage close to arc.  Variable with change in arc length.  Too low, electrode ‘stubs’ into weld pool.  Too high, spatter, porosity, excess penetration, undercut, burn-through.

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Copyright © TWI Ltd 2015

MMA Welding Parameters Current  Range set by electrode, diameter, material type and thickness.  Approximately 35A per mm diameter.  Too low – poor start, lack of fusion, slag inclusions, humped bead shape.  Too high – spatter, excess penetration, undercut, burn-through. Polarity  Can be DCEP, DCEN, AC.  Determined by operation and electrode type. Copyright © TWI Ltd 2015

11-3

MMA Welding Parameters Travel speed:  Controlled by welder.  Often measured as run-out length as time to burn single rod fairly standard at constant current.  Too low – wide bead, excess penetration, burn-through.  Too high – narrow bead, lack of penetration, lack of fusion, difficult slag removal.

MMA – Parameter Setting left to right  Good conditions.  Current too low.  Current too high.  Arc length too short.  Arc length too long.  Travel too slow.  Travel too fast.

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Copyright © TWI Ltd 2015

Typical Current Ranges Type Cellulosic, mild steel Rutile, mild steel allpositional Rutile,mild steel high recovery, 160% Basic, low alloy

Rutile, stainless

Basic, Cu 7Sn

EN Specification

Dia. mm 3.2 E38 0 C 11 4.0 5.0 2.0 2.5 3.2 E 35 2 R12 4.0 5.0 6.0 2.5 3.2 E42 0 RR73 4.0 5.0 6.0 2.0 2.5 E69 4 Mn2NiCrMo B42 3.2 H5 4.0 5.0 1.6 2.0 2.5 E19 9 LR12 3.2 4.0 5.0 2.5 3.2 4.0

Heat Input

Current Range, A

 Total energy put in weld bead in unit time.  Calculated as: HI (kJ/mm) = 60iVk/1000S.

90 – 120 120 – 160 135 – 200 40 – 70 75 – 100 95 – 125 135 – 180 155 – 230 185 – 300 85 – 125 130 – 170 180 – 230 250 – 340 300 – 430 50 – 75 70 – 110 100 – 150 135 – 210 180 – 260 35 – 45 35 – 65 50 – 90 70 – 130 90 – 180 140 – 250 60 – 90 90 – 125 125 – 170

Where:  i = current in amps.  V = voltage in volts.  S = travel speed in mm/min.  k = thermal efficiency factor.  k = 0.8 for MMA, MIG/MAG and FCAW.  k = 0.6 for TIG and plasma.  k = 1.0 for SAW.

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Stringer or Weave Weave:  Lateral swings as well as moving along joint.  Useful to assist side wall fusion.  Run-out is shorter so heat input is higher.  Slows cooling rate, poorer toughness. Stringer Bead:  Run weld bead in straight line along joint.  Lower heat input per unit length.  Can be too low – martensite in steel so poor toughness. Copyright © TWI Ltd 2015

Copyright © TWI Ltd 2015

Multipass or Block Welding  In thick material, typical bead won’t fill groove.  Move slowly allowing metal to build but limited in flat position.  Block welding very high HI so poor properties.  Use multiple layers – multipass welding.  Need good cleaning of slag between runs.  Excellent properties, each bead heat treats one below. Can give with high toughness.

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11-4

Skip or Back-Step Welding  Technique to minimise distortion.  30-50mm weld made then move ~150mm along seam and lay another short run.  Continue to end of seam.  Return to start and make 30-50mm welds in gaps.  Repeat until seam completely welded.  Large number of starts and stops may have defects like porosity or cracking.

Preheat  Ferritic steels must not have hydrogen diffusing and inducing cracking.  Can apply preheat to slow rate of cooling giving hydrogen time to be released as process more susceptible to MICC.  Preheat may be with gas torch and large nozzle or electrically heated blankets.  Preheat specified as a minimum. Parent plate near weld must be heated. Check with probe or temperature sensitive crayons.

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Interpass Temperature  In multipass welding must avoid heat build up. Can lower strength and toughness.  Maximum interpass may be specified.  Note preheat still applicable so may have minimum interpass temperature (equivalent to original preheat) and maximum.

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Operating Factor for MMA  Welder needs time to change rods.  Also has to de-slag weld bead and grind any imperfections.  May be required to observe interpass temperatures.  Inspection will be required.  On long runs welder has to reposition.  All reduce time weld metal is deposited.  Arc time % to total time is operating factor for MMA this is rarely above 30%.

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Typical Welding Defects Most caused by:  Lack of welder skill.  Incorrect settings of equipment.  Incorrect use or treatment of electrodes. Typical Defects:  Slag inclusions.  Arc strikes.  Porosity.  Undercut.  Shape defects. (overlap, excessive root penetration, etc.) Copyright © TWI Ltd 2015

Copyright © TWI Ltd 2015

Advantages and Disadvantages Advantages:  Field or shop use.  Range of consumables.  All positions.  Portable.  Simple equipment.

Disadvantages:  High welder skill.  High levels of fume.  Hydrogen control (flux).  Stop/start problems.  Low productivity.

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11-5

Welding Consumables Welding consumables are any products that are used up in the production of a weld. Welding consumables may be:  Covered electrodes, filler wires and electrode wires.  Shielding or oxy-fuel gases.  Separately supplied fluxes.  Fusible inserts.

Welding Consumables TWI Training & Examination Services

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Welding Consumable Standards MMA (SMAW)  BS EN 499: Steel electrodes.  AWS A5.1 non-alloyed steel electrodes.  AWS A5.4 chromium electrodes.  AWS A5.5 alloyed steel electrodes.

MIG/MAG (GMAW) TIG (GTAW)  BS 2901: Filler wires.  BS EN 440: Wire electrodes.  AWS A5.9: Filler wires.  BS EN 439: Shielding gases. SAW  BS 4165: Wire and fluxes.  BS EN 756: Wire electrodes.  BS EN 760: Fluxes.  AWS A5.17: Wires and fluxes.

Welding Consumables TIG/PAW rods

Welding fluxes (SAW)

Cored wire

SAW strips

SAW solid wire

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MIG/MAG solid wire

Courtesy of ESAB AB

Covered electrodes Copyright © TWI Ltd 2015

Welding Consumable Gases

Welding Consumables Each consumable is critical in respect to: Size. Classification/supplier. Condition. Treatments eg baking/drying. Handling and storage is critical for consumable control.  Handling and storage of gases is critical for safety.

Welding gases:

     

 GMAW, FCAW, TIG, Oxy- Fuel.  Supplied in cylinders or storage tanks for large quantities.  Colour coded cylinders to minimise wrong use.  Subject to regulations concerned handling, quantities and positioning of storage areas.  Moisture content is limited to avoid cold cracking.  Dew point (the temperature at which the vapour begins to condense) must be checked.

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12-1

Welding consumables:  Filler material must be stored in an area with controlled temperature and humidity.  Poor handling and incorrect stacking may damage coatings, rendering the electrodes unusable.  There should be an issue and return policy for welding consumables (system procedure).  Control systems for electrode treatment must be checked and calibrated; those operations must be recorded.  Filler material suppliers must be approved before purchasing any material.

Welding Consumables

MMA Covered Electrodes

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Copyright © TWI Ltd 2015

MMA Welding Consumables Welding consumables for MMA:  Consist of a core wire typically between 350-450mm in length and from 2.5-6mm in diameter.  The wire is covered with an extruded flux coating.  The core wire is generally of a low quality rimming steel.  The weld quality is refined by the addition of alloying and refining agents in the flux coating.  The flux coating contains many elements and compounds that all have a variety of functions during welding.

MMA Welding Consumables Function of the electrode covering:  To facilitate arc ignition and give arc stability.  To generate gas for shielding the arc and molten metal from air contamination.  To de-oxidise the weld metal and flux impurities into the slag.  To form a protective slag blanket over the solidifying and cooling weld metal.  To provide alloying elements to give the required weld metal properties.  To aid positional welding (slag design to have suitable freezing temperature to support the molten weld metal).  To control hydrogen contents in the weld (basic type).

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Copyright © TWI Ltd 2015

MMA Welding Consumables The three main electrode covering types used in MMA welding.  Cellulosic - deep penetration/fusion.  Rutile - general purpose.  Basic - low hydrogen.

MMA Welding Consumables Plastic foil sealed cardboard box  

Rutile electrodes. General purpose basic electrodes.

Courtesy of Lincoln Electric

Courtesy of Lincoln Electric

Quality Assurance

Tin can 

Cellulosic electrodes.

Vacuum sealed pack 

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Extra low hydrogen electrodes.

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12-2

MMA Welding Consumables  Cellulosic electrodes:  Covering contains cellulose (organic material).  Produce a gas shield high in hydrogen raising the arc voltage.  Deep penetration / fusion characteristics enables welding at high speed without risk of lack of fusion.  Generates high level of fumes and H2 cold cracking.  Forms a thin slag layer with coarse weld profile.  Not require baking or drying (excessive heat will damage electrode covering!).  Mainly used for stove pipe welding.  Hydrogen content is 80-90 ml/100 g of weld metal.

MMA Welding Consumables Cellulosic electrodes Disadvantages:  Weld beads have high hydrogen.  risk of cracking (need to keep joint hot during welding to allow H to escape).  Not suitable for higher strength steels - cracking risk too high (may not be allowed for Grades stronger than X70).  Not suitable for very thick sections (may not be used on thicknesses > ~ 35mm).  Not suitable when low temperature toughness is required (impact toughness satisfactory down to ~ -20°C).

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Copyright © TWI Ltd 2015

MMA Welding Consumables Cellulosic electrodes  Advantages:  Deep penetration/fusion.  Suitable for welding in all positions.  Fast travel speeds.  Large volumes of shielding gas.  Low control.

MMA Welding Consumables Rutile electrodes:

Disadvantages: High in hydrogen. High crack tendency. Rough weld appearance.  High spatter contents.  Low deposition rates.    

 Covering contains TiO2 slag former and arc stabiliser.  Easy to strike arc, less spatter, excellent for positional welding.  Stable, easy-to-use arc can operate in both DC and AC.  Slag easy to detach, smooth profile.  Reasonably good strength weld metal.  Used mainly on general purpose work.  Low pressure pipework, support brackets.  Electrodes can be dried to lower H2 content but cannot be baked as it will destroy the coating.  Hydrogen content is 25-30 ml/100g of weld metal.

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Copyright © TWI Ltd 2015

MMA Welding Consumables

MMA Welding Consumables Rutile electrodes

Rutile electrodes Disadvantages:  They cannot be made with a low hydrogen content.  Cannot be used on high strength steels or thick joints - cracking risk too high.  They do not give good toughness at low temperatures.  These limitations mean that they are only suitable for general engineering - low strength, thin steel. Copyright © TWI Ltd 2015

Advantages: Easy to use. Low cost/control. Smooth weld profiles.  Slag easily detachable.  High deposition possible with the addition of iron powder.    

    

Disadvantages: High in hydrogen. High crack tendency. Low strength. Low toughness values.

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12-3

MMA Welding Consumables

MMA Welding Consumables

High recovery rutile electrodes

High recovery rutile electrodes

Characteristics:

Disadvantages:

 Coating is bulked out with iron powder.  Iron powder gives the electrode high recovery.

 Same as standard rutile electrodes with respect to hydrogen control.

 Extra weld metal from the iron powder can mean that weld deposit from a single electrode can be as high as 180% of the core wire weight.

 Large weld beads produced cannot be used for all-positional welding.

 Give good productivity.

 The very high recovery types usually limited to PA and PB positions.

 Large weld beads with smooth profile can look very similar to SAW welds.

 More moderate recovery may allow PC use.

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Copyright © TWI Ltd 2015

MMA Welding Consumables

MMA Welding Consumables

Basic covering:

Basic electrodes

 Produce convex weld profile and difficult to detach slag.  Very suitable for for high pressure work, thick section steel and for high strength steels.  Prior to use electrodes should be baked, typically 350°C for 2 hour plus to reduce moisture to very low levels and achieve low hydrogen potential status.  Contain calcium fluoride and calcium carbonate compounds.  Cannot be re baked indefinitely!  Low hydrogen potential gives weld metal very good toughness and YS.  Have the lowest level of hydrogen (less than 5ml/100g of weld metal).

Disadvantages:  Careful control of baking and/or issuing of electrodes is essential to maintain low hydrogen status and avoid risk of cracking.  Typical baking temperature 350°C for 1-2hours.  Holding temperature 120-150°C.  Issue in heated quivers typically 70°C.  Welders need to take more care/require greater skill.  Weld profile usually more convex.  De-slagging requires more effort than for other types.

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Basic electrodes Advantages  High toughness values.  Low hydrogen contents.  Low crack tendency.

Disadvantages  High cost.  High control.  High welder skill required.  Convex weld profiles.  Poor stop/start properties.

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Copyright © TWI Ltd 2015

BS EN 499 MMA Covered Electrodes

Compulsory

Optional

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12-4

BS EN 499 MMA Covered Electrodes E 50 3 2Ni B 7 2 H10 Covered Electrode Yield Strength N/mm2 Toughness Chemical composition Flux Covering Weld Metal Recovery and Current Type Welding Position Hydrogen Content

BS EN 499 MMA Covered Electrodes  Electrodes classified as follows:  E 35 - Minimum yield strength 350 Tensile strength 440-570 N/mm2  E 38 - Minimum yield strength 380 Tensile strength 470-600 N/mm2  E 42 - Minimum yield strength 420 Tensile strength 500-640 N/mm2  E 46 - Minimum yield strength 460 Tensile strength 530-680 N/mm2  E 50 - Minimum yield strength 500 Tensile strength 560-720 N/mm2

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Symbol

Weld metal recovery (%)

1

105

AC/DC

2

105

DC

3

>105 125

AC/DC

4

>105 125

DC

5

>125 160

AC/DC

6

>125 160

DC

7

>160

AC/DC

8

>160

DC

Symbol 1

N/mm2 N/mm2 N/mm2

AWS A5.1 Alloyed Electrodes E 60 1 3

Welding position designation Type of current

N/mm2

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BS EN 499 Electrode Designation Recovery and type of current

N/mm2

Welding position All positions

2

All positions except vertical down

3

Flat butt/fillet, horizontal fillet

4

Flat butt/fillet

5

Flat butt/fillet, horizontal fillet, vertical down

Covered electrode Tensile strength (p.s.i) Welding position Flux covering

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Copyright © TWI Ltd 2015

AWS A5.5 Alloyed Electrodes E 70 1 8 M G

MMA Welding Consumables Types of electrodes (for C, C-Mn Steels) BS EN 499 AWS A5.1  Cellulosic

Covered electrode Tensile strength (p.s.i)

E XX X C

EXX10 EXX11

 Rutile

E XX X R

Flux covering

 Rutile heavy coated

E XX X RR EXX24

Moisture control

 Basic

E XX X B

Welding position

Alloy content

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EXX12 EXX13 EXX15 EXX16 EXX18

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12-5

AWS A5.1 & A5.5 Alloyed Electrodes

Moisture Pick-Up

 Example AWS electrode flux types:  Cellulosic: flux-ends in 0 - 1 Examples: E6010, E6011, E7010, E8011  Rutile: flux-ends in 2 - 3 - 4 Examples: E5012, E6012, E6013, E6014  Basic: flux-ends in 5 - 6 - 7 - 8 Examples: E6016, E7017, E8018, E9018

Moisture pick-up as a function of:

 Temperature.  Humidity.

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Copyright © TWI Ltd 2015

Electrode Efficiency up to 180% for iron powder electrodes

Covered Electrode Treatment Baking oven:  Need temperature control.

Mass of weld metal deposited Electrode efficiency = Mass of core wire me lted

75-90% for usual e lectrodes

 Requires calibration. Heated quivers:  Only for maintaining moisture out of electrodes after baking.

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Copyright © TWI Ltd 2015

Covered Electrode Treatment Cellulosic electrodes

Use straight from the box - No baking/drying!

Rutile electrodes Vacuum packed basic electrodes

Covered Electrode Treatment Basic electrodes

Baking in oven 2 hours at 350°C!

If necessary, dry up to 120°C- No baking!

Limited number of rebakes!

After baking, maintain in oven at 150°C

Use straight from the pack within 4 hours No rebaking!

If not used within 4 hours, return to oven and rebake!

Use from quivers at 75°C

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Weld

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12-6

Covered Electrode Inspection 1 Electrode size (diameter and length)

2 Covering condition: adherence, cracks, chips and concentricity

3 Electrode designation EN 499-E 51 3 B

Arc ignition enhancing materials (optional!) See BS EN ISO 544 for further information Copyright © TWI Ltd 2015

Questions Welding consumables:  QU 1. Why are basic electrodes used mainly on high strength materials and what c ontrols are required when using basic electrodes?  QU 2. Name ten functions of an MMA flux?  QU 3. Why are cellulose electrodes commonly used for the welding of pressure pipe lines?  QU 4. What type of issues need to be considered when using cellulostic electrodes? Copyright © TWI Ltd 2015

12-7

History of the Process

Submerged Arc Welding TWI Training & Examination Services

 In 1929 Robinoff in the US patented continuous wire process with flux – but visible arc.  In 1935 Union Carbide/Linde patented fully submerged arc – called Unionmelt.  Licensed around world with fused flux.  Used for Liberty Ships, T2 Tankers in WW2.  In 1949 Lincoln offered agglomerated flux.

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Copyright © TWI Ltd 2015

SAW Principle of Operation

Process Characteristics  Arc between bare wire and parent plate.  Arc, electrode end and the molten pool submerged in powdered flux.  Flux produces gas and slag in lower layers under heat of arc giving protection.  Wire fed by voltage-controlled motor driven rollers to ensure constant arc length.

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Process Characteristics  Flux fed from hopper in continuous mound along line of intended weld.  Mound is deep to submerge arc. No spatter, weld shielded from atmosphere, no UV on welder.  Unmelted flux reclaimed for use.  Only for flat and horizontal-vertical positions.

SAW Basic Equipment

Power return cable Power control panel

Transformer / Rectifier Welding carriage control unit Welding carriage

Granulated flux

Electrode wire reel

Granulated flux

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13-1

Types of Equipment

SAW Equipment Wire reel

Slides

Hand-held gun

Flux hopper

Tractor

Wire feed motor

Feed roll assembly

Torch assembly Column and boom

Tracking system

Gantry

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Copyright © TWI Ltd 2015

Tractor Units  For straight or gently curved joints.  Ride tracks alongside joint or directly on workpiece.  Can have guide wheels to track.  Good portability, used where piece cannot be moved.

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Gantry  2D linear movement only.  For large production.  May have more than one head.

Contact tip

Column and Boom  Linear travel only.  Can move in 3 axis.  Workpiece must be brought to weld station.  Mostly used in workshop.

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Power Sources Power sources can be:  Transformers for AC.  Transformer-rectifiers for DC. Static characteristic can be:  Constant voltage (flat) – most popular.  Constant current (drooping) – used for high current.

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13-2

Wire    

Usually 2-6mm diameter. Copper coated to avoid rusting. 25 or 30kg coils. Can be supplied in bulk 300-2000kg.

Fused Fluxes  Original Unionmelt design – manganese, aluminium and calcium silicates.  Non-hygroscopic, no need to bake.  Good for recycling, composition doesn’t vary  Some can accept up to 2000A.  Very limited alloying and property control.  Cannot make basic fused flux.

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Copyright © TWI Ltd 2015

Bonded or Agglomerated Flux       

Powdered minerals pelletised with silicate. Baked to high temperature but hygroscopic. Flexible composition, can alloy, make basic. Can add deoxidants for good properties. Composition can vary as particle breakdown. Need to extract fine granules when recycling. Can add Mn and Si to weld so separate formulae for single or multipass.

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Welding Current Controls penetration and dilution

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SAW Operating Variables       

Welding current. Current type and polarity. Welding voltage. Travel speed. Electrode size. Electrode extension. Width and depth of the layer of flux.

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Setting Current  Too high  excess weld metal, increased shrinkage, more distortion.  Excessively high  digging W
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13-3

Current Type and Polarity

Welding Voltage

 DCEP - deep penetration; better for porosity.

 Controls arc length.  Increase gives flatter, wider bead.  Increase also in flux consumption and alloying transfer.  Increase reduces porosity.  Can bridge root gaps.

 DCEN - higher deposition rate; reduce penetration; surfacing use.  AC used to avoid arc blow; can give unstable arc.

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Setting Voltage  Low voltage - stiffer arc penetration in deep groove and resists arc blow.  Excessive low voltage - high narrow bead  difficult slag removal.

Setting Voltage  Excessively high voltage:  Produces hat-shaped bead – tendency to crack.  Increases undercut, slag removal difficult.  Produce concave fillet weld subject to cracking.

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Copyright © TWI Ltd 2015

Setting Travel Speed Increase gives:  Low heat input .  Less filler metal applied per unit of length.  Less excess weld metal.  Smaller weld bead.

W
Setting Travel Speed  Excessively high speed leads to undercut, arc blow and porosity.  Excessively low speed produces hatshaped beads  cracking.  Excessively low speed produces rough beads and slag inclusions.

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13-4

Electrode Size At same current, small electrodes have higher current density so higher deposition rates.

Electrode Extension Increased extension:  Adds resistance  Increases deposition  Decreases penetration and bead width  Helps prevent burn-through  Increase voltage to control weld shape Excessive extension:  difficult to position tip

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Copyright © TWI Ltd 2015

Depth of Flux  Influences appearance of weld.  Usually, depth of flux is 25-30mm.  If too deep:  

Arc too confined so rough rope-like top surface. Gases trapped so pool surface distorted.

 If too shallow:  

Effect of Electrode Angle on Bead Shape

Flashing and spattering. Poor appearance and porous weld.

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Weld Backing

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Starting/Finishing the Weld

Backing strip

Backing weld

Copper backing

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13-5

Potential Defects Porosity:  Oil, paint, grease, etc decompose in the arc to give elongated wormhole porosity.  Flux must be dry. Manufacturer's give drying temperatures.  Compressed air flux recovery units need dry air.  Insufficient flux burden can expose arc and pool to atmospheric contamination.

Solidification Cracking  Control composition, susceptibility predictor 230C + 190S + 75P + 45Nb - 12.3Si - 5.4Mn – 1.  Add Mn and Si to counter C, S and P, either in wire or through flux.  Depth to width ratio important:   

W much greater than D – surface cracks likely D much greater than W – centreline cracks likely D similar to W – sound welds

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Copyright © TWI Ltd 2015

Solidification Cracking a In the root beads of a multi-run weld. b Caused by high speed giving a long deep weld pool in first pass.

Solidification cracking

Mushroom shaped weld penetration resulting from high voltage combined with low speed.

c Caused by high restraint and root gap.

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13-6

History Many cite Hopkins (USA) as inventor in 1939. Paton Institute developed process in 1950s. Shrubsall (USA) consumable guide in 1957. Much used in the US buildings in 1960s, 1970s.  Apparently very poor toughness led to ban is the US.  Earthquake 1994 showed no problem to ESW.  Ban lifted in 2000.    

Electroslag Welding TWI Training & Examination Services

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Copyright © TWI Ltd 2014

Principle  Thick vertical plates, square edge, large gap.  Copper shoes on either side make a well to hold molten metal in place.  Wire fed to bottom, usually through tube that also melts (consumable guide).  Flux covers wire end.  Initial arc melts wire and flux.  Molten flux conductive, floods arc so wire melts through resistive heating of flux.  Weld completed in single pass.

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Process Characteristics  After initiation arc extinguishes, wire melted rapidly by resistive heating.  Welds up to 300mm made in single pass.  Copper guide tube used in standard process. Oscillated, slowly lifted as weld progresses.  Tubular consumable guide not lifted so melts into pool. Not usually oscillated either.  Very slow cooling, near equilibrium structure  PWHT to gain properties.

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Variants of ESW

Guide tube system Consumable guide

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Materials Welded  Mostly used on C and C-Mn steel.  Has been used on stainless and Ni alloys by Paton Institute.  Also claimed to weld Ti successfully.  Al is possible but not welded commercially.  Process developed for rail track joining but although better quality than thermite did not gain favour.

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14-1

Advantages and Disadvantages Advantages:  Speed ~1 hr per m whatever thickness.  No angular distortion.  Low lateral distortion.  Defect-free.  Simple flame-cut square edge.  Can be used for cladding (major application now).

Disadvantages:  Grain growth gives very large grains and poor toughness.  Limited to vertical or near vertical position.  Except cladding modification – flat.  Difficult to examine with NDT.

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14-2

Description of Processes

Thermal Cutting and Gouging TWI Training & Examination Services

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Summary of Processes Process operations

Thermal process

Primary

Secondary

Oxyfuel gas flame

Cutting Gouging

Grooving Chamfering

Ferritic, cast iron

Manual metal arc

Gouging

Grooving Chamfering

Ferritic, stainless, cast iron, Ni alloys

Air carbon arc

Gouging

Grooving Chamfering

Ferritic, cast iron, Ni alloys, Cu alloys, Al

Plasma arc

Cutting Gouging

Chamfering Grooving

Ferritic, stainless, Al

Laser

Cutting

Chamfering Drilling

Ferritic, stainless

Metals

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Gouging

 Thermal cutting and gouging are essential parts of welding fabrication.  Thermal cutting severs metal, creating two pieces or a specific shaped single piece.  Gouging form of cutting removing metal to leave groove as weld preparation.  Torches and parameters different for each.  Material locally heated and molten metal ejected - usually by blowing it away.  Flame, laser or arc processes can be used.

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General Safety  Cutting and gouging forcibly eject molten metal, often over large distance.  Must take appropriate precautions to protect operator, other workers and equipment.  Protective clothing, enclosed booth or screens, fume extraction, removal of all combustible material.

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Typical Applications of Gouging

 Like cutting but not severing into two pieces.  Reverse side of welds, removal of tacks, temporary welds, and weld imperfections:  

 



Repair and maintenance of structures. Removal of cracks, blow holes and sand traps in forgings and castings. Preparation of plate edges for welding. Removal of surplus metal - excess weld bead profiles, temporary backing strips. Removal of temporary welded attachments such as brackets, strongbacks, lifting lugs.

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15-1

Oxyfuel Gas Cutting  Most widely applied industrial thermal cutting process.  Can cut thicknesses from 0.5-250mm.  Low cost equipment can be manual or mechanised.  Several fuel gas and nozzle design options.

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Four Basic Requirements  Ignition temperature lower than melting point.  The oxide MPt must be lower than metal so that it can be blown away by jet.  Reaction between O2 and metal must give heat to maintain ignition temperature.  Minimal gas products so as not to dilute the cutting O2.

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Propane  Highest heat energy in outer flame.  Flame unfocussed, (speed 3.3m/s).  Slower preheating than acetylene but effective.  Once at ignition temperature, O2 reaction is same so cutting speed same.

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Process Fundamentals  Mixture of O2 and fuel gas used to preheat metal to its ignition temperature .  O2 jet then directed into preheated area.  Exothermic reaction between O2 and metal to form iron oxide or slag.  Jet blows away slag so it can pierce through the material and continue to cut.

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Acetylene  Highest temperature so fastest preheat.  Highest heat energy in inner flame reduces HAZ width and distortion.  High flame speed (7.4m/s), good piercing.  Lowest ratio of O2.

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MAPP  Methylacetylene and propadiene.  High flame temperature (second to acetylene), good flame energy levels.  Can be readily compressed.  Choice for underwater cutting.

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15-2

Cutting Quality Oxyfuel typically:  Large kerf (<2mm).  Low roughness values (Ra<50µm).  Poor edge squareness (>0.7mm).  Wide HAZ (>1mm).

Cutting Speed  Left – too slow, top face melting, irregular cut.  Centre – optimum.  Right – too fast, metal and oxide not fully expelled.

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Preheating  Left - too little, deep gouges low on face.  Centre - optimum.  Right - too much, top face melts.

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Advantages and Disadvantages Advantages:  Faster than machining.  Shapes can be cut economically.  Equipment costs low.  Portable equipment.  Can follow small radius easily.  Can mechanise torch for large plates.  Economical for edge preparation.

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Powder Cutting  Can inject flux into flame to remove oxide from stainless making cut possible.  Can inject Fe powder giving exothermic reaction makes cuts in stainless, Cu, Ni possible.  Cut quality usually poor.

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Disadvantages:  Not precision cut.  C and low alloy steel.  Fire and burn hazards.  Need fume control and ventilation.  Can give distortion and residual stress.

Flame Gouging  Cutting principle adapted to gouging.  Curved nozzle.  Quick, efficient removal on steel.  Low noise, ease of use, all positional.  Nozzle size changes gouge dimensions.

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15-3

MMA Gouging

MMA Gouging

 Similar to welding but electrode has very high arc force to eject metal.  Used at low angle to push molten pool away from groove.  DC or AC on standard MMA power source.  Can cut thin material but poor quality.  Gouge not as smooth as gas processes.  Mild steel electrode used for all materials.

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Air Carbon Arc Gouging  Arc between tip of carbon electrode and workpiece.  Metal melts and high velocity air jet blows it away, leaving clean groove.  Simple, uses MMA equipment.  High metal removal rate and gouge profile can be closely controlled.  Can be used on wide range of metals.

Process Characteristics  DCEP for steel and stainless steel. AC for cast iron, Cu and Ni alloys.  Graphite electrode with Cu coating to reduce electrode erosion.  Diameter selected for depth and width.  Molten metal/dross kept to minimum.  Standard MMA CC power source. Electrode different for AC vs DC.  Air from compressor or bottle used.

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Air Carbon Arc Gouging

Advantages and Disadvantages Advantages:

Disadvantages:

Low equipment cost. Economical to run. Easy to operate. Fast, easy to control. Defects visible. No slag issues. Compact, can work in confined areas.  Use on all materials.  Can be automated.

 Air jet ejects metal large distances.  Very noisy.  Needs large volume air.  C increase, grinding usually needed.  Sparks, ejected metal, fumes, noise and intense light.

      

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15-4

Plasma Arc Cutting  Basic process uses same torch as plasma welding.  Keyhole range plasma arc pierces metal.  Conditions set to avoid pool formation so becomes cutting tool.  No oxidation reaction, usable on any metal.  Introduced for stainless and Al.  Cut quality similar to oxyfuel.  Variants developed with different torches.

Plasma Cutting Variants Water shroud or immersion  Shroud cuts fume and noise.  Bath cuts noise 11570dB.  No effect on top edge rounding. Air plasma  Air as plasma gas, cheap.  Needs Hf electrode.  Used for manual cutting thin steel.

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Plasma Gouging

Advantages and Disadvantages Advantages:

 Standard torch may be used.  Air plasma also possible.  Use low angle.  Forces metal away from groove by power of plasma.

       

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Noise can be high. Fairly expensive. Cut edges tapered. Air plasma limited to 50mm thick plate.  Arc glare.  High consumable costs.    

Laser Cutting      

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Disadvantages:

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Laser Cutting  First done in 1967 at TWI.  O2 jet with laser in centre.  CO2 laser then only high power, now Yb fibre or Nd-YAG possible.  Nd-YAG good for Al, Cu.

Cuts non-conductors. Faster than oxyfuel. Instant start-up. HTPAC has high quality cut edges. Narrow HAZ. Air plasma no gas cost. Ideal for thin sheet. Water bath reduces fumes.

Very quick, especially on thin sheet. Now used for automotive door panels. Growing use in shipbuilding. Automated with programmed pattern. Complex and very fine detail possible. Can also drill very fine holes.

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15-5

Advantages and Disadvantages Advantages:  Very fast speed.  No preheating.  Readily automated and can follow three dimensional tracks.  Can cut polymers and other nonmetallic materials.  Good quality squareedged kerf.

Disadvantages:  High cost of equipment.  Need to isolate personnel from laser.

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15-6

Why Surface? Number of reasons including:  Repair build-up. 

Replace worn surface with matching weld metal.

 Hard-facing.

Surfacing and Spraying





TWI Training & Examination Services

Give soft material wear, abrasion resistance.

 Cladding. Give corrosion or oxidation resistant surface.

 Buttering. 

Put layer of weld metal onto face of preparation before making full welded joint.

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Surfacing Methods Solid-state bonding:  Join two layers by pressure or pressure and heat.  Rolled clad plate.  Explosive bonding .  Friction can bond new material onto surface.  Diffusion bonding. Electrically melted:  Arc welding and electroslag strip cladding. Spraying:  Oxy-fuel, plasma, laser –also cold spraying.

Friction Surfacing  Rotate solid bar with one end pressed hard material.  Lateral movement of substrate deposits plasticised material.  No melting so no dilution, same composition as consumable.  Limited practical use.

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Surfacing by Welding  Most processes possible, deposition rates vary.  Can be manual or mechanised.

Surfacing by Welding Process

Deposition rate, kg.hr -1

Manual or mechanised

MMA

1-3

Manual

MIG

2-6

Both

TIG, micro plasma

1-2

Both

2-10

Mechanised

10-40

Mechanised

Plasma transferred arc SA or ES strip cladding

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16-1

Practical Examples Repair:  Cast iron tooling (nickel alloys).  Injection molds (martensitic steels).  Hot work tool steels (high speed steels). Engine exhaust valves (Co alloys):  Wear plate for earth moving, mineral moving.  Concast rolls (FeCr + carbide).  Gate valves (Co alloys).

Thermal Spraying  Apply powder or wire as spray of fine molten or semi-molten droplets to give coating.  Heat from oxy-fuel or arc.  Low energy, MIG or flame:  

 High energy, plasma, HVOF and detonation:   

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Processes Comparison Attribute Flame temp. °C

Flame Spray >3000

Wire Arc >3000

Air Plasma >5000

HVOF ~3000

Particle speed, m/s

50-100

50-150

100-400

400-800

Gas flow, l/min

100-200

500-3000

100-200

Gas types

O 2 , C 2 H2

Air, N2, Ar

N2, Ar, H2, He

Power, kW Particle size, µm Feed rate, kg/hr

20 5-100 2-10

2.5 Wire only 3-18

40-200 5-100 3-6

Typical materials

Metals, ceramics

Metals, cermets (cored wire)

Ceramics, metals

Coating density, % Porosity, % Oxides, %

85-90 10-15 10-20

80-95 5-10 10-20

90-95 5-10 1-3

400-1100 CH4, C2H2, H2, C 3 H6 , O 2 150-300 5-45 1-4 Ceramics, metals, cermets > 95 1-2 1-2

Bond strength, MPa

50

50

> 80

> 80

Thickness, mm

0.2-10

0.2-10

0.2-2

0.2-2

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Metallisation. Reclamation, corrosion resistant surfaces. High density coating. Thick coatings possible. High mechanical properties.

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Benefits of Thermal Spraying  Coating can be metal, ceramic and polymer, in the form of powder, rod or wire.  Substrate <300°C. Can be plastic.  Up to 10mm thick coatings.  Can create freestanding structures for netshaped manufacture.

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16-2