Welding Metallurgy Iiw Presentation Anb Program Dec 2011

Mr. R.D.Pennathur

Metallurgy of Welding

Mr. R.D.Pennathur

Metallurgy of Welding Introduction Welding and heat treatment Physical changes Metallurgical property changes Conclusion

Mr. R.D.Pennathur

Welding A Major Fabrication Process  General Engineering  Construction - Earthmoving equipment, cranes

 Infrastructure - Buildings , bridges , roads, flyovers, tunnels  Projects - Refineries, fertilizers, steel plants, chemical & petrochemical plants

 Automotive sector - 2- wheelers, cars, trucks, buses  Railways - Coaches, locomotives, wagons  Shipbuilding and aircraft

 Power plants & pressure vessels  Consumer durable - Refrigerators, ACs, Almirahs  Defense - Tanks, APCs, Aircraft, Rockets

 Food processing - Dairy, brewery, cooking etc.

Mr. R.D.Pennathur

Why Should Welding Technologists Learn Metallurgy?  Welding is mostly done for fabrication of metals and alloys  The final properties of the welded assembly will depend on the metallurgical structure of the parent metal and the weld.  All welding processes involve heating and cooling of the components being welded  Thus to ensure a satisfactory welded component, it is necessary to understand metallurgical structures and how they and the weld thermal cycle, determine the properties of the weld joint.

Mr. R.D.Pennathur

Joining of Materials Joining Mechanical Fasteners

Soldering

Metallurgical Brazing

Welding

Bolting Riveting

Adhesive Bonding

Mr. R.D.Pennathur

Welding Metallurgy

Prior to welding as a useful fabrication, riveting was extensively used for joining Mr. R.D.Pennathur

Mr. R.D.Pennathur

Joining of Materials Mechanical Joining- Demerits  Interfaces- fretting fatigue, fretting corrosion, crevice corrosion….  Extra manufacturing steps  Additional weight- energy inefficient  Cost- materials, manufacturing and above all….  Stress concentration!

Mr. R.D.Pennathur

Mr. R.D.Pennathur

Mr. R.D.Pennathur

Mr. R.D.Pennathur

Joining of Materials Metallurgical Joining- Scores over Mechanical joining due to  No interface  Less weight  Faster  Ease of operation, in situ welding…..  Suitable for different joint types  Cost effective

Mr. R.D.Pennathur

Welding Metallurgy Welding dispenses with the disadvantages Is it totally free of disadvantages? No, but advantages often outweigh such demerits- faster, safer, cost effective…

Mr. R.D.Pennathur

Welding Metallurgy Welding almost always requires application of heat

Temperatures as high as 1000 to 1600 deg C! Mr. R.D.Pennathur

Mr. R.D.Pennathur

Welding Metallurgy

Physical Changes- effect of local heat Distortion Residual stresses

Mr. R.D.Pennathur

Mr. R.D.Pennathur

Much more profound is the metallurgical structure change

Mr. R.D.Pennathur

Inherent Heat Treatment in Welding Welding causes Melting at the interface- fusion zone or weld zone (WZ) Adjoining regions experience temperatures up to but not exceeding MP, called Heat Affected Zone (HAZ)

Mr. R.D.Pennathur

Dendrites in fusion zone

Is grain size change/orientation only the change? Does not „heat treatment‟ take place? Yes! Mr. R.D.Pennathur

Heat Treatment and Welding Heat treatment is applied to whole component Temperatures and soaking times are well defined Heating and cooling rates are well defined Intended properties are predictable/ achievable

Mr. R.D.Pennathur

Heat Treatment in Welding Localized heat application Peak temperatures vary at different regions Time at temperature also varies Heating and cooling rates vary widely due to the above High level of unpredictability

Mr. R.D.Pennathur

Heat Treatment and Welding The joint cools after welding The cooling rate depends on Heat input Base metal thermal conductivity Geometry of the joint Thickness, Type of joint Ambient temperatures

Mr. R.D.Pennathur

Cooling Rate  Cooling rate depends on  R∞ 1/T0*H Where R is cooling rate, ºC/sec T0 is preheat temperature, ºC H is Heat Input, kJ/mm

Mr. R.D.Pennathur

Heat Input During Welding  Is calculated from the Arc energy divided by the welding speed Arc voltage X Welding current ----------------------------------------------Welding speed ( mm / sec ) X 1000

kJ / mm

 For other welding process divide by following factors SAW ( single wire ) - 0.8 GTAW - 1.2 GMAW - 1.0 Mr. R.D.Pennathur

Heat Treatment During Welding Under such conditions, what is the response of base material?  Chemical composition  Mechanical properties  Prior microstructure

Hence, it is time to understand the principles underlying heat treatment!

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After Heat Treatment

Before Heat Treatment

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In metals the atoms are arranged in well defined geometric arrangements/ patterns that get repeated in all three directions. These are called crystal structures

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Single Crystal

Unit Cell Mr. R.D.Pennathur

Poly-crystal

Grain boundary

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Grain Boundary

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Crystal boundary or Grain boundary  In these regions there exists a film of metals, some three atoms thick, in which atoms do not conform to any pattern  This crystal boundary is of amorphous nature  Metallic bond acts within and across the crystal boundary and therefore not necessarily an area of weakness  Impurity atoms has got tendency to segregate at grain boundary or crystal boundary.  Depending on the nature of impurity atom they may strengthen or weaken the boundary Mr. R.D.Pennathur

Defects in Metals - Dislocations  Any real crystal always has defects in its structure and deviates from perfect periodicity  These defects are called Lattice defects / Lattice imperfections / Dislocations  Metals and alloys get deformed when dislocations are forced to move by the application of force  Any solute atom, phase or inter-metallic that resists the flow of dislocations are the strengthening agents in any alloy system Mr. R.D.Pennathur

Structural Changes Metal/alloy may be of: Single crystal structure up to melting point More than one structure within the solid state

It is pertinent to discuss only STEEL here since it is the most important industrial alloy extensively used for welding

Mr. R.D.Pennathur

•

Body centered cubic crystal (BCC)Structure of iron at RT- α iron up to 910 deg C

• • • •

• •

Face centered cubic crystal (FCC)‫‏‬High temperature structure of ironγ iron between 910 deg and 1400deg C Reverts back to BCC (δ iron) between 1490 and 1530 deg C Melts at 1530 deg C

Body centered tetragonal (BCT) Structure of martensite - metastable Mr. R.D.Pennathur

A plane of atoms of Iron

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Principles of Heat Treatment Steel is an alloy with principally carbon and other elements. How do these elements participate in the crystal structure?

Mr. R.D.Pennathur

Principles of Heat Treatment Two ways by which the atoms of the element can participate without destroying the arrangement of crystal.

Either they can substitute the iron atoms in their equilibrium sites or can settle in the “gaps” between the iron atoms. Substitutional and interstitial solid solutions Mr. R.D.Pennathur

Substitutional Solid Solution Nickel

Iron Mr. R.D.Pennathur

Interstitial Solid Solution Iron

Carbon Mr. R.D.Pennathur

Crystal Structure Is there a limit to solid solubility? What happens if it is limited? What happens beyond its solid solubility? Alloying elements beyond solubility limits are present as different phases/compounds/precipitates The limit varies with temperature

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Principles of Heat Treatment What changes to these arrangements happen when we start heating the steel? Are there any re-arrangements? What is the extent to which solid solution can take place? What happens beyond that? A Phase diagram or Equilibrium diagram explains all these.

Mr. R.D.Pennathur

STEEL The uniqueness of steel as the most widely used engineering material depends on its ability to get heat treated to different levels of strength and toughness! Heat treatment itself is made possible because of two factors: Different crystal structures in solid state Solubility variation of these structures Mr. R.D.Pennathur

Mr. R.D.Pennathur

Various Regions In HAZ Formed During Welding

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Mr. R.D.Pennathur

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a. Temp. below A1: a. Mixture of ferrite & pearlite grains; hence microstructure not affected.

b. Temperature below A3: a. Pearlite transformed to Austenite, A3 temp is not exceeded, hence not all ferrite transforms to Austenite. On cooling, only the transformed grains will be normalized. Mr. R.D.Pennathur 49

c. Temperature just exceeds A3, thereby causing full Austenite transformation. a. On cooling all grains will be normalized.

d. Temperature significantly exceeds A3 line permitting grains to grow. 1. On cooling, ferrite will form at the grain boundaries, and a coarse pearlite will form inside the grains. 2. A coarse grain structure is more readily hardened than a finer one, therefore if the cooling rate between 800°C to 500°C is rapid, a hard microstructure will be formed –(brittle fracture may occur in this region) Mr. R.D.Pennathur 50

Microstructure & Hardness Of HAZ In Steel

Preheating helps reduce hardness of HAZ by extending time it spends between 800-500deg C Mr. R.D.Pennathur

Carbon < 0.80%

Carbon 0.80%

Carbon > 0.80%

Slow cooling condition is called equilibrium rate of cooling. But, do we get such condition in heat treatment or welding? Far from that!! Mr. R.D.Pennathur

Temperature – Time – Transformation T-T-T Diagrams

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Time-Temperature-Transformation (TTT) Diagram Mr. R.D.Pennathur

 Heat treatment enables different structures to be obtained from the same material

Figure 2 Microstructure of medium carbon steel resulting from normalizing heat treatment, showing ferrite and pearlite

Figure 4 Martensite microstructure of medium carbon steel resulting from water quenching

Mr. R.D.Pennathur

Heat Treatment

Normalized

After Spheroidizing

Microstructure of 0.40% carbon steel Mr. R.D.Pennathur

Martensite

Martensite :  Very hard and brittle phase.  Formed on rapid cooling below Ms temperature Tempered Martensite :  However has a good combination of strength and toughness and is a useful structure and is developed by reheating martensite  Hardness depends on carbon content of steel Carbon %

0.1

0.2

0.3

0.4

0.5

0.6

0.8

Hardness Rc

38

44

50

57

60

63

65

Mr. R.D.Pennathur

Bainite Formed in alloyed steels when austenite is cooled rapidly passed the nose of the C-curve . Extremely fine mixture of ferrite + carbide but not lamellar like pearlite Formed between 500 – 220 C Upper Bainite or lower Bainite depending on temp. Has higher hardness and toughness than pearlite

Mr. R.D.Pennathur

How is the HT Discussion Relevant for Welding? Martensite is the hardest condition of any steel Primarily used for high strength and wear resistance, but lacks toughness Fabrication requires good formability- derived from ductility- hence lower carbon steels are used Fabricated structures require not wear resistance but good toughness Mr. R.D.Pennathur

Micro – Alloyed HSLA steels  Fine dispersion of alloy carbides results in strengthening by precipitation hardening  Small amounts of carbide forming elements eg. Nb, V, Ti etc added Total amount 0.20% max as such called Micro-alloyed steels  Controlled rolling at low finish roll temperatures results in very fine grain size ASTM 12 – 14. Also improves strength.  Range of medium and high tensile steel developed to give improved strength and toughness without impairing weldability. Covered by IS:8500 - 1991  Gives comparatively lower elongation but better toughness than low alloy HSLA steels  Properties : UTS 600 – 650 MPa YS 400 – 500 MPa Elongation 20 – 22 %

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Properties Of Typical Micro-alloyed Steels Grade / Trade name

%C

% Mn

% Si

% MA

YS MPa

UTS MPa

ASTM A633 Gr C

0.20

1.50

0.50

0.05 Nb

350 min

600 min

SAILMA 410

0.25

1.50

0.50

Nb+V+Ti =0.20

410 min

540 - 660

SAILMA 450

0.25

1.50

0.50

Nb+V+Ti =0.20

450 min

570 - 720

SAILMA 450HI

0.20

1.50

0.50

Nb+V+Ti =0.20

450 min

570 – 720 CVN = 19.6J Min at – 20C

TISTEN 60

0.20

1.80

0.50

0.20

440 min

590 min

Mr. R.D.Pennathur

Welded & Higher Strength Structures Introduction of welded structures implied  High heat input of the welding arc / heat source and influence of arc atmosphere  Solidification of the molten filler metal and fused portion of base metal into a separate weld zone  Parent metal on both sides of the weld affected by the weld thermal cycle – Heat affected zone ( HAZ )  Metallurgical effects on both reheating and cooling

Introduction of higher strength steels to reduce weight and cost of structure  Alloying elements added to develop strength  Lead to more complex metallurgical changesMr. R.D.Pennathur

Toughness Welded structures require good toughness, ability to absorb impact Measured by Charpy test Charpy values are specified for welds requiring good toughness – RT as well as at subzero temperatures Mr. R.D.Pennathur

How is the HT Discussion Relevant? Since low carbon steels have poor hardenability, martensite seldom forms during welding But when hardenable steels have to be used, precautions have to be used during welding

Mr. R.D.Pennathur

What if Martensite forms? Affects toughness, possibility of brittle failure Susceptible to hydrogen cracking at vulnerable regions of HAZ in toe and under bead; if hardenable steel is to be welded precautions to be taken Preheat to slow down the cooling rate  Post heat to temper the martensite

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Hydrogen Cracking

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Mechanism of HAZ cracking 3 factors causing Hydrogen induced cold cracking  A brittle martensitic micro-structure produced by rapid cooling in HAZ area heated above A1 line  Presence of Hydrogen from the welding process  Presence of contractional and residual stresses Mechanism  Hydrogen absorbed by the weld pool diffuses to the fusion zone and HAZ as the weld solidifies and cools  Forms pockets of molecular hydrogen which exerts additional stress on the susceptible microstructure  In combination with existing stresses causes cracking generally in HAZ but can also take place in multi-pass welds Mr. R.D.Pennathur

Factors influencing HICC  Presence of Hydrogen – Process  Presence of stress – Weld design  Formation of hard microstructure Chemical composition ( intrinsic to material ) Cooling rate - Combined thickness of joint - Heat input of process - Degree of preheat if any and inter-pass temp    

Chemical composition expressed in terms of carbon equivalent C.E. is the measure of the susceptibility of the material to form a hard microstructure ( martensite ) Thus Carbon Equivalent has become synonymous with Weldability of a steel  C.E. = %C + % Mn / 6 + % (Cr + Mo + V ) / 5 + % (NI + Cu) / 15

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Weldability Steels with Carbon Equivalent (C.E.) value less than 0.40% have good weldability This means that no detrimental hard microstructures result in WZ and HAZ Such steels do not require any pre or post heating C.E.> 0.40% require either pre or post heating or both. Mr. R.D.Pennathur

Why Preheating?  The cooling rate, particularly from 800 deg to 500 deg C (ΔT800-500), is important that decides the microstructure, from TTT diagrams The nose of the TTT curve is shifted towards right for hardenable steels and hence harder microstructures tend to form

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Post Heating When martensite cannot be avoided, post heating is carried out PWHT reduces the brittleness by tempering the martensite

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Combined Thickness Of Joints  Butt welds & corner welds of equal thickness - T1 + T2  Butt welds & corner welds of unequal thickness  Av of T1 over 75 mm + T2  Fillet welds – T1 + T2 + T3

 Directly opposed simultaneous fillet welds – T1 + T2 + T3 / 2  Two rods - D1 + D2 / 2 Mr. R.D.Pennathur

Hydrogen Levels For Different Processes And Consumables  Scale A : Above 15 ml / 100 gm diffusible hydrogen content in weld – Rutile electrodes, LH electrodes which have been exposed to moisture  Scale B : 10 – 15 ml / 100 gm diffusible hydrogen content - LH electrodes redried at 250 C  Scale C : 5 – 10 ml / 100 gm diffusible hydrogen content – Gas Metal arc welding ( MIG ) process, LH electrodes redried at 350 C  Scale D : below 5 ml / 100 gm diffusible hydrogen content – Gas Tungsten Arc welding ( TIG ) process, LH electrodes re-dried at 450 C Mr. R.D.Pennathur

Practical Requirements Of Welding Engineer Given a steel of known composition or C.E.  Upto what combined thickness can be welded with normal rutile electrodes, without danger of HAZ cracking  Upto what thickness can be welded using Low Hydrogen electrodes  Upto what thickness can be welded using Low Hydrogen electrodes properly re-dried as per manufacturers recommendations  Above what thickness pre-heat is required and degree of pre-heat.  Is it necessary to impose any restrictions on heat input by the welding process and parameters used Mr. R.D.Pennathur

Combined Influence Of Base-metal Thickness And Carbon Content On Weldability

Both Preheat & PWHT required

Highest carbon content of carbon steel base metal %

Only Preheat is required No Preheat & PWHT required Greatest single thickness of carbon steel base metal Mr. R.D.Pennathur 75

 Weldability is defined as the capacity of a metal to be welded under the fabrication conditions imposed, into a suitable designed structure, and to perform satisfactorily in the intended service

Weldability

 Weldability is the ease with which a metal can be welded to give the required service  Weldability is the number of problems you face to weld a material

Macrograph of a weld joint & HAZ Mr. R.D.Pennathur

Metallurgical Zones In A Typical Weld

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WELDABILITY PROBLEMS Cracking - In the weld – solidification cracks -- micro-fissuring - In the HAZ – Hydrogen cracking - Liquation cracks Porosity Oxidation of reactive metals Reduced joint strength - In the weld - In the HAZ Reduced corrosion resistance Mr. R.D.Pennathur

Problems In Welding Structural Steels Hydrogen induced cold cracking ( HICC )  HAZ cracking  Delayed cracking Hot cracking  Solidification cracking  Centerline cracking  Due to high S & P levels which produce low melting films at grain boundaries  Reduced by higher Mn content

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Solidification Cracking

Steels having unfavourable Mn-S ratio are prone to such cracking. Mr. R.D.Pennathur

Lamellar Tearing  Is generally associated with welding of fairly large highly restrained structures  Occurs predominantly in plate material  Due to presence of non – metallic inclusions  Difficult to detect by NDT techniques. Maybe assessed by STRA of tensile test in short transverse direction  Cracks can occur in parent plate / HAZ and generally run parallel to the plate surface Mr. R.D.Pennathur

Lamellar tearing

Microstructure susceptible to lamellar tearing

Lamellar tearing near a C-Mn steel weld

Prevention: Use joint designs that minimise transverse constraint & butter with a softer layer Mr. R.D.Pennathur

Stainless Steel Welding Austentic – Extensively Used Ferritic

Martensitic Precipitation Hardening Duplex Mr. R.D.Pennathur

Sensitization in SS Welding  Chromium in solid solution gives corrosion resistance  If slow cooled from 950 to 400 deg C, Cr23 C6 precipitates and segregates to grain boundaries depleting the matrix of Cr, particularly close to GBsensitization

 Under corrosive environs, the GB gets attacked

Mr. R.D.Pennathur

Sensitization in SS Welding  Normally SS is quenched from high temperature to retain the Cr23C6 in solid solution- called „solution annealing‟  The treatment is carried out at 1050 deg C in vacuum or hydrogen atmosphere  If SS is welded and allowed to slow cool, Cr23C6 precipitates  If assembly permits, it can be resolutionized Mr. R.D.Pennathur

Sensitization in SS Welding

HAZ of weld of 316 where the grain Boundaries show Cr23 C6 precipitation.600X

Intergranular corrosion of sensitized SS.500X

Corroded Heat Exchanger Tube

Mr. R.D.Pennathur

Sensitization in SS Welding  If it is not possible, use low carbon or extra low carbon SS grades, to prevent Cr23 C6 formation  Alternately use stabilized grades having Nb or Ti  These with better affinity for C forms the respective carbides which h precipitate within the grains and Cr is not affected Mr. R.D.Pennathur

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