Gmaw Of Carbon Steel

GAS METAL ARC WELDING OF CARBON STEEL

TABLE

OF

CONTENTS

Section

Topic

Page

1

Introduction

1

Section

Topic

Page

6

Metal Transfer

26

A. Short-Arc 2

Base Metals

2

B. Globular Transfer

A. Alloying Elements

C. Spray Transfer

B. Carbon Steels

D. Pulsed Spray Transfer

C. Alloy Steels 7 3

Electrical Characteristics

8

Welding High Strength Steels

33

A. Select the Proper Filler Metal B. Minimize Hydrogen

A. Power Supply Basics

Contamination

B. Constant Voltage Power

C. Control Heat Input

Supply Controls

D. Use the Correct

C. Electrical Stick Out

Technique

D. Constant Voltage Power Supply Characteristics 8

1. Slope

Technique and Equipment

2. Inductance

Set-Up

3. Heat Input

A. Torch Angle

40

B. Feed Roll Tension 4

Shielding Gases

14

C. Burnback

A. Shielding Gas Functions

D. Arc and Puddle Position

B. Flow Rates

E. Vertical Down Welding

C. Gas Losses

F. Gaps G. Crater Filling

5

Electrodes

21

H. Arc Starting

A. Alloying Additions B. 1. Solid Wire Designations

9

Weld Discontinuities

45

and Problems

and Chemistry

A. Lack of Fusion

B. 2. Metal-Cored Wire

B. Porosity

Designations and Chemistry

C. Burn-Through

C. Flux-Cored “Tubular” Wire

D. Undercut

Designations D. Slag and Gas Formation

E. Spatter

E. Solidification of the Weld Puddle

F. Cracking 10

Conclusion

50

SECTION

1

1 Introduction

▲ This training program was written to give

As you learn more about GMAW, it will

you a better understanding of the MIG

become apparent that this is a sophisti-

welding process. MIG is an acronym for

cated process. Welders that have used

Metal Inert Gas, which is not technically

“stick” welding (Shielded Metal Arc

correct for steels, because shielding gases

Welding or SMAW) are sometimes of the

for steels contain an active gas such as

opinion that the GMAW process is sim-

oxygen or carbon dioxide. The correct

pler; but to deposit a high quality bead

term according to the American Welding

requires as much knowledge, or probably

Society (AWS) is Gas Metal Arc Welding

more, than the SMAW process. The reason

(GMAW). We will use the correct termi-

for this is the number of variables that

nology as defined by the AWS and also

affect the arc and the degree of control the

explain the slang used so that you will be

operator has over those variables.

familiar with all the terms applicable to The purpose of this manual is to make you

this process.

a better welder by increasing your knowledge of how the GMAW process works. A more knowledgeable welder can be more

Wire 10%

productive by working smarter, not harder. Gas 5.0%

Figure 1 shows why your company is interested in your education. Your labor and overhead account for about 85% of the cost of depositing weld metal. Any knowledge you gain from this course not only helps you, but also helps to make your company more competitive in a very

Overhead and Labor 85%

tough marketplace. If you should have any question in the future that this manual or your supervisor cannot answer, please free to have him contact your Praxair regional engineering staff for further assistance.

Figure 1 – Breakdown of welding costs

1

SECTION

2

2 Base Metals

▲ The two main categories of steel fabricated today are carbon and alloy steels. Carbon

Unit cell (9 iron atoms)

steels basically contain carbon and manganese. Alloy steels contain carbon, manganese, and a variety of other elements to give the base metal the required properties. Alloy steels cover a wide range of materials, such as the stainless and tool steels. As weight becomes an issue in the transportation industry, there has been a substitution of high strength low alloy steels

Figure 2 – Iron body centered

(HSLA) in applications that previously

Cubic Unit Cell

used carbon steels. The higher tensile and yield strengths allow the cross-sectional

than the iron atoms, and they fit into the

area weight of the structural members to

open areas between the iron atoms. In

be reduced. The resulting weight reduction

carbon steel, manganese is the other

allows better fuel economy and a stiffer

alloying element. In the matrix, a small

body in the case of automobiles, or an

portion of the iron atoms is replaced by

increase in payload in the case of heavy

manganese atoms. The layers above and

trucks. Stainless and tool steels are two

below the first layer are arranged identi-

more familiar categories of alloy steels.

cally, except that they are shifted on a

This manual will concentrate on the low

45 degree angle to fall into the areas where

alloys such as the “construction steels”,

the first layer of balls intersect. Now each

(A514, for example, referred to by USX as

ball in the second layer is touching four

their T1 alloys). In order to understand

balls in its layer and four balls in the layers

alloy steels, it helps to understand a little

directly above and below it. The orderly

bit about the metallurgy involved with the

manner in which metals are arranged in

elements added to increase the strength of

crystals is one of the reasons that they are

these materials.

so strong. When a metal yields, or deforms plastically, the planes of atoms slip in

2

Metals are crystals, which mean that the

relation to the adjacent planes and at the

atoms are arranged in an ordered matrix.

grain boundaries. The only single crystal

An easy way to visualize a metal is to think

materials used today are for turbine

of layers of balls with each ball in the layer

blades. They are extremely strong due to

touching its four neighbors (see figure 2).

the orderliness of the matrix. They are also

The balls represent the iron atoms of the

extremely expensive to make.

metal. Carbon atoms are much smaller

The steels that are used in fabrication are

The carbon forms a compound called iron

actually made up of grains or groups of

carbide (Fe3C). The size of the iron carbide

crystals. During welding grains begin to

molecule (a molecule is a combination of

grow into the molten puddle from the solid

atoms held together to form a compound)

base metal at the edge of the weld. Each

is considerably larger than the surrounding

grain continues to grow until it meets

iron atoms. When the atomic layers begin

another grain. The area where they meet is

to slip, the larger carbide molecule resists

called a grain boundary. In the steels of

this slip by “pinning” the layers together

most interest, a lot of the “slip” or shifting

due to their difference in size. There is a

of atoms, occurs at these grain boundaries.

number of other carbide forming elements

Because of grain boundaries, the actual

that work in a similar manner to strength-

strength of these steels is typically 25% to

en alloy steels.

50% of the theoretical strength of a single crystal of iron.

The following section lists some of the elements that are added to steel to yield

When carbon is added to a steel, the

the desired properties.

strength of the steel goes up dramatically.

▲ A. Alloying Elements

Carbon

nese oxide (MnO). The manganese also

As mentioned previously, carbon in steel

combines with sulfur to form manganese

forms iron carbide (Fe3C) in the matrix.

sulfide (MnS) (these are sometimes known

The larger size of the carbide compound

as “stringers” and can cause welding

pins the layers within the metal matrix and

problems in certain steels). The reason

makes it much more difficult for the mater-

sulfur is detrimental is that it solidifies at

ial to yield. This raises the tensile, yield

a low temperature. The liquid sulfur is

strength, and hardness of the steel consid-

carried to the grain boundaries and

erably.

reduces the strength of the weld. Any manganese remaining after formation of

Silicon

MnO and MnS will form manganese

Silicon is added mainly as a deoxidizer. It

carbide (Mn3C) which strengthens and

combines with oxygen to form SiO2. This

toughens the matrix. A special category

silicon dioxide, (also known as glass), floats

of steel containing greater than 10%

to the surface of the weld puddle in com-

manganese, are called “Hadfield steels”

bination with manganese oxide to form the

after the metallurgist that discovered them.

brown slag islands seen in the weld surface.

Hadfield steels harden very quickly to

Silicon can also be added as an alloying

high hardness levels. They are also very

element; this is very beneficial in electrical

abrasion resistant. The most common

steels used in transformers.

application of these materials is in railroad crossings or “frogs”. Hadfield steels are

Manganese

also used in rock crushers, dredging pumps,

Manganese is used in small amounts in

and dippers for power shovels.

most steels to deoxidize, desulfurize, and improve material properties. The manganese reacts with oxygen and forms manga3

Copper

material to resist corrosion; these materials

Copper is added in very small amounts to

are known as stainless steels. In chrome

increase the hardness of the steel. Copper

plating, a very thin, tight layer of chro-

can also be added to improve the corrosion

mium oxide is formed that resists further

resistance of weathering steels such as

oxidation. As the layer grows in thickness,

“Cor-Ten”. These materials can be seen in

the color begins to change to a straw color,

the unpainted condition on bridges, guard

and then to blue. This “bluing” can be seen

rails, and light poles. The copper aids in the

on motorcycle exhaust pipes.

formation of a very tight oxide that is not as prone to flaking as carbon/manganese

Nickel

steel; this slows the corrosion rate of the

While nickel does not form any carbide in

material.

a steel matrix, hardenability, ductility and toughness are all improved when nickel is

Molybdenum

added. Nickel is added to austenitic stain-

Molybdenum is another carbide forming

less steels (300 series) in concentrations of

element, and is added to most alloy steels

7-35%. Nickel is also an important element

from .5% to 1.5%. Molybdenum improves

found in the materials used for storage

yield strength and resistance to high

of cryogenic fluids. An alloy containing

temperature creep (deformation due to

9% nickel is used to fabricate the inner

high temperature and stress). It also helps

vessel in liquid nitrogen, argon, and

to maintain the strength of the steel after it

oxygen tanks.

undergoes stress relief. Molybdenum is added to stainless steels to reduce pitting

Boron

(highly localized corrosion) in corrosive

Boron is added in very small amounts to

environments.

increase the hardenability of steels. The amount added is typically below .01%.

Chromium

King pins, used on semi-trailers, are made

Chromium is added to increase the

from boron-containing steels.

strength, wear resistance, heat resistance, corrosion resistance, and hardness of steel.

Vanadium

Chromium is also a carbide former, and

Vanadium is a strong carbide former, and

forms chromium carbide (Cr3C). In steels,

increases the hardenability of the steel.

chromium can also form complex carbides,

Vanadium is expensive, so it is usually

Fe2CrC and Cr2FeC.

used in percentages of less than .2%. This element reduces the grain size and

Bearing steels typically contain about

increases the toughness of the material.

1% carbon and 1.5% chromium. When

Vanadium steels are used to make axles,

chromium levels rise to about 4%, and

connecting rods, hand tools, and engine

tungsten and molybdenum are added, tool

crankshafts.

steels are the result. These materials are made into the high speed cutting tools used in machining. Increasing the chromium level above 12-13% causes the

4

Titanium

Sulfur

Titanium is a strong carbide former that

Sulfur, also considered an impurity like

will also form oxides and nitrides. A large

phosphorus, is usually specified as a maxi-

use of titanium is to stabilize certain

mum allowable concentration. Sulfur in

grades of the stainless steels. Titanium

the puddle moves to the grain boundaries

combines with any carbon in the matrix to

of the solidifying weld metal because of its

form carbides before chromium carbide

low melting temperature. This segregation

precipitation can occur. When chromium

in the grain boundaries reduces the

forms carbides, the corrosion resistance of

strength of the material. Manganese is

the material will deteriorate. Titanium also

added to prevent this as it combines with

helps to reduce grain growth in high

the sulfur (Mn + S = MnS) before it can

strength steels, improving strength and

react with iron. Certain steels called free

toughness.

machining steels, contain up to .3% sulfur; these alloys are difficult to weld and have

Phosphorus

poor strength characteristics.

Phosphorus is generally considered an impurity in steels; a maximum percentage

With this greater understanding of the

is generally listed. Phosphorus tends to

alloys that are added to steel, let’s look at

segregate forcing carbon into the sur-

carbon and alloy steels.

rounding matrix. This can lead to brittle materials.

5

▲ B. Carbon Steels

Carbon steels are categorized as low, medium, and high carbon. The main alloying elements are carbon and manganese. Typical products made of these three categories of materials are: Low Carbon Steel

1. Auto frames and bodies

(.005 - .3 C)

2. Auto and truck wheels 3. Structural shapes (I-beams, channel, angle)

Medium Carbon Steel

1. Machine parts, pins

(.3 - .6 C)

2. Tools

High Carbon Steel

1. Railroad rail

(.6 - 1.0 C)

2. Dies 3. Springs

Carbon is a very powerful alloying element

real change in most of the alloys is in

because it forms iron carbide (Fe3C).

carbon content. At the right of the chart

Manganese also forms carbides, but has

notice that the tensile strength rises rapidly

much less of an effect on strength and

as the carbon level is increased with little

hardness. The following chart shows two

or no change in the manganese level.

carbon steels from the low, medium, and high carbon groups. Notice that the only Table 1 –

SAE/AISI

Carbon

Manganese

P (max)

S (max)

Chemical

1008

.08 max

.25 - .40

.04

.05

43,000

1018

.14 - .21

.6 - .9

.04

.05

58,000

1040

.36 - .45

.6 - .9

.04

.05

76,000

1050

.47 - .55

.6 - .9

.04

.05

90,000

1070

.64 - .76

.6 - .9

.04

.05

102,000

1090

.89 - 1.4

.6 - .9

.04

.05

122,000

Compositions of Carbon Steels and Tensile Strengths

Tensile

▲ C. Alloy Steels

Alloy steels are generally classified by

there are. The carbon steels, discussed

alloying additions. There are currently

earlier, are in the first classification. There

38 different classifications of steels that

are 15 to 20 different carbon steels avail-

are recognized by both organizations that

able, and there are 38 different classifica-

categorize steels (AISI and SAE). The

tions listed. Table 2 gives specification

following table is included to give a better

numbers and alloy classification.

understanding of how many steel alloys 6

Table 2 –

Specification

Alloy Steel

Number

Classification and

10XX

Carbon Steels

Specification

11XX

Carbon Steels, Resulfurized

Numbers

12XX

Carbon Steels, Resulfurized & Rephosphorized

13XX

Manganese Steels

2XXX

Nickel Steels

31XX

Nickel-Chromium Steels

33XX

High Nickel-Chromium Steels

40XX

Carbon-Molybdenum Steels

41XX

Chromium-Molybdenum Steels

43XX

Chromium-Nickel-Molybdenum Steels

46XX

Nickel-Molybdenum Steels

48XX

High Nickel-Molybdenum Steels

50XX

Low Chromium Steels

51XX

Chromium Steels

52XXX

Carbon-Chromium Steels

61XX

Chromium-Vanadium Steels

86XX

Low Nickel-Chromium-Molybdenum Steels

92XX

Silicon-Manganese Spring Steels

92XX

Silicon-Manganese-Chromium Spring Steels

93XX

Nickel-Chromium-Molybdenum Steels

98XX

Nickel-Chromium-Molybdenum Steels

XXBXX

Boron Containing Steels

XXBVXX

Boron-Vanadium Containing Steels

WX

Water-Hardening Steels

SX

Shock-Resisting Steels

OX

Oil-Hardening Steels

AX

Air-Hardening Steels

DX

High Carbon-High Chromium Tool Steels

HXX

Hot Work Tool Steels

TX

High Speed Tungsten Based Tool Steels

Classification

MX

High Speed Molybdenum Based Tool Steels

LX

Special Purpose Tool Steels

FX

Carbon-Tungsten Tool Steels

PX

Mold Steels

2XX

Chromium-Nickel-Manganese Stainless Steels

3XX

Chromium-Nickel Stainless Steels

4XX

Chromium-Stainless Steels

5XX

Low Chromium Heat Resisting Stainless Steels

7

SECTION

3

3 Electrical Characteristics

▲ A. Power Supply Basics

The power supply (or welding machine)

Congress is made of:

connected to the torch is basically a big transformer/rectifier. Its purpose is to

SENators And REPresentatives

take high voltage (440v or 220v) and low current (20-50 amps/leg) AC power and

Straight Electrode Negative

transform it to low voltage (16-40v), high current (80-500 amp) DC power.

Reverse Electrode Positive

To change AC to DC, a device called a rectifier is used. Direct current provides a much more stable arc. Most GMAW power

Almost all GMAW power supplies are

supplies are set-up using a reverse polarity

constant voltage machines, where Stick

connection. Reverse polarity is designated

Electrode (SMAW) machines are constant

as DCEP, which means Direct Current,

current.

Electrode Positive. An easy way to remember this is:

+

DC Low Voltage High Current

AC High Voltage Low Current

– Work

Reverse Polarity - DCEP (DC - Electrode Positive) Straight Polarity - DCEN (DC - Electrode Negative)

Figure 3 – Typical GMAW Power Supply

8

▲ B. Constant Voltage Power Supply Controls

All constant voltage power supplies have

supply or on the remote control. Increasing

at least two operator-adjustable settings:

wire feed increases current proportionally

current and voltage. Current is set by

so that enough current is available to melt

adjusting the wire feed rate; voltage is set

the wire and deposit it in the weld pool.

with a voltage adjustment on the power

Voltage adjusts the length of the arc. Some power supplies also provide the

Figure 4 –

options of adjustable slope and inductance.

Power Supply

The purpose of these controls will be

Adjustments

discussed in the section on power supply characteristics. Figure 4 shows a power supply with the standard voltage (arc length) and wire feed speed (current)

A

adjustments. This power supply also allows the operator to change the inductance and slope.

Flat

V

A

V

I

+

Steep

–

▲ C. Electrical Stick Out

Electrical stick out (ESO) is the distance

1. Electrode preheat

measured from the contact tip in the torch

2. Burning off of drawing lubricants

to the workpiece as figure 5 shows. ESO

3. Determination of current level

is very important, and can affect the following:

ESO

Figure 5 – Measuring Electrical Stick Out

9

The wire in the GMAW process is called

penetration, because as current increases,

an electrode because it conducts electricity.

so does the depth of penetration. By using

The current is transferred to the wire in

a slightly longer stickout, more weld metal

the contact tip. The energy resulting from

can be deposited without burning through

the welding current is distributed to two

thinner parts. Increasing ESO makes the

different places in the welding circuit;

arc harder to start because less current is

(1) resistance heating of the electrode,

available at the arc due to resistance heat-

and (2) penetration into the base metal as

ing. As more resistance is put into the

figure 6 shows. The electrode acts like the

welding circuit (increased ESO), the effec-

elements in a home toaster. As current

tive slope of the system is also increased.

passes through it, resistance heating occurs

This also tends to reduce the short-circuit

and its temperature rises. The increased

current.

temperature burns off drawing lubricants used in the manufacturing of the wire. The

ESO also affects shielding gas coverage. As

temperature rise also helps make it easier

the distance increases from the contact tip

to melt the electrode. This is the reason

to the work (also called TWD – tip to work

that deposition rate increases as ESO

distance), you reach a point where the

increases. As ESO increases, current is

shielding gas cannot effectively blanket the

decreased. This also helps to keep the

molten weld puddle. This will be covered

contact tip cooler at higher deposition

in more detail in the shielding gas section.

rates. This is a big help in controlling

Current from the power supply is distributed to:

1. Resistance heating of the electrode (ESO dependent) 2. Penetration into base metal

Figure 6 – Current Distribution

10

▲ D. Constant Voltage Power Supply Characteristics

1. Slope

This means that for each increase of

The characteristics of a power supply are

100 amps, the power supply will produce

determined by the components used in its

2 volts less at the same voltage setting.

design. The performance of a typical mach-

The lower slope line is 6 volts/100 amps,

ine is described by a graph such as figure 7.

and is about the maximum slope seen in

Most constant voltage power supplies

a constant voltage power supply (steep

without slope adjustment are factory

slope). A few machines are still available

preset at about 2 volts/100 amps

with continuously adjustable slope; others

(flat slope).

have external or internal taps to switch between slopes. Increasing the slope of a

Figure 7 -

power supply to control short-arc welding

Power Supply

at low currents is necessary because the

Characteristics

short-circuit current is limited. This

Curves

reduces the tendency to burn-through on thinner materials and decreases spatter on arc starts. This will be explained further in

32

lat S

30 28

Volts

the section on metal transfer.

CV-F

100

p

op

in

22

Sl

A review of this graph helps to explain

op

ro

Figure 8 shows a typical 2 volt/100 amp

A)

slope power supply characteristic curve.

tee

-D

24

(2V/

CV -S

CC

26

lope

e

(6

why the arc changes during welding. As

V/

g

10

an example, select a welding condition of

0A

)

20

27 volts and 250 amps. As welding contin-

18

ues, if the stickout (ESO) is reduced the 100

150 200 250 Current (Amps)

300

350

welding conditions change. As that change is made, the spatter level begins to increase. As ESO is decreased, less current

Figure 8 –

goes into preheating the wire and more

The Effect of Slope on

goes into the arc. Suppose the current

Current and Voltage

increases 50 amps, which is easily done with a small torch movement of about 1/4". This moves the operating point to the second point in figure 8; here the voltage Flat Slope 2V/100A

22

decreases to 26v while the current increases to 300 amps. This voltage is at the minimum for spray transfer; this would

Volts

21

account for the slight increase in spatter 20

that is observed. This will be explained in more detail in Section 6.

19 18 17 225

250 275 Current (Amps)

300

11

Measuring Actual Welding Voltage

measure as high as 5 volts which makes starting the arc difficult.

Measuring actual welding voltage is a useful way to be certain that the condition is

2. Inductance

within the range specified by the welding procedure. Hard starts can result from

Inductance is an adjustment that is provid-

bad connections in the welding circuit.

ed more frequently than slope on CV

The voltage drop due to bad connections

power supplies. Inductance is another

increases the slope of the system, and

method for controlling the arc. This is done

reduces the available short-circuit current.

by controlling the rate at which the weld-

Comparing the voltage at the power

ing current reaches the setting selected.

supply terminals and between the feeder

Figure 10 shows a plot of inductance vs.

and the work (figure 9) will give the

time. The top curve shows what happens

voltage drop due to resistance. It can

with no additional inductance and the current rises as quickly as the power sup-

Figure 9 –

ply will allow it to rise. This can result in

Measuring Actual

Volt Meter

Welding Voltage

27.5

very hot starts or even in the wire exploding at these very high current levels. Inductance should be kept low for spray

+

and more stable arc at high currents. High inductance settings can make it hard to

A

V

transfer. This produces better arc starting

–

Connect to + terminal on feeder and work piece and read while welding

A

initiate the arc because it limits the maximum short circuit current available for this purpose. Referring again to figure 7, as the elec-

V Flat

+

I Steep

Work

trode first touches the work to strike an arc, the voltage falls from open circuit

–

voltage (40-50v) to 0 arc volts. At 0 volts (no arc length or a dead short), the power supply produces the maximum shortcircuit current. For a machine rated at

Figure 10 –

450 amps (at 100% duty cycle) this might

The Effect of Increasing Inductance

be 550-600 amps. A 600 amp machine might produce up to 800 amps during this initial start. This is more than enough

Current (amps)

Least Inductance

current to explode the wire and make the

600

arc difficult to start. If the arc start is too

500

hot, either the slope can be increased or

400

More Inductance

300

inductance added to reduce the current at arc initiation. Inductance can be beneficial when welding with low current short-arc,

Most Inductance

as it makes the puddle more fluid and allows it to better wet the base material.

12

Time (milliseconds)

3. Heat Input

The heat input formula is also used for when welding high alloy materials, but it

A useful formula often used by welding

also helps in understanding the uses of the

engineers is the heat input equation. If

power supply characteristic curve.

you have been welding high strength or corrosion resistant alloys, you may already

The heat input formula can assist in pre-

be familiar with this formula. It allows you

paring welding procedures where similar

to calculate the amount of heat delivered

material thicknesses and joint configura-

to the workpiece.

tions are welded. After determining the

The formula is:

range of heat inputs that produce an acceptable weld, the range of heat inputs

Amps X Volts X 60

can be calculated. This is really helpful in increasing speeds for robotic welding. The

Heat Input = Travel Speed (in/min)

required travel speed can be calculated as wire feed speed (current) and voltage are

Heat input is measured in units of energy/

increased. The formula is also helpful in

unit of length (joules/inch). A joule is a

controlling distortion. Conditions that put

unit of energy equal to 1 watt of energy

less energy into the metal can be calcu-

into the workpiece per second. While a

lated, which reduces distortion.

joule is not a familiar unit of measurement, there are a lot of uses for the heat input formula itself. For example, suppose that welding is done at the second condition used as an example in figure 8. That condition was 300 amps and 26 volts. For this example, say the travel speed is 10 in/min. This works out to a heat input of 46,800 joules/in. If we know that the welding involves bridging a gap, ESO can be increased to reduce current and increase voltage while reaching the first condition indicated. That was 250 amps and 27 volts, still at 10 in/min. The heat input is now 40,500 joules/in. By increasing ESO (stickout), heat input has been reduced by almost 15%.

13

SECTION

4

4 Shielding Gases

▲ At high temperature, all metals commonly

has very high moisture content. The

used for fabrication will oxidize in the

moisture produces oxygen and hydrogen in

presence of the atmosphere. Every welding

the arc environment.

process provides shielding from the atmosphere by some method. When welding

Submerged-arc welding shields the puddle

steels, we want to exclude oxygen, nitro-

by a different method. As the puddle

gen, and moisture from the area above

progresses, the intense heat melts the flux

the molten puddle.

in the joint area; this forms a slag that covers the weld and excludes the

In the Oxy-fuel process, the weld pool

atmosphere.

is shielded from the atmosphere by the combustion by-products of carbon monox-

GMAW (MIG) and GTAW (TIG) are

ide (CO) and carbon dioxide (CO2). In

both gas shielded processes in which the

stick welding (SMAW), CO and CO2 are

shielding gas is provided from an outside

also the shielding gases. The 60XX type of

source. No fluxing agents are included in

electrode uses a cellulosic coating, which

the filler metal of solid wires.

For the purpose of this discussion the

As was mentioned earlier, the atmosphere

GMAW process will be emphasized

must be displaced while the puddle is

because it constitutes the greatest portion

cooling or oxidation will occur rapidly.

of welding done in industry. A good

This appears as a gray surface on the weld

portion of this information is applicable

bead. One cause of porosity is the result of

to GTAW too.

poor shielding when atmospheric oxygen

▲ A. Shielding Gas Functions

combines with carbon in the puddle. As The major functions of a shielding

the weld metal cools, porosity occurs as

gas are to:

this carbon monoxide escapes from the center of the bead. If air is aspirated into

1. Protect the puddle from the atmosphere

the shielding gas line through a leak,

2. Provide arc plasma

nitrogen and moisture will also contami-

3. Provide oxygen for wetting

nate the shielding gas. Nitrogen, while very

(ferrous alloys)

will cause porosity as it escapes during

5. Affect arc stability

cooling of the weld bead.

6. Control welding costs.

14

soluble in the puddle at high temperatures,

4. Control type of metal transfer

The shielding gas also provides a portion

shows. This is accomplished by “ionizing”

of the arc plasma, which transfers the

the gas, which frees electrons to transfer

welding current across the gap between

the current from the work to the electrode.

the electrode and the work as figure 11

Metallic and argon ions (atoms stripped of an electron) transfer the positive charge

Figure 11 –

across the arc. This explains in part why

Transfer of Current

the arc becomes very unstable when a

Across the Arc

torch is hooked up using straight polarity

Plasma

(DCEN) rather than reverse polarity (DCEP). In DCEN, the positive current is trying to remove iron atoms from the plate, which are much harder to melt than a

+

small diameter electrode. DCEP

e–

Fe+

In steels (carbon and stainless), oxygen stabilizes the arc and reduces the surface

–

tension of the weld metal. Oxygen is obtained from direct additions of oxygen

Ar+

or carbon dioxide to the shielding gas. Surface tension, the force that causes water to bead up on a waxed car surface, is not desirable when depositing a weld bead. If pure argon is used instead of a mixed gas, the bead does not wet out and appears as though it is sitting on top of the part surface (convex bead). Figure 12 shows the basic gases used in

Gas

Characteristic

Ionization

shielding GMAW. The ionization potential

Potential (eV)

is the amount of energy (in electron volts)

Ar

Totally Inert (Cool)

15.759

He

Totally Inert (Hot)

24.587

O2

Highly Oxidizing

13.618

CO2

Oxidizing (Dissociates

13.769

H2

Highly Reducing

13.598

required to establish an arc. The use of a helium rich gas such as Praxair’s HeliStar™ A-1025 blend (for short-arc welding of stainless steel), requires 3-5 volts more than an Ar/CO2 mixture at the same current. The shielding gas used also has a pronounced effect on the type of metal transfer obtained.

Figure 12 – Gases Used for GMAW Shielding 15

There is a “best gas” for almost every

Argon/oxygen mixtures are also very

application, but there may be 2 or 3 gases

stable, and are used in steel welding

that will do a very reasonable job. Gases

applications. Pure carbon dioxide provides

are selected on the basis of performance,

a less stable arc plasma, but its additions

availability, cost, and many other variables.

to argon can be very beneficial where

Further discussion of gases and metal

depth and width of penetration need

transfer is found in section 6.

to be controlled. Some shielding gases also use additions of oxygen and carbon

Stability of the arc plasma is another factor

dioxide in one mixture (Praxair's Stargon™

influenced by the shielding gas. Pure argon

gas blend).

provides a stable arc, and is used when welding reactive metals such as aluminum.

▲ B. Flow Rates

Once the shielding gas is selected, it is

Regulator-flowmeters can vary the inlet

critical to make sure that the flow rate is

pressure to the flowmeter. As inlet pres-

within certain limits. For low current short-

sure falls when a cylinder gets low, the flow

arc applications, 25-35 scfh (standard cubic

rate is actually skewed to a higher reading.

feet per hour) is adequate if ESO is held

For example, a regulator-flowmeter,

from 3/8" to 1/2". For high current short-

installed in a low pressure line (20 psig),

arc and the spray transfer mode, flow rates

showed a flow reading of 70+ scfh, but

need to be increased to the 35-45 scfh

the actual flow rate was 15 scfh. The

range. Figure 13 shows the best way to

regulator had reduced the pressure in the

measure the flow rate using a torch

flowmeter to about 5 psig instead of the

flowmeter.

correct design pressure of 50 psig. Flow rates must be kept in a controlled range so that the shielding gas column does not

70 60 50 40 30 20 10 0

become unstable and mixes with air at both low and high flow rates or when forced to flow past an obstruction in the nozzle such as spatter. This type of flow is

•

called turbulent (non-axial flow). 30 - 70 scfh measured at the nozzle

Figure 13 – Measuring Actual Flow Rate

16

Testing has shown that, the shielding gas

shielding gas. The erratic quality of the

column stays in laminar flow from 30 scfh

shielding can provide a weld that looks

up to about 70 scfh with a 400 amp gun

satisfactory, but can contain subsurface

(using a 5/8" diameter nozzle). Above

(honeycomb) porosity. As deposition

70 scfh, the flow becomes turbulent and

rates increase, a 35-45 scfh flow rate is still

mixes with air. A cigarette in an ashtray

satisfactory unless there are breezes or

can illustrate the difference between

drafts. In high deposition MIG welding,

laminar and turbulent flow. The smoke

a .045 wire, can be run at 1300 ipm

initially leaves the tip, in a tight orderly

(35 lb/hr) at 50-60 scfh with no problems.

laminar flow. A few inches above, the flow

Fans and drafts will displace a shielding

becomes turbulent and the smoke mixes

gas, and may require increasing flow rates

with the air rapidly as shown in figure 14.

to 50-70 scfh. Reducing cup-to-work

This same thing happens with a column of

distance can also improve shielding.

Figure 14 – Laminar and Turbulent Flow

Air

Turbulent Flow

Turbulent Flow

Laminar Flow Laminar Flow

Air

▲ C. Gas Losses

Loss of shielding can cause problems in GMAW. Inadequate gas coverage can result in an oxidized surface or porosity. The first step in troubleshooting what you think may be a gas loss is the use of a torch flowmeter. This flowmeter fits over the nozzle on a torch and measures the actual flow rate of your shielding gas (figure 13). Compare the actual reading with that of the station flowmeter (if used). The two readings should be very close to each other. If not, there may be some potential problem areas. 17

Threaded Connections are notorious for

If the leak is large enough, no amount of

leaking if not properly sealed. A pipeline

flow can give a good quality weld. Use a

with shielding gas at 50 psi can leak a lot of

liquid leak detector to find any leaks.

expensive gas. It will also allow air to diffuse in, as figure 15 shows.

O2 Concen

Hose Wall

N2, O2, H2O Process Gas

Air

Figure 15 –

Velocity

N2, O2, H2O

Hose Wall

Back Diffusion Allows Pressurized Line

Distance CL

Wall

Wall

Air to Leak into a

A few shops use quick disconnects for

problem by disconnecting the supply hose

their shielding gasses. There are also prime

at the wire feeder. Use the torch flow

suspects when investigating leaks. Use a

meter to check the flow out of the hose. If

liquid leak detector solution to check

the flow meter is correct, check if a leaking

them. If the flow measurement at the torch

fitting or if the o-rings in the back of the

indicates a much lower flow than the

liner have been damaged (figure 16). These

station flowmeter, find the source of the

can be easily replaced, and should be lightly coated with silicone grease to avoid

Lube O-Ring with silicone grease before installing

damaging them during reinstallation. Another place where shielding gas flow can be disrupted is in the diffuser. The gas diffuser is found at the point where the contact tip is mounted. Its purpose is to distribute the gas evenly to produce laminar flow out of the gas nozzle. If spatter builds up on the diffuser, it can clog it and reduce the gas flow enough to provide poor shielding. If the diffuser is only partially blocked, the entire gas flow

Figure 16 –

may try to exit the holes still open and

O-Rings Seal the Gas

create unbalanced turbulent flow. This in

Ports at the Feeder

itself will aspirate air into the shielding gas column and may once again, cause

18

porosity.

Figure 17 –

Holding the torch at too small an angle can

Air Aspirated by the

also create a venturi effect between the

Venturi Effect can

plate and the nozzle. This will also con-

Contaminate the

taminate the shielding gas stream with air

Shielding Gas

and cause porosity (figure 17). Some welders clean spatter from nozzles by lightly tapping the gas cup against Venturi effect pulls air into the shielding gas

something which knocks the spatter out. This can create a problem by eventually causing the cable to gooseneck connection to loosen. When this happens, it is possible

Figure 18 –

to lose your gas coverage in the torch

Pressure Correction

handle and aspirate air into the shielding

Formula for Flowmeters

gas stream. One final thing to check is incorrect inlet

Example: A flowmeter calibrated at 20 psig on a 50 psig line indicating 40 scfh

pressure to the flowmeter. All flowmeters are calibrated for one specific inlet

Actual Flow Rate

Indicated

= Flow Rate x

Actual Pressure (psia)

pressure, and the actual flow reading will

Calibration Pressure (psia)

be incorrect if the inlet pressure does not match the calibration pressure. Figure 18

Actual Flow Rate

= 40 scfh

x

50 + 14.7

shows what happens when a 20 psig

20 + 14.7

calibrated flowmeter is attached to a 50 psig line. The actual flow is 26% higher

= 40 x 1.37 = 54.6 scfh

than indicated on the flowmeter. The easiest way to check this is by using a torch flowmeter, because it is calibrated for atmospheric pressure at the outlet.

Figure 19 – Weld Terminology

Penetration Patterns

and Penetration

The penetration pattern of a weld can be

Measurement

examined by cutting and etching the weld as shown in figure 19. To examine a weld, cut through it at 90 to the face. The sample is easiest to prepare when a saw is

Effective Throat

used. The cut face is then sanded with Face

progressively finer sandpaper until a 320 grit paper is reached. An etchant reveals the penetration pattern. For steels, there are many different etchants available. For macro-etching, a mixture of 10% nitric acid in methanol works well. This etchant is called nital-10%.

Toe Root

19

When the etchant is swabbed on the weld

rapidly. The cooling is caused by the

area, the different microstructures react at

quenching effect of the colder base metal.

different rates. The darkest area will be the

By measuring from the face of the weld to

heat-affected zone, or HAZ. This is base

the root, we find the effective throat of the

metal that is adjacent to the weld metal.

fillet weld as shown in figure 19.

The HAZ has been heated to near the melting temperature and then cooled very

Different gases will change the shape and depth of the penetration pattern. Oxygen

Figure 20 –

additions will reduce the diameter of the

Different Gases

plasma, and lead to a deep and narrow

Provide Different

penetration pattern as shown in figure 20.

Penetration Patterns

This is sometimes called “finger-like” penetration. Additions of carbon dioxide

C-25

O-5

and helium increase the diameter of the

C-15/C-8

plasma because of their increased thermal conductivity. This tends to decrease the effective throat and broaden the penetration pattern. Figure 21 shows the penetration patterns of tubular (flux-cored or metal cored) and solid wires at the same power (current and voltage) level. The tubular wires are made with a solid sheath and filled with a powder; most of the current is carried by the sheath. This increases the diameter of the plasma, and makes for a less penetratFigure 21 –

ing or softer arc. Because of the axial metal

Penetration Patterns

transfer characteristics in spray transfer

of Solid and Tubular

with solid wire, the penetration pattern is

Wires in Spray

narrower and deeper than that of a tubular

Transfer

wire. Because the power of the two arcs is the same in the example, the areas melted in the base metal are equal, but different in

Solid

Tubular

shape. In some welding applications where a solid wire produces burn-through on thinner parts, a tubular wire may be a good choice because the larger diameter plasma spreads the heat over a larger area and reduces the tendency to burn-through.

20

Equal Areas

SECTION

5

5 Electrodes

▲ A. Alloying Additions

Although a welder doesn’t often get the

The list shown in figure 22 outlines the

chance to select a filler material, this

elements added to steel and the reasons

section is included for information.

for their addition. These are the same

Knowing how wires are alloyed and why,

alloying elements that are added to the

can sometimes be helpful when a problem

base metals, proportions differ slightly for

arises. We will concentrate on a few carbon

filler metals.

steel wires, because they are used in the biggest portion of GMAW welding.

1. Carbon – The addition of carbon to iron has a very strong influence on its proper-

Figure 22 –

ties. Mild steels, 1010 and 1020 for instance,

Steel Alloying

have low carbon contents (0.1 and 0.2%

Additions

respectively.) Carbon is a very potent strengthener, and when added above about

Steel is iron that is alloyed with

0.3%, requires special welding procedures to keep the material from cracking

Carbon

Strength

(preheat, interpass and post heat, etc.)

Manganese

Deoxidation & Strength

Most common wires are low in carbon

Silicon

Deoxidation

content.

Aluminum

Deoxidation

Zirconium

Deoxidation

Titanium

Deoxidation

2. Manganese – This element is added for three reasons: (1) Deoxidation. Manganese combines with oxygen in the weld metal before the carbon does so there is little or no oxidation of carbon in the weld puddle to produce carbon monoxide (and cause

Figure 23 – Deoxidation Reactions

porosity. (2) Desulfurization. Manganese combines with sulfur to form manganese

Liquids

sulfides before the sulfur can segregate to

Si + 2O -> Si O2 Mn + O -> MnO

the grain boundaries and form low melting point iron sulfides. Iron sulfides can cause hot cracking in steels. (3) Strengthening. Manganese remaining after these other

Gases

reactions combine with carbon to form

C + O -> CO C + 2O -> CO2

manganese carbides, which strengthen the weld deposit.

21

3. Silicon – This element is mainly added as

4. Aluminum – The main function of this

a deoxidizer. Silicon combines vigorously

element is also deoxidation. It is a very

with oxygen in the weld puddle and forms

strong deoxidizer and forms aluminum

a silicon dioxide (see figure 23) slag.

oxide (Al2O3). A secondary function is that

Beach sand is silicon dioxide. When the

of a grain refinement, which produces a

silicon combines with the oxygen, heat is

stronger, tougher deposit.

generated because of this oxidation reaction. This is one reason why a wire

5. Zirconium – This is also a deoxidizer,

higher in silicon will provide a more fluid

and is used in only a few wires.

puddle. The brown glassy solid that forms on the weld deposit is a combination of

6. Titanium – This element is also a

silicon dioxide and manganese oxide.

deoxidizer in low carbon steels.

▲ B. 1. Solid Wire Designations and Chemistry

With some understanding of why alloy additions are made to low carbon wires, the compositions of some commonly used wires are more meaningful. Figure 24 Rod

shows the American Welding Society

Solid

(AWS) nomenclature used for solid wires. Table 3 shows the chemistries of the

ER70S-X Electrode

Figure 24 –

70,000 UTS 70 - 120 ksi

AWS Solid Wire

available ER70S- electrodes.

Chemistry 2, 3, 4, 5, 6, 7, 8, 9, 10, G

Designation

22

Table 3 –

AWS

ER70S - Wire

Electrode

Chemistries

Class.

Carbon

Manganese

Silicon

Phos.

Sulfur

(max)

(max)

ER70S-2

.07 Max

.9 - 1.4

.4 - .7

.025

.035

ER70S-3

.06 - .15

.9 - 1.4

.45 - .7

.025

.035

ER70S-4

.07 - .15

1.0 - 1.5

.65 - .85

.025

.035

ER70S-5

.07 - .19

.9 - 1.4

.3 - .6

.025

.035

ER70S-6

.07 - .15

1.4 - 1.8

.8 - 1.15

.025

.035

ER70S-7

.07 - .15

1.5 - 2.0

.5 - .8

.025

.035

Other

TI,ZR,AL

AL

These wire designations are set by the

fourth characters indicate the minimum

AWS, to standardize welding electrodes

tensile strength of the weld metal in

and filler metals. The classification system

thousand psi’s. An ER70S-X wire would

is based on chemical composition and

have a tensile strength of 70,000 psi. The

strength of the deposited weld metal. A

fifth character, S, indicates that this is a

typical solid wire designation would be

solid wire. The number after the dash

ER70S-3. The E designates a wire can be

indicates the composition classification of

used as an electrode, meaning it can carry

the alloy. These numbers run from 2 to 7

current. The second character, R, indicates

and G. The G classification (stands for

that this alloy is available as rod. Rods are

general) indicates a chemistry agreed upon

usually the 36" straight lengths and are

by the supplier and purchaser.

used for GTAW (TIG). The third and

▲ B. 2. Metal-Cored Wire Designations and Chemistry

Since metal-cored wires perform like solid

Where the “C” Indicates a “cored” wire

wires during welding, they have recently

and the “X” indicates the type of shielding

been included for classification purposes in

gas used in qualification of the wire

the AWS specification for solid GMAW

(C=100% Carbon Dioxide, M=Mixed

wires, but they follow a classification

Gas of 75-80% Ar/balance CO2).

process more like a flux-cored wire than a solid wire. The two basic wire types are:

E70C-3X E70C-6X ▲ C. Flux-Cored “Tubular” Wire Designations

A second classification document covers

current. Notice there is no R, because

flux cored and wires. An example of the

straight lengths of these wires are not

designation for these wires is E70T-X

usually practical. The second character is

(see figure 25). As with solid wires, E

the tensile strength multiplied by 10,000

designates an electrode that carries

psi. The third character is either a “0” or a “1”. A “0” indicated that the electrode is for use only in the flat and horizontal

Electrode

Tubular

welding positions. A “1” indicated that the electrode is suitable for all position work. The T in the fourth position designates this

E70T-X

as a tubular electrode. The digit after the dash indicates the type of shielding required with this wire.

Figure 25 – AWS Tubular Wire Classification

70,000 UTS 70 - 120 ksi 70 = flat only 71 = all position

Shielding Type 1,4,5,6,7,8,11,G 1,2,5 - with gas 3,4,6,7,8, - w/o gas

For Example, an

E70T1 = 70,000 tensile, flat position E71T1 = 70,000 tensile, all position 23

Deoxidation of the weld pool is very

S-6 or even an S-2 solid electrode is made.

important in metal joining. There are at

To determine the deoxidation potential of

least 5 elements added for deoxidation,

a flux-cored wire, the manufacturer’s

each doing a slightly different job. As a

literature must be consulted. Flux-cored

general rule, the rustier or more mill-

wires contain increasing amounts of

scaled a plate is, the more deoxidation

deoxidizers to remove the oxygen being

required from the electrode. The shielding

deposited in the weld puddle by the

gas is also a source of oxidation. If a rusty

shielding gas and by any mill scale or rust.

plate is welded with a gas of high oxidation

Both mill scale and rust are iron oxides

potential, a cleaner deposit would be

(FeO- mill scale, and Fe2O3) .

obtained if a change from an S-3 to an S-7,

▲ D. Slag and Gas Formation

Silicon and manganese are oxidized in the molten weld puddle. Silicon forms silicon

The SiO2 and the MnO are liquids that

dioxide and manganese forms manganese

float to the surface of the puddle. Upon

oxide. These reactions occur to remove the

cooling, they are commonly referred to as

oxygen and keep the carbon and iron from

slag islands. Higher oxygen contents can

being oxidized. Higher oxidizing potential

remove much of the silicon and manganese

gases, such as pure carbon dioxide will

so that the oxygen will then begin to

oxidize more of the alloying additions

combine with carbon. Carbon monoxide

and produce a deposit of slightly lower

and carbon dioxide result, and most of the

strength and toughness. The oxygen in the

gas formed is carbon monoxide. The gases

weld puddle will also combine with the

will evolve from the molten puddle, but if

carbon to form carbon monoxide if all

cooling is rapid and the gas concentration

of the manganese and silicon have been

is high gas bubbles will be trapped and

oxidized. Porosity in a weld is usually

create porosity. Nitrogen is also soluble

caused by the evolution of carbon mon-

in the puddle and will cause porosity if

oxide. The reactions that occur in the

shielding is inadequate.

molten puddle are:

24

▲ ▲ ▲ ▲

Si + O2 2Mn + O2 2C + O2 C + O2

SiO2 2MnO 2CO CO2

▲ E. Solidification of the Weld Puddle

Figure 26 shows how a weld puddle cools

available for deoxidization (and also

and solidifies. At the weld puddle to base

possibly nitrogen), these gases will be

metal interface, crystals begin to grow into

pushed to the centerline as the puddle

the molten weld pool. This is very similar

freezes. This causes porosity along the

to the growth of ice crystals on a window

solidification line and is known as

seen in time-lapse photography. The

centerline porosity. Larger amounts of

crystals are called grains, and where they

contamination can cause gross porosity in

meet and stop growing is called the grain

the weld and lead to the condition shown

boundary. As the metal solidifies, the

in the bottom illustration in figure 26.

solubility of gases decreases greatly. If there is just slightly more oxygen in the

As the weld is finished, the cooling rate

puddle than manganese and silicon

at the crater increases because the weld is losing heat in all directions. This rapid

Figure 26 –

cooling rate leaves less time for the gases

Solidification of

to leave the puddle, and a hollow gas

a weld

cavity can form at the crater. Calculation of Deposition Rates A very useful piece of information needed when calculating the cost of welding is the deposition rate. The deposition rate is usually stated in pounds of wire per hour of weld time. Figure 27 shows the multipliers that can be used to determine deposition rate for different diameters of solid wires. The multiplier is a factor that takes into account the cubic inches of wire per hour consumed and the density of steel, to arrive at the rate in pounds per hour. To calculate the deposition rate of an

Wire Diameter

Multiplier

.045" diameter wire at 500 ipm wire feed

.030

.012

speed, multiply the 500 ipm times the

.035

.0163

.027 multiplier, and determine a rate of

.045

.027

13.5 lbs/hr of arc-on time. To determine the

.052

.0361

.0625

.0521

Example: an .045 wire at 500 ipm

actual amount of metal deposited, multiply this weight by the duty cycle (% of an hour that the arc is actually on) and the deposition efficiency of the process.

500 ipm x .027 = 13.5 lbs/hour

Figure 27 – Calculating Deposition Rates for Solid Electrodes 25

SECTION

6

6 Metal Transfer

▲ An understanding of metal transfer is very

be obtained.” In this section, electrical

helpful when trying to solve a welding

characteristics, wires and shielding gases all

problem such as “why is there so much

come together. There are four major types

spatter?”, or “how can more penetration

of metal transfer that will be discussed. They are:

Figure 28 –

1. Short-Arc

The Pinch Effect

2. Globular Transfer

Tries to Pinch Off

3. Spray Transfer

the End of the

4. Pulsed Spray Transfer

Electrode

Figure 28 shows the pinch effect. The pinch effect is a function of current, and tries to P

P

P Short-Arc

Globular

P Spray

The pinch effect is a function of current

pinch off the molten tip of the electrode. Higher currents and smaller areas increase the pinch effect and give cleaner metal transfer with less spatter.

▲ A. Short Arc

Short-arc, or short-circuit transfer, is

thin material and sheet metal and has been

basically a low heat input, low penetration

used extensively for out-of-position MIG

process. Currents range from 40-50 amps

welding. Short-arc is also a good choice

(.023" wire) up to 250-275 amps (0.052"

where bridging gaps is a problem.

diameter wire). Voltage ranges from 14-21v. This process is a good choice on

This form of metal transfer is called shortarc because the wire does electrically short

Figure 29 –

to the workpiece. When the wire touches

In Short-Arc Transfer,

the base material, the arc goes out, and the

the Electrode Shorts

current flowing through the wire begins to

60-120 Times Per

rapidly raise the temperature of the wire.

Second

As seen in the power supply characteristic curve, at 0 volts the power supply tries to produce a maximum current output. When the wire reaches its melting point, it flows Current Voltage

into the puddle and the arc reignites. This

26

shorting takes place very rapidly, from 60 to 120 times per second. Figure 29 shows the droplet of molten weld metal pinching off just before the arc reignites in the Time (milliseconds)

third illustration.

After adjusting the wire feed to do the job,

A power supply that has an inductance

the voltage can be fine-tuned to where the

control is easier to use for short-arc

sound from the arc becomes smooth and

transfer. As shown in figure 10 adding

very regular. Electrical stick-out must be

inductance slows the rate of current rise.

closely controlled as it has a great impact

With this available, after wire feed and

on current levels. At the low-end condition

voltage are adjusted, inductance should be

of 40-50 amps and 14-15 volts with a .023"

increased to the point where the metal

electrode, stick-out should be about 1/4".

begins to transfer smoothly. By increasing

With a .035" electrode at 80-90 amps and

the inductance, the current rises at a slower

15-16 volts, ESO should be about 3/8". At

rate, and is at a lower level when the cycle

the high end of short-arc transfer with a

begins again than it would be without the

.045" electrode, current will be at 225-235

inductance in the circuit. Limiting short

amps and 20-22 volts. Because of the high

current reduces explosive, harsh metal

current levels, we increase the ESO to

transfer. Most short arc welding is done

preheat the electrode and reduce the

with .023", .035" or .045" electrodes.

current. The ESO with these conditions

Larger wires require too much current

would be in the 5/8"-3/4" range. This

for most applications.

method gives a very controllable arc. If the arc is unstable, the usual cause is that

Gases that work well in short-arc

the ESO is too long or voltage is too high.

span from C-8 through C-25 mixtures

There should be very little spatter with

(Praxair's StarGold™ blends), straight CO2

this process, regardless of shielding gas.

and Praxair's Stargon™ gas blend. For

Argon mixtures, however, provide smaller

low current applications carbon dioxide

droplets with better gap bridging and arc

is sometimes a good choice. The arc is

stability as a result.

hotter than with an argon mixture, and at low current levels, there is not much

To adjust the power supply for short-arc

spatter. For increased deposition rates and

transfer, two variables can help, if they

travel speeds, C-25, C-15, or C-8 mixtures

are available. The slope on some machines

will usually provide better results with

is adjustable, either externally, or with

decreased spatter levels. Reducing the

internal taps. Figure 7 shows that the

amount of carbon dioxide makes the

steeper curve will limit the maximum

puddle less fluid and easier to control.

current that the power supply can deliver,

Burn-through is also reduced. On thinner

which is a real benefit when short-arc

materials, gases lower in carbon dioxide

welding at low currents on thin materials.

content work best by minimizing burn-

If the majority of your work is in the short-

through and permitting higher currents

arc range, it is probably worthwhile to

and travel speeds. For welding higher

change an internal tap if the machine has

alloys, like stainless steel, helium is some-

one. An internal tap isn’t something you

times added to the shielding gas to

change for each job.

increase heat input at lower deposition rates.

27

▲ B. Globular Transfer

Globular transfer is usually not the

The arc is continuously moving to the

recommended way to deposit weld metal

place where the glob of metal is closest to

because of the inefficiency of the process.

the work, where the minimum voltage is

This type of transfer produces the most

required to sustain the arc. This creates the

spatter. Depending on the current range,

instability that you see and hear in the arc.

shielding gas, and power supply settings,

When the surface tension and the force of

globular transfer can waste 10-15% of the

the arc are finally overcome by gravity the

weld metal as spatter. Because of the

glob transfers. As the glob of metal hits the

inefficiency of the process, slower travel

work, it tends to splash, throwing spatter

speeds or smaller bead sizes result at wire

out of the puddle onto the work.

feeds comparable to spray or short-arc Globular transfer occurs when voltages

transfer.

and currents exceed that of the short-arc When the tip of the wire begins to melt in

range. Other than short-arc, this is the

globular transfer as shown in figure 30, it

only type of transfer you get with carbon

only shorts to the workpiece occasionally

dioxide in the current range used in

due to higher voltages. The inconsistent

industry. If you are using gas blends like

cracks and pops you hear are the breaking

Praxair's Stargon™ and StarGold™ (Ar/O2

of the short circuits. Unlike short-circuit

or Ar/CO2), globular transfer is what you

transfer, an arc is present most of the time,

get when voltage or current falls out of the

and the metal begins to form a ball on the

spray transfer range. This is where spatter

end of the wire. This ball is held by the

develops when a spray arc mixture is used

surface tension and the force of the arc.

improperly.

Unstable Arc High Spatter Levels

Figure 30 – Globular Transfer Produces High Levels of Spatter

28

▲ C. Spray Transfer

Spray transfer is a very clean, high effi-

Spray transfer can be used on materials as

ciency process. All wire diameters can be

thin as 14 and 16 gauge metals with the

used. For most applications in the 175 amp

right wire diameter (.023"). Thicker

to 500 amp range, .035" to 1/16" wires

section welding is where spray really gains

work well. When the welding equipment is

an advantage, especially in the flat and

set up properly, there is almost no spatter

horizontal positions. This type of metal

and 97-98% of the filler weld is deposited

transfer can be used out of position but

in the weld puddle (deposition efficiency).

wire diameter should be smaller and the operating conditions less than in the flat

In spray transfer, the tip of the electrode

position. All steels (carbon and stainless),

becomes pointed as figure 31 shows.

and most other materials, can be GMAW

Because the tip is so small, the current

welded in spray transfer.

density (amps/square inch) and the pinch force are very high. This pinches off metal

The gases used for spray are lower in

droplets that are smaller than the diameter

active gases (CO2 and O2) than gases for

of the wire. The droplets are accelerated by

short-arc and globular transfer. Most con-

the magnetic field, around the arc instead

tain from 85-90% argon, and some blends

of transferring by gravity as in globular

contain both carbon dioxide and oxygen.

transfer. The small droplets are absorbed

Some of the newer gases also contain small

into the weld pool rather than splashing.

additions of helium (Praxair's HeliStar™ gas blends) to increase the energy in the arc.

Droplets smaller than diameter of electrode Very low spatter Minimum voltage and current required

Figure 31 – Spray Transfer is a Very High Efficiency Process

29

Transition Currents (Steel and Stainless Steel) To set a welding system for spray, there are Figure 32 –

minimum voltages and currents required.

Transition Currents

Voltages range from 24 v (small diameter

for 95% Ar/5% O2

with Ar/O2 ) to 30 v (hi-deposition with

Shielding Gas

He). A good place to start is around 26-27 volts. To estimate what the minimum transition current for spray transfer would be, multiply the wire diameter (in

.035

x 10,000 = 350/2 = 175 amps

.045

x 10,000 = 450/2 = 225 amps

.052

x 10,000 = 520/2 = 260 amps

thousandths of an inch) by 10,000 and divide by two as figure 32 shows. This approximation is accurate for a 95% Ar/5% O2 shielding gas. If you

.0625 x 10,000 = 625/2 = 312amps

are using a gas with 10% CO 2, a closer approximation is made by adding the % CO2 to the transition current calculated above and tabulated in figure 33. For example, a .045" wire with C-10 would produce spray transfer at approximately

Figure 33 –

225 + 10 = 235 amps and about 27 arc volts.

Transition Currents

Figures 34 and 35 show the spray transfer

with Various

ranges for 95% Ar/5% O2 (figure 34) and

Shielding Gases

the short-arc and spray transfer regions for Ar/8% CO2 (figure 35).

30

Wire

O-5

C-5

C-10

C-15

.035

175

180

185

190

.045

225

230

235

240

.052

260

265

270

275

.0625

320

320

325

330

Figure 34 – Spray Transfer Ranges for 95% Ar/5% O2 .035" and .045"

36

.035" and .045" wire with O-5 gas

Electrodes

Hiss

.045

Spatte r

Voltage

32

28

.035 tter

Spa 24

22

18 100

200

300

400

Current

Figure 35 – Short-Arc and Spray Transfer Ranges for

.045" wire with C-8 gas

Ar/8% CO2 (C-8) with 0.35" and 0.45"

40

Electrodes

Hiss

.045

Spatte

Voltage

r

35

30

.035 tter

Spa 25

20

Short-arc 15 100

200

300

400

Current 31

▲ D. Pulsed Spray Transfer

Pulsed spray transfer is a process that

Due to its low heat input, pulsed spray is

combines the lower heat inputs associated

beneficial for out of position work and for

with short-arc with the clean metal transfer

filling gaps. Since it can produce high peak

and good penetration associated with

currents, a larger wire can usually be used

spray transfer. A graph of current vs. time

at lower deposition rates. A larger wire

(figure 36) shows the shape to be a square

(.045 instead of .035) will usually reduce

wave. The current at the top of the square

wire costs and reduce wire feeding prob-

wave is called the peak current, and the

lems, especially for materials such as

current at the bottom of the square wave is

aluminum.

called the background current. The background current keeps the arc lit, but at

Recent research has shown that inverter

very low currents – typically 20-40 amp.

pulsed power supplies with very rapid

When the current rises to the peak current,

current rise can reduce the fume associ-

one droplet is transferred in spray transfer.

ated with higher current GMAW welding.

Because of the small size of the droplet,

The fuming is caused by superheating the

spatter is minimized and penetration is

molten tip of the wire and causing the

maximized due to the spray transfer.

metal to boil. The very rapid current rise reduces the superheating, leading to the reduced fume generation rates.

Peak Current

Current

275

Average Current

140

20 Background Current Time (milliseconds)

Figure 36 – Pulsed Spray Transfer Produces Low Heat Inputs With Very Clean Transfer

32

SECTION

7

7 Welding of High Strength Steels

▲ Higher tensile and yield strengths can be

the ASTM. The ASME (American Society

achieved by increasing carbon content,

of Mechanical Engineers) grades for these

adding alloys, or a combination of both. In

alloy steels are ASME SA517, grades B

the section on materials, it was seen that

and F.

there are hundreds of different steels available today. US Steel’s “T1” construc-

A comparison will be made between T1

tion alloy will be used as an example of an

steel and a carbon steel of comparable

alloy steel. Most of the major steel produc-

strength (SAE 1080) in table 4 to look at

ers now make similar High Strength Low

the procedures required to weld the two

Alloy (HSLA) steels, which are designated

different materials. It is usually easier to

A514 and A517 grades B, Q, H, and F by

weld an alloy steel than a carbon steel of equivalent strength.

Table 4 – Mechanical Properties Comparison of 1080 High Carbon

Tensile Yield

1080

T1

T1A

T1B

T1C

112,000

110,000

110,000

110,000

110,000

61,000

100,000

100,000

100,000

100,000

Elongation

10%

18%

18%

18%

18%

Construction Alloy

%RA*

25%

40%

40%

40%

40%

Steels

* Reduction of Area

Steel and 4 T1

The tensile strengths of the four materials

deform (stretch), and the structural

are all in the 110,000 to 112,000 psi range.

members would take a permanent set

Most products are designed using the

instead of returning to their original shape.

material’s yield strength; in this case

Elongation and reduction in area are

there is a dramatic difference in the yield

measures of the ductility of the material.

strengths of the materials. Yield strength is

A ductile material will deform instead of

where the material begins to plastically

fracture under severe loading.

33

Table 5 –

T1

T1 A

T1 B

T1 C

1080

Carbon

.1 - .2

.12 - .21

.12 - .21

.14 - .21

.78 - .89

Manganese

.6 - 1.0

.7 - 1.0

.95 - 1.3

.95 - 1.3

.6 - .9

Phos (max)

.035

.035

.035

.035

.04

of Equal Tensile

Sulfur (max)

.04

.04

.04

.04

.05

Strengths

Silicon

.15 - .35

.2 - .35

.2 - .35

.15 - .35

Nickel

.7 - 1.0

Copper

.15 - .5

Chrome

.4 - .65

.4 -.65

.4 - .65

1.0 - 1.5

Molybdenum

.4 -.6

.15 - .25

.2 - .3

.4 - .6

Vanadium

.03 -.08

.03 - .08

.03 - .08

.03 - .08

Boron

.0005 - .006

.0005 - .005

.0005 - .005

Comparison of Chemical Compositions of 1080 and T1 Alloys

Titanium

1.2 - 1.5

.01 - .03

Welding of high strength steels is different than welding low carbon steels. The welder must pay a lot more attention to detail when welding high strength steels.

▲ A. Select the Proper Filler Metal

If standard T1 steel were to be welded the tensile strength of the material should be considered. From table 4 it is seen that the tensile strength is 110,000 psi. For the GMAW process, an ER110S-1 electrode could be used. The 110 indicates a tensile strength of 110,000 psi. Some manufacturers also develop electrodes that do not exactly fit in the AWS classifications. These electrodes can also be used, but a welding engineer should look at the mechanical properties to ensure compatibility. If the FCAW process were to be used, an E110T5-K3 (Mn-Ni-Cr) or an E110T5-K4 (Mn-Ni-Cr-Mo) would be selected according to the mechanical properties required.

34

▲ B. Minimize Hydrogen Contamination

Molten weld metal is capable of absorb-

3. Moisture

ing considerable amounts of hydrogen.

Moisture can come from surface contami-

Hydrogen in weld metal causes two

nants such as rust and mill scale, and it can

problems, porosity and cracking. Hydrogen

be adsorbed on the surface of a clean sheet

can come from a variety of sources, and all

of steel. Bringing a piece of high strength

of them can cause problems. Hydrogen is

steel inside in the winter will sometimes

normally found in nature as a diatomic

cause moisture to condense on the surface.

molecule, H2.

For T1, it is recommended that the material temperature be at least 70 F before

In the arc, the hydrogen immediately

welding begins.

dissociates to monatomic hydrogen (H), which is the smallest atom known. A single

Moisture can also come from the shielding

hydrogen atom is about 1/100,000 the

gas if there is a leak in the supply line as

diameter of an iron atom. The hydrogen

discussed in the shielding gas section. Air

atoms are very soluble in the weld metal.

contains oxygen, nitrogen, and moisture, all

Some of the hydrogen sources are:

of which are detrimental to the integrity of the weld metal.

1. Grease, Oil, Oxidation or Paint on the Part or Electrode

As the puddle freezes, the solubility of

Grease and oil are both hydrocarbons, and

hydrogen in the puddle decreases. The

in the heat of the arc (about 10,000ºF), will

hydrogen tries to escape from the weld

rapidly dissociate to produce hydrogen,

metal by two different mechanisms.

carbon, and other contaminants in the puddle. The carbon can over-harden the weld metal with possible carbon increases

• Porosity Formation If the concentration of hydrogen in

of .1 to .25%. Hydrogen goes into solution

the matrix exceeds its solubility in the

in the weld metal. Some paints are also

solidified weld metal, the excess hydro-

hydrocarbon based, leading to the same

gen can form bubbles of hydrogen gas.

problems as oil and grease. Mill scale and

These bubbles can show up as porosity in

rust can also contain moisture. When

an x-ray. At very high concentrations of

the arc heats these materials, the water

hydrogen they can cause visible porosity

molecules are dissociated into hydrogen

at the face of the weld.

and oxygen. Both of these gases go into solution in the weld metal and can cause weld problems such as porosity and cracking. 2. Excess Drawing Lubricants on the Electrode Drawing lubricants on the wire electrode can also contain hydrocarbons. When they are exposed to the arc heat, they dissociate and contaminate the puddle.

35

• Slowly Diffuses Out of the Weld Metal

hydrogen content is high enough, there

After welding is completed, hydrogen

are visible streams of bubbles rising from

continues to rapidly diffuse out of the

the coupon. The rest of the diffusible

weld metal. In lab tests for measuring

hydrogen will escape within 20 to 30

diffusible hydrogen, the welded coupon

days. The remaining hydrogen in the

is quickly inserted in a bath of liquid

weld metal is called residual hydrogen,

nitrogen. The sample is then placed in

and it can cause cracking problems after

mercury, and the quantity of hydrogen

welding.

released is measured over time. If the During the welding process, the base Figure 37 –

metal in contact with the molten puddle

Hydrogen is Driven

was subjected to temperatures very close

Into the HAZ by

to the melting temperature of the material.

Stress Gradients

In that period, hydrogen atoms also diffuse into the base metal in what is referred to as the heat affected zone (HAZ - see figure 37). The solubility of hydrogen increases with temperature even when the metal remains a solid. The diffusion of the hydrogen into the base metal in conjunction with the very rapid cooling rate can lead to underbead cracking or delayed brittle fracture.

HAZ

▲ C. Control Heat Input

36

To understand the welding of the higher

and the HAZ is very important in deter-

strength steels, it is necessary to know a

mining the properties of the material.

little about what happens when steels

A familiar microstructure found in metal

solidify. As molten metal cools, it under-

files is called martensite. Martensite is

goes a transformation from a phase called

very strong, with a tensile strength of

austenite to a number of different struc-

200,000 psi or more, but the microstructure

tures. A low carbon steel like a 1008 grade

is very brittle. Martensite can be formed

changes from austenite to ferrite, which is

by very rapid cooling of the material. If a

a very soft and ductile microstructure.

file is heated to 1300 F - 1400 F and then

Adding slightly more carbon and cooling

cooled very slowly, a hole can be drilled in

more slowly forms a microstructure called

it. If it is then reheated and quenched in

pearlite, which is ferrite and cementite

oil, martensite will reform and the file can

(Fe3C). When carbon and other alloying

be used again. There is also an intermedi-

elements are added, the transformation

ate microstructure called bainite, between

from austenite is modified to form harder

pearlite and martensite, which has proper-

and sometimes more brittle microstruc-

ties that fall between the two.

tures. The cooling rate of the weld metal

Metallurgists use charts called continuous

temperature. The cooling rate of the HAZ

cooling transformation (CCT) diagrams to

is much faster than the weld metal because

determine the cooling rates required to

the base metal is acting like a quenching

obtain certain microstructures. A CCT

medium. This is the reason that the heat

diagram tells what microstructure to

input must be controlled during the

expect when cooling occurs at different

welding of the high strength steels. For T1,

rates. At the bottom of the chart are three

higher heat inputs lead to grain growth in

different scales for air, oil, and water

the HAZ, and the strength of the material

quenching. Air cooling produces the

is reduced.

slowest cooling rate with a water quench being the fastest. Welding on a piece of

To control the cooling rate, minimum

base metal that has just been brought in

preheat and interpass temperatures are

the shop from the yard on a very cold day

specified to avoid a brittle crack sensitive

can produce some very rapid cooling rates.

HAZ and weld metal.

The reason why a CCT diagram is men-

Table 6 below shows the maximum heat

tioned is that weld metal and the heat

inputs for T1 for different thicknesses and

affected zone (HAZ) also go through the

preheat and interpass temperatures.

same cooling process after welding. The HAZ is the base metal right next to the weld metal that almost reached its melting

Table 6 –

Preheat and Interpass Temperature ( F)

Maximum

70

150

200

300

400

27

23

21

17

13

1/4"

36

32

29

24

19

in Kilojoules Per

1/2"

70

62

56

47

40

Inch of Weld

3/4"

121

107

99

82

65

1"

Any

188

173

126

93

1 1/4"

Any

Any

Any

175

127

1 1/2"

Any

Any

Any

Any

165

2"

Any

Any

Any

Any

Any

Allowable Heat Input for T1 Steel

Thickness 3/16"

37

The thicker the material is the higher the

Any hydrogen in the weld metal will

allowable heat input. Some of the thicker

diffuse into the highly strained microstruc-

materials have no maximum heat input

ture and could lead to underbead cracking.

because there is sufficient material to

Hydrogen in a strained microstructure can

ensure a rapid cooling rate with currently

also lead to a problem called delayed

available welding processes. To check

brittle fracture. Delayed brittle fracture is

interpass temperature, use a temperature

a weld defect that does not show up

indicating crayon 1/2" to 1" away from the

immediately. The product can be in service

joint. It is possible to contaminate the weld

for a period of time, and then fail at loads

with the temperature stick, and it is the

well below the yield strength of the

temperature of the base metal, not the

material. Since the hydrogen atom is about

previous pass, that is important.

1/100,000 the size of an iron atom, it can diffuse through the metal easily. Stress

If the preheat and interpass temperatures

gradients in the material cause the hydro-

for T1 steel are compared with those for

gen atom to migrate and concentrate in

1080, a real difference is seen. The 1080

areas of high triaxial (3 dimensional)

steel has a carbon equivalent of up to

stress. In the matrix, these areas would be

1.03%. The steel with 1% carbon would

at the tip of a microcrack, a grain bound-

require preheat and interpass tempera-

ary imperfection, or at the base of a

tures of between 600 F and 800 F depend-

surface imperfection, such as undercut.

ing on thickness. A post weld heat treat-

When the hydrogen concentration reaches

ment at high temperature would also

a critical level, the crack grows slightly to

probably be required to prevent cracking.

relieve the stress. The diffusion mechanism

Compared to the preheat requirements

begins again at the tip of the new crack,

(70 F) in table 6, it is easy to see that T1

and the process repeats itself. As the crack

steel is much easier to weld than the

continues to grow, the material resisting

equivalent strength carbon steel. The

the load decreases. At some point, rapid

alloying additions in T1 steel undergo

crack growth occurs, and there is cata-

transformation to carbides at higher

strophic failure.

temperatures than the carbon steels, which allows the greatly reduced preheat and interpass temperatures. For materials that require a slow cooling rate to avoid cracking, it is also very important to control preheat and interpass temperature. A highly strained microstructure, such as that created by rapid cooling, is very sensitive to hydrogen.

38

▲ D. Use the Correct Technique

The correct technique incorporates all of

When beginning to weld, it is very good

the topics just mentioned, such as control-

practice to use the back step technique.

ling hydrogen and watching heat input.

The back step technique involves starting

With high strength materials, it is also very

to weld 1/4"-3/8" ahead of where the

important to watch torch angle. The ideal

beginning of the weld is required, and then

fillet has a flat face and the toes blend into

backing up to the beginning. The beginning

the side wall smoothly. No undercut can

of a weld usually cools too rapidly because

be tolerated in high strength materials,

the base metal has not been preheated

because undercut acts just like a notch to

from the heat of the arc. Back stepping

greatly reduce the stress at which the

begins to preheat the material, and greatly

component fails. A bolt that fails in the

reduces the possibility of lack of fusion

first thread illustrates how undercut can

because the material where you strike the

affect weld properties. The thread, just like

arc will be re-melted.

undercut, (called a stress riser) is basically the beginning of a crack. Undercut will

Weaving is also not recommended on

also increase the strain in the material

higher strength materials. A stringer bead

directly below it and make it more sensi-

technique gives better results, and helps

tive to any residual hydrogen that may be

control heat input. If heat input were

in the material.

calculated using a weaving technique, it would probably be difficult to control the maximum heat input due to the slow travel speeds associated with weaving.

39

SECTION

8

8 Technique and Equipment Set-Up

▲ Technique is very important when welding

burnback, arc and puddle position, vertical

any type of material, and gets more

down welds, gaps, crater filling, and arc

important as the material strength in-

starting will be discussed here.

creases. Torch angle, feed roll tension,

▲ A. Torch Angle

There is a specific amount of energy

welded. Because of the elevated tempera-

available from the arc to heat and melt the

ture of the base metal, the bead will cool

base metal. Torch angle plays a very

more slowly. This allows the face of the

important role in the shape of the bead

weld to come to equilibrium and will give a

and the depth of penetration into the base

relatively flat face. If a lagging (or drag)

metal as figure 38 shows. A leading (or

angle is used, very little of the arc energy

push) angle will use some of the arc energy

goes into preheating the base metal, and

to preheat the base metal before it is

deeper penetration is the result. Because of the lack of preheat, the bead will tend to be convex (humped) because the weld will

Travel

cool more quickly. If a line were drawn between the toes of the weld, the weld metal above that line is wasted. If a fracture were to occur, it would start at the toe of the weld, not through the thick section. In fatigue service, a humped bead Lead Angle (Push)

Lag Angle (Pull)

actually reduces the service life of the component. The decreased reentry angle (angle that the bead face makes with the base metal) tends to raise the stress level at the toe of the weld. A flat bead face distributes the stress more evenly across the joint. A slight drag angle does work well in a deep groove, and also in the first pass of a multi-pass weld. As a first pass, the benefit of increased penetration is

Figure 38 –

obtained, but it is critical to be sure to melt

Torch Angle Affects

out the toes of the first pass when the cap

Penetration and

passes are put in. A lack of fusion at the

Bead Shape

toe of the first pass is a defect that will reduce the service life of the component.

40

▲ B. Feed Roll Tension

Setting feed roll tension is important to

pressure on the wire to deform it between

improving consumable life and reducing

the feed rolls. Figure 39 shows the shape of

downtime due to feeding problems. How

the wire as it leaves a two U-grooved roller

many times have you seen a welder have a

setup with excessive pressure. The “fin”

feeding problem “corrected” by increasing

will be scraped off as it feeds through the

feed roll pressure? There are two main

liner, and will make it more difficult to

types of feed roll designs used for solid

feed the wire as the excess material begins

wires, one grooved and one flat roll, or two

to clog the liner. Excessive pressure on the

grooved rolls. Increasing feed roll tension

feed roll will wear out the feed rolls, plug

on either design can actually put enough

up the liners, and usually lead to a burnbacks at the tip.

Figure 39 – Feed Roll Tension

The best method of adjusting feed roll

Adjustment

tension involves running about a foot of wire out of the gun. Bend the wire 180 to Too much feed roll pressure deforms the wire and causes slivers to form

form a curved end and run the wire into a

Slivers clog the liner and make the wire harder to feed

adjust the feed roll pressure until the wire

Apply just enough tension that the wire feeds into a gloved hand and exits at 180

this feed roll pressure, the wire will feed

gloved hand. Pull the trigger and slowly will make the turn and feed smoothly. At without deforming and the rolls will slip if you get a burnback instead of “bird nesting”.

▲ C. Burnback

The burnback control is designed to reduce the stick-out at the end of a weld and keep the wire from sticking in the puddle. When welding begins, the trigger

Wire feed speed

Burnback is a timer that controls the ESO for the start of the next weld

on the gun closes the contacts in the welder, opens the gas solenoid, and starts Contactor off

When you release the trigger at the end of the weld, the gas solenoid and the contactor can react very quickly. The motor has the inertia of the armature, the gearbox, and the feed rolls to overcome, so it does

Time Gas on contactor closed feed motor on

the wire feed motor as shown in figure 40.

Motor up to speed

Motor off

Gas off

not stop instantly. What a burnback control does is put a timer in the circuit that delays the opening of the contactor and the closing of the gas solenoid. This allows the motor to spin down and the wire to continue to burn off so you don’t have to try

Figure 40 –

to start the next weld with too much stick-

The Burnback

out. A short stick-out greatly improves

Control Is Essentially

the starting of the arc because more

a Timer

current is available.

41

▲ D. Arc and Puddle Position

It is very important to watch the position of your puddle in relation to the arc. In order to get good fusion into both pieces of base metal, the heat from the arc must be directed onto the base metal (figure 41).

If you get on top of the puddle, lack of fushion and coldlap can result

If a very large fillet is attempted in one pass, the tendency is to try to hold the puddle back with the arc. The puddle will continue to advance, and trying to hold the puddle back with the arc will lead to incomplete fusion into the side wall. When the puddle rolls under the arc, the heat of the arc is no longer being put into the base metal, it is going into the puddle. This increases the temperature of the puddle and reduces the fusion into the walls. This

Figure 41 –

is sometimes called “slugging” the joint.

Stay in Front of the

This is a very poor practice and leads to

Puddle with the Arc

welds that will fail at well below their design strength.

▲ E. Vertical Down Welding

Vertical down welding (figure 42) can be done properly, but the weld parameters Use a drag angle

must be set very carefully. The problem

Stay in front of the puddle

with vertical down welding is that it is very

Use short-arc pulsed spray or low current spray transfer

If you have watched a welder putting in a

Use a smaller diameter electrode

metal hitting the floor, this is very similar

easy to get lack of fusion into the side wall. vertical down weld and seen the molten to “slugging” a joint mentioned before in that the only path to the floor is between the arc and the base metal. If the weld metal is taking this path, the arc energy cannot be going into the base metal.

Very little metal should fall past the puddle

A high quality vertical down weld can be made in either short-arc or spray transfer, but the arc must stay in front of the puddle. Here is one more place where a lagging angle is beneficial, because the

Figure 42 –

arc force can be used to hold the puddle

Vertical Down

in place as it begins to cool.

Welding Can Produce a High Quality Joint 42

▲ F. Gaps

Gaps can cause a lot of problems because

Reducing the electrode diameter reduces

they vary in consistency and also change

the current due to the higher resistance

the maximum heat input that the part can

of a smaller wire. A .035" (.9mm) wire

handle. A real problem arises on critical

will give the same deposition as a .045"

parts where depth of penetration is

(1.2 mm) at about 20 amps less because

important, and, therefore, the heat input is

of the higher resistance of the smaller

near the upper end of the range that the

electrode. Shielding gas mixtures with

part can handle. As soon as a gap appears,

lower carbon dioxide contents can also

the heat input must be reduced, or burn-

reduce the energy in the arc and make

through will occur.

gaps easier to bridge. If a C-25 mixture is used, try C-8 for more gap tolerance.

The best solution is to fix the parts so the

Consider also, changing to short-arc

gaps don’t exist, but, this is usually not

transfer if spray is currently used. Short-

possible. In a manual welding operation,

arc is a lot better at filling gaps due to the

the solutions are a lot easier than in a

reduced heat input. Another very good

robotic operation. A welder is a lot smarter

solution is to reduce heat input with the

than a robot, and can quickly adjust

use of pulsed spray transfer. Pulsed spray

conditions to reduce heat input. Increasing

allows low heat inputs with almost no

the electrical stick-out can reduce the heat

spatter, and makes it easier to fill gaps.

input enough in some cases to eliminate the burn-through. Remember that increasing ESO reduces current and increases voltage slightly, reducing heat input.

Use short-arc pulsed spray or low current spray transfer Use smaller electrodes Reduce the CO2 in the shielding gas Increase ESO (stick-out)

Figure 43 – Welding Gaps

43

▲ G. Crater Filling

The crater is the last bit of weld metal to

little more time filling in the crater before

freeze at the end of a weld. Due to the

ending the weld. This will add a little more

shrinkage that occurs as the weld metal

weld metal to increase the reinforcement

cools, the crater will sometimes appear

and also put some additional energy into

concave. This can cause problems because

the weld to slow the cooling rate. Slowing

it is highly strained weld metal with very

the cooling rate will allow more time for

little reinforcement because of the concav-

the gases in solution to escape through the

ity. Highly strained weld metal with very

face of the weld.

little reinforcement can develop cracks very easily. Porosity in the crater is also possible. The solution to this is to spend a

▲ H. Arc Starting

Initiating the arc is mainly a function of

The current rise begins when the arc starts,

the current available. The biggest cause of

is slowed by the additional resistance in

poor arc starting is too much stick-out.

the welding circuit. Replacement of the

The extra resistance heats the wire and

poor connections will restore arc starting.

reduces the current available for arc

Sealing the exposed copper with a liquid

initiation. When the wire has heated

electrical tape will prevent oxidation from

enough to soften it buckles and the arc

occurring.

goes out. Then the whole process begins again, leaving little stubs of wire at the

At higher current levels, also consider the

start of the weld.

slope and inductance setting on the power supply. A steep slope or high inductance

Another problem seen on a regular basis is

setting is designed to limit short-circuit

poor connections on the work and the hot

current. If the material to be welded is not

leads. When copper is new, it is bright and

too thin, a flat slope and/or minimum

highly conductive. As it ages, the copper

inductance will improve the arc starting

surface oxidizes and acts as an insulator.

by increasing the rate of current rise.

This resistance causes the cables to get hot during welding.

44

SECTION

9

9 Weld Discontinuities and Problems

▲ The AWS defines discontinuities as

defect when the component is rendered

interruptions in the typical structure of a

unserviceable.

weldment. A discontinuity only becomes a

▲ A. Lack of Fusion

Lack of fusion, is little or no penetration

a good chance for lack of fusion. As the

and tie-in to the base metals, as shown in

torch moves back into the puddle, the

figure 44. This can be caused by many

molten metal doesn’t stop and wait. It

different reasons. Some of the common

continues to advance, but without the

ones are:

benefit of the intense heat of the arc to melt the base metal. The puddle just lies

1. Torch Not Centered: This concentrates

on top of the work instead of fusing to it.

the heat of the arc on only one of the

Exaggerated oscillation produces the

pieces. If both pieces to be joined are not

same problem.

melted by the arc, the molten puddle will have a tendency to lie on top of the second

3. Excessive Travel Speed: If the travel

piece without fusing to it.

speed is too high, it is possible to not spend enough time to allow fusion with the base

2. Poor Torch Oscillation: If a welder

metal. There is a minimum amount of heat

makes “little circles” with the torch, it is

required for every welding joint, because

easier to make a pretty bead, but there is

the base metal can cool the puddle very rapidly after it is deposited. 4. Vertical Down Welding is notorious for lack of fusion at the toes (where the weld metal intersects the base metal) and very little penetration. This lack of fusion can also occur in flat and horizontal positions where it is called overlap, or cold lap. Lack of fusion in vertical down welds is usually caused by getting on top of the weld puddle. This prevents the arc from adequately heating the base material to produce proper fusion. An old slang term for this defect is fingernailing because if it is bad enough, you can insert a fingernail beneath it.

Figure 44 – Lack of Fusion

45

5. Travel Speed Too Slow: It is similar to

thicker sections, where the base metal pulls

improper torch oscillation. The heat from

heat out of the puddle very rapidly.

the arc is used to further heat the already molten puddle and raise its temperature

7. Incorrect Welding Parameters is the

instead melting the base metal.

last major cause for lack of fusion. This category would include all the variables

6. Current Too Low can also cause lack of

covered plus incorrect ESO (electrical

fusion. This is usually only a problem on

stick out), torch angle wrong, incorrect shielding gas, etc.

▲ B. Porosity

Porosity (figure 45) can be a significant

Another form of contamination can come

problem, and not easily solved. The biggest

from the base metals. It is against better

causes are probably contamination of the

welding practice to weld over paint, mark-

shielding gas, followed by filler metal and

er lines, water, rust, oil, and heavy mill

base metal contamination. A leak any-

scale. Paint and markers are typically made

where in the distribution system from a

of hydrocarbons. The heat from the arc

leaking fitting in the ceiling to a loose hose

breaks down these compounds to release

fitting at the feeder will allow gas to leak

hydrogen, carbon and other contaminants

out and air to diffuse into the shielding gas.

into the weld pool. Mill scale and rust

Molten weld metal holds a lot more

can also contain a lot of moisture that is

nitrogen, oxygen, and hydrogen (from

dissociated in the arc to form oxygen and

moisture in the air) than solid metal. As

hydrogen. Although this is not such a big

the weld puddle freezes, the gases come

problem on low carbon steels, on higher

out of solution and form porosity.

alloys it can cause cracking that sometimes doesn’t show up until the component is in service. Additions of higher thermal conductivity gases (He & CO2) and higher alloy content wires (ER70S-6, S-7) can help to reduce porosity. Helium and carbon dioxide increase the energy being put into the puddle, so the cooling rate is slower. The slower cooling rate gives more time for the gases to leave the puddle instead of causing porosity. The increased deoxidizer levels in some wires will also reduce porosity if the cause of the porosity is associated with oxygen levels (not high nitrogen levels due to gas leaks). These deoxidizers

46

Figure 45 –

will tie up the oxygen and float it to the

Porosity in a

surface as liquid oxides that freeze to form

Weld

slag islands.

Porosity can also be caused by excessive

Barriers must sometimes be put between

tip to work distance. This can create tur-

the welding area and an open door. This

bulence in the shielding gas column,

can usually be something as simple as a

aspirating oxygen and nitrogen from the

welding screen.

atmosphere that then react with the high temperature weld metal.

As mentioned under gas losses, a dirty torch can cause porosity by blocking and

This also occurs when the torch angle from

disturbing the laminar flow of the shielding

vertical is too great. When the torch angle

gas. Spatter adherence to the nozzle is an

is too severe, a venturi is set up between

obvious culprit, as is spatter on the gas

the gas nozzle and workpiece. This pulls in

diffuser.

a great deal of atmospheric air contaminating the shielding gas.

If the operating voltage is too high problems can occur because the arc will have a

Additional causes of porosity are shielding

tendency to wander, especially on fillets.

gas flow too low and shielding gas flow too

As the arc seeks the closest point, it can

high. At low rates, the gas cannot exclude

disturb the gas column enough to cause

the atmosphere. At high flows, turbulence

turbulence and entrain air.

in the gas column causes mixing with the atmosphere.

A further cause of porosity is excessive torch oscillation. Too much torch oscilla-

Drafts, winds and fans can also cause

tion can cause porosity because it is

porosity. All of these sources disturb the

possible to either induce turbulence or

shielding gas column and must be con-

even run out from under the gas column.

trolled. Sometimes just moving a fan a few degrees is enough to solve a problem.

▲ C. Burn-Through

Burn-through is usually caused by excessive current for the application, as shown in figure 46. Sometimes an easy solution is to increase your ESO (electrical stick out.) This reduces heat input into the part. Another problem can be that the travel speed is too low. Traveling too slowly increases heat input greatly. When possible, increase the travel speeds and make multiple passes, if necessary.

Figure 46 – Burn-Through

47

Incorrect wire diameter is also a cause of

Excessive gaps can also be a cause of burn-

burn-through. Reducing the wire diameter

through, as every manual welder knows.

allows you to weld at lower currents but at

This is easy to forget on robots and mecha-

the same deposition rate with reduced heat

nized welds, and sometimes, mysterious

input. One more variable that can cause

burn-through occurs because the robot

additional heat is using the incorrect

can’t compensate by changing conditions

shielding gas. The shielding gas can be

when faced with gaps like a welder can. If

tailored to the application to help control

the parts can’t be made to fit better, gaps

the heat input.

can be pre-welded or “stringered” with a short-bead prior to final welding.

▲ D. Undercut

Undercut (figure 47) has three main causes. The first is excessive voltage, which can cause the arc to wander. The melted area of the basemetal is too large for the puddle to flow into and fill the undercut area. High travel speeds can lead to undercut for the same reason. The heat input is too low to allow the weld metal to flow and the bead is generally convex. On horizontal fillets, undercut can be caused by incorrect torch position. If the torch is positioned on the vertical leg, there is no way for the weld metal to flow uphill against the force of gravity. Repositioning the torch 1-2 wire diameters out from the root on the horizontal plate will eliminate the undercut. Undercut is a very serious

48

Figure 47 –

defect on high strength materials because

Undercut on the

it greatly increases the strain in the mater-

Upper Leg of a

ial just below the notch and increases its

Fillet Weld

sensitivity to hydrogen cracking.

▲ E. Spatter

Spatter is usually caused by operating

In spray transfer, too low a voltage will

outside acceptable parameters for the

also cause spatter. For the same .045"

desired metal transfer mode. For example,

diameter electrode and C-8, the mini-

operating in the short arc mode with an

mum conditions are about 235 amps and

.045" diameter electrode and a C-8 gas,

27 arc volts. The key here is arc volts. The

the hottest condition would be about

power supply may say that it is putting out

223-235 amps and 20 to 21 volts. This is a

32 volts, and there is still spatter. Use a volt

sliding scale, so at 150 amps it might be

meter and measure the voltage from the

17 or 18 volts. As the voltage climbs into

positive connection at the wire feeder to

the 23-25 volt range the spatter level

the work piece while welding. It is possible

increases rapidly because the transfer

that there is 100 feet of work leap with two

mode is now globular transfer.

frayed, oxidized connections. The 32 volts at the machine may be only 25 volts at the arc due to the resistance in the circuit.

▲ F. Cracking

Cracking (figure 48) in low carbon steels is not usually a problem because the materials are so ductile. In high strength materials, cracking can be caused by: High Restraint – A highly restrained part will not allow the material to move to relieve the stresses of the weld metal during cooling. This also leads to local stresses beyond the yield strength of the material. A highly restrained joint using high strength material can require preheat and interpass temperatures be increased to slow the cooling rate. Post-weld heat treating may also be necessary. Poor Bead Shape – A convex bead will greatly increase the stress at the toes of the

Figure 48 –

weld. The increased thickness of the center

Centerline and

of the bead forces the stress into the toes,

Underbead

and can rapidly exceed the yield strength

Cracking

of the material. A concave bead may not have the required effective throat to withstand the design forces, and crack through the throat. The optimum fillet shape is a flat face with the toes blending into the side walls very smoothly.

49

SECTION

10

10 Conclusion

▲ Hopefully this course has given you a little better understanding of the Gas Metal Arc Welding Process. With the knowledge you have, and this booklet to use as a reference, it is hoped that you’re now a more informed welder and your job has been made easier. Please call your local Praxair engineering representative with any questions that you have in the future.

50

WORLD HEADQUARTERS Praxair, Inc. 39 Old Ridgebury Road Danbury, CT 06810-5113 Tel: 1-800-PRAXAIR (1-800-772-9247) (716) 879-4077 Fax: 1-800-772-9985 (716) 879-2040 Internet: www.praxair.com e-mail: [email protected] The information contained herein is offered for use by technically qualified personnel at their discretion and risk, without warranty of any kind. PRAXAIR, the FLOWING AIRSTREAM design, HELISTAR, STARGOLD and STARGON are trademarks or registered trademarks of Praxair Technology, Inc. in the United States and other countries. Copyright © 1996, 1997, 1998, 1999 Praxair Technology, Inc. All Rights Reserved.

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