Gas Metal Arc Welding ofCarbon Steel

Section/Topic Page

Section 1 1

Introduction

Section 2 2

Base Metals

A. Alloying Elements 3B. Carbon Steels 6C. Alloy Steels 6

Section 3 8

Electrical Characteristics

A. Power Supply Basics 8B. Constant Voltage Power

Supply Controls 9C. Electrical Stick Out 9D. Constant Voltage Power

Supply Characteristics 111. Slope 112. Inductance 123. Heat Input 13

Section 4 14

Shielding Gases

A. Shielding Gas Functions 14B. Flow Rates 16C. Gas Losses 17D. Shielding Gas Selection 20

Section 5 23

Electrodes

A. Alloying Additions 23B. 1. Solid Wire Designations 24

and ChemistryB. 2. Metal-Cored Wire Designations 25

and ChemistryC. Flux-Cored Tubular Wire 25

DesignationsD. Slag and Gas Formation 26E. Solidification of the Weld Puddle 27

T A B L E O F C O N T E N T S

Section/Topic Page

Section 6 28

Metal Transfer

A. Short-Arc 28B. Globular Transfer 30C. Spray Transfer 31D. Pulsed Spray Transfer 34

Section 7 35

Welding High Strength Steels

A. Select the Proper Filler Metal 36B. Minimize Hydrogen Contamination 37C. Control Heat Input 38D. Use the Correct Technique 40

Section 8 42

Technique and Equipment Set-Up

A. Torch Angle 42B. Feed Roll Tension 43C. Burnback 43D. Arc and Puddle Position 44E. Vertical Down Welding 44F. Gaps 45G. Crater Filling 46H. Arc Starting 46

Section 9 47

Weld Discontinuities and Defects

A. Lack of Fusion 47B. Porosity 48C. Burn-Through 49D. Undercut 50E. Spatter 51F. Cracking 51

Section 10 52

Conclusion

S E C T I O N 1

Introduction

1

Figure 1

Breakdown of

welding costs

This training program was written to giveyou a better understanding of the MIGwelding process. MIG is an acronym forMetal Inert Gas, which is not technicallycorrect for steels, because shielding gasesfor steels contain an active gas such asoxygen or carbon dioxide. The correct termaccording to the American Welding Society(AWS) is Gas Metal Arc Welding (GMAW).We will use the correct terminology asdefined by the AWS and also explain theslang used so that you will be familiar withall the terms applicable to this process.

As you learn more about GMAW, it willbecome apparent that this is a sophisticatedprocess. Welders that have used stickwelding (Shielded Metal Arc Welding orSMAW) are sometimes of the opinion thatthe GMAW process is simpler; but to deposita high quality bead requires as much know-ledge, or probably more, than the SMAWprocess. The reason for this is the number ofvariables that affect the arc and the degree ofcontrol the operator has over those variables.

The purpose of this manual is to make you abetter welder by increasing your knowledgeof how the GMAW process works. A moreknowledgeable welder can be more produc-tive by working smarter, not harder. Figure 1shows why your company is interested inyour education. Your labor and overheadaccount for about 85%of the cost of deposit-ing weld metal. Any knowledge you gainfrom this course not only helps you, but alsohelps to make your company more compe-titive in a very tough marketplace. If youshould have any question in the future thatthis manual or your supervisor cannot ans-wer, please free to have him contact yourPraxair regional engineering staff for furtherassistance.

Wire10%

Overheadand Labor85%

Gas5.0%

1

The two main categories of steel fabricatedtoday are carbon and alloy steels. Carbonsteels basically contain carbon and manga-nese. Alloy steels contain carbon, manga-nese, and a variety of other elements to givethe base metal the required properties. Alloysteels cover a wide range of materials, suchas the stainless and tool steels.

As weight becomes an issue in the transpor-tation industry, there has been a substitutionof high strength low alloy steels (HSLA) inapplications that previously used carbonsteels. The higher tensile and yield strengthsallow the cross-sectional area weight ofthe structural members to be reduced. Theresulting weight reduction allows better fueleconomy and a stiffer body in the case ofautomobiles, or an increase in payload inthe case of heavy trucks. Stainless and toolsteels are two more familiar categories ofalloy steels. This manual will concentrate onthe low alloys such as the constructionsteels, (A514, for example, referred to byUSX as their T1 alloys). In order to under-stand alloy steels, it helps to understand alittle bit about the metallurgy involved withthe elements added to increase the strengthof these materials.

Metals are crystals, which mean that theatoms are arranged in an ordered matrix. Aneasy way to visualize a metal is to think oflayers of balls with each ball in the layertouching its four neighbors (see figure 2).The balls represent the iron atoms of themetal. Carbon atoms are much smaller thanthe iron atoms, and they fit into the open

2

S E C T I O N 2

Base Metals

2

areas between the iron atoms. In carbonsteel, manganese is the other alloying ele-ment. In the matrix, a small portion of theiron atoms is replaced by manganese atoms.The layers above and below the first layerare arranged identically, except that they areshifted on a 45 degree angle to fall into theareas where the first layer of balls intersect.Now each ball in the second layer is touch-ing four balls in its layer and four balls inthe layers directly above and below it. Theorderly manner in which metals are arrangedin crystals is one of the reasons that they areso strong. When a metal yields, or deformsplastically, the planes of atoms slip in re-lation to the adjacent planes and at the grainboundaries. The only single crystal materialsused today are for turbine blades. They areextremely strong due to the orderliness ofthe matrix. They are also extremely expen-sive to make.

Figure 2 Iron body centered

Cubic Unit Cell

Unit cell(9 iron atoms)

The steels that are used in fabrication areactually made up of grains or groups ofcrystals. During welding, grains begin togrow into the molten puddle from the solidbase metal at the edge of the weld. Eachgrain continues to grow until it meets an-other grain. The area where they meet iscalled a grain boundary. In the steels ofmost interest, a lot of the slip or shiftingof atoms, occurs at these grain boundaries.Because of grain boundaries, the actualstrength of these steels is typically 25%to 50% of the theoretical strength of asingle crystal of iron.

When carbon is added to a steel, the strengthof the steel goes up dramatically. The car-bon forms a compound called iron carbide(Fe3C). The size of the iron carbide molecule(a molecule is a combination of atoms heldtogether to form a compound) is consider-ably larger than the surrounding iron atoms.When the atomic layers begin to slip, thelarger carbide molecule resists this slip bypinning the layers together due to theirdifference in size. There is a number of othercarbide forming elements that work in asimilar manner to strengthen alloy steels.

The following section lists some of theelements that are added to steel to yieldthe desired properties.

CarbonAs mentioned previously, carbon in steelforms iron carbide (Fe3C) in the matrix.The larger size of the carbide compound pinsthe layers within the metal matrix and makesit much more difficult for the material toyield. This raises the tensile, yield strength,and hardness of the steel considerably.

SiliconSilicon is added mainly as a deoxidizer. Itcombines with oxygen to form SiO2. Thissilicon dioxide, (also known as glass), floatsto the surface of the weld puddle in combi-nation with manganese oxide to form thebrown slag islands seen in the weld surface.Silicon can also be added as an alloyingelement; this is very beneficial in electricalsteels used in transformers.

ManganeseManganese is used in small amounts in moststeels to deoxidize, desulfurize, and improvematerial properties.

A.AlloyingElements

The manganese reacts with oxygen andforms manganese oxide (MnO). The man-ganese also combines with sulfur to formmanganese sulfide (MnS) (these are some-times known as stringers and can causewelding problems in certain steels). Thereason sulfur is detrimental is that it solidi-fies at a low temperature. The liquid sulfur iscarried to the grain boundaries and reducesthe strength of the weld. Any manganeseremaining after formation of MnO and MnSwill form manganese carbide (Mn3C) whichstrengthens and toughens the matrix. Aspecial category of steel containing greaterthan 10% manganese, is called Hadfieldsteels after the metallurgist that discoveredthem. Hadfield steels harden very quicklyto high hardness levels. They are also veryabrasion resistant. The most common appli-cation of these materials is in railroad cross-ings or frogs. Hadfield steels are also usedin rock crushers, dredging pumps, anddippers for power shovels.

3

4CopperCopper is added in very small amounts toincrease the hardness of the steel. Coppercan also be added to improve the corrosionresistance of weathering steels such asCor-Ten. These materials can be seen inthe unpainted condition on bridges, guardrails, and light poles. The copper aids in theformation of a very tight oxide that is not asprone to flaking as carbon/manganese steel;this slows the corrosion rate of the material.

MolybdenumMolybdenum is another carbide formingelement, and is added to most alloy steelsfrom .5% to 1.5%. Molybdenum improvesyield strength and resistance to high tem-perature creep (deformation due to hightemperature and stress). It also helps tomaintain the strength of the steel after itundergoes stress relief. Molybdenum isadded to stainless steels to reduce pitting(highly localized corrosion) in corrosiveenvironments.

ChromiumChromium is added to increase the strength,wear resistance, heat resistance, corrosionresistance, and hardness of steel. Chromiumis also a carbide former, and forms chrom-ium carbide (Cr3C). In steels, chromium canalso form complex carbides, Fe2CrC andCr2FeC.

Bearing steels typically contain about1% carbon and 1.5% chromium. Whenchromium levels rise to about 4%, andtungsten and molybdenum are added, toolsteels are the result. These materials aremade into the high speed cutting toolsused in machining.

Increasing the chromium level above12-13% causes the material to resist cor-rosion; these materials are known as stain-less steels. In chrome plating, a very thin,tight layer of chromium oxide is formed thatresists further oxidation. As the layer growsin thickness, the color begins to change to astraw color, and then to blue. This bluingcan be seen on motorcycle exhaust pipes.

NickelWhile nickel does not form any carbide ina steel matrix, hardenability, ductility andtoughness are all improved when nickel isadded. Nickel is added to austenitic stain-less steels (300 series) in concentrations of7-35%. Nickel is also an important elementfound in the materials used for storage ofcryogenic fluids. An alloy containing 9%nickel is used to fabricate the inner vesselin liquid nitrogen, argon, and oxygen tanks.

BoronBoron is added in very small amountsto increase the hardenability of steels.The amount added is typically below .01%.King pins, used on semi-trailers, are madefrom boron-containing steels.

VanadiumVanadium is a strong carbide former, andincreases the hardenability of the steel.Vanadium is expensive, so it is usually usedin percentages of less than .2%. This ele-ment reduces the grain size and increases thetoughness of the material. Vanadium steelsare used to make axles, connecting rods,hand tools, and engine crankshafts.

TitaniumTitanium is a strong carbide former that willalso form oxides and nitrides. A large use oftitanium is to stabilize certain grades of thestainless steels. Titanium combines with anycarbon in the matrix to form carbides beforechromium carbide precipitation can occur.When chromium forms carbides, the corro-sion resistance of the material will deterio-rate. Titanium also helps to reduce graingrowth in high strength steels, improvingstrength and toughness.

PhosphorusPhosphorus is generally considered an im-purity in steels; a maximum percentage isgenerally listed. Phosphorus tends to seg-regate forcing carbon into the surroundingmatrix. This can lead to brittle materials.

SulfurSulfur, also considered an impurity likephosphorus, is usually specified as a maxi-mum allowable concentration. Sulfur in thepuddle moves to the grain boundaries of thesolidifying weld metal because of its lowmelting temperature. This segregation in thegrain boundaries reduces the strength of thematerial.

Manganese is added to prevent this as itcombines with the sulfur (Mn + S = MnS)before it can react with iron. Certain steelscalled free machining steels, contain up to.3% sulfur; these alloys are difficult to weldand have poor strength characteristics.

With this greater understanding of the alloysthat are added to steel, lets look at carbonand alloy steels.

5

B.CarbonSteels

Carbon steels are categorized as low,medium, and high carbon. The mainalloying elements are carbon and manga-nese. Typical products made of thesethree 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 elementbecause it forms iron carbide (Fe3C).Manganese also forms carbides, but hasmuch less of an effect on strength andhardness. The following chart shows twocarbon steels from the low, medium, and

high carbon groups. Notice that the onlyreal change in most of the alloys is in carboncontent. At the right of the chart notice thatthe tensile strength rises rapidly as the car-bon level is increased with little or nochange in the manganese level.

SAE/AISI Carbon Manganese P (max) S (max) Tensile1008 .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

Table 1

Chemical

Compositions of

Carbon Steels and

Tensile Strengths

Alloy steels are generally classified byalloying additions. There are currently38 different classifications of steels thatare recognized by both organizations thatcategorize steels (AISI and SAE). Thefollowing table is included to give a betterunderstanding of how many steel alloys

there are. The carbon steels, discussedearlier, are in the first classification. Thereare 15 to 20 different carbon steels available,and there are 38 different classificationslisted. Table 2 gives specification numbersand alloy classification.

C.AlloySteels

6

Specification ClassificationNumber

10XX Carbon Steels11XX Carbon Steels, Resulfurized12XX Carbon Steels, Resulfurized and Rephosphorized13XX Manganese Steels2XXX Nickel Steels31XX Nickel-Chromium Steels33XX High Nickel-Chromium Steels40XX Carbon-Molybdenum Steels41XX Chromium-Molybdenum Steels43XX Chromium-Nickel-Molybdenum Steels46XX Nickel-Molybdenum Steels48XX High Nickel-Molybdenum Steels50XX Low Chromium Steels51XX Chromium Steels52XXX Carbon-Chromium Steels61XX Chromium-Vanadium Steels86XX Low Nickel-Chromium-Molybdenum Steels92XX Silicon-Manganese Spring Steels92XX Silicon-Manganese-Chromium Spring Steels93XX Nickel-Chromium-Molybdenum Steels98XX Nickel-Chromium-Molybdenum SteelsXXBXX Boron Containing SteelsXXBVXX Boron-Vanadium Containing SteelsWX Water-Hardening SteelsSX Shock-Resisting SteelsOX Oil-Hardening SteelsAX Air-Hardening SteelsDX High Carbon-High Chromium Tool SteelsHXX Hot Work Tool SteelsTX High Speed Tungsten Based Tool SteelsMX High Speed Molybdenum Based Tool SteelsLX Special Purpose Tool SteelsFX Carbon-Tungsten Tool SteelsPX Mold Steels2XX Chromium-Nickel-Manganese Stainless Steels3XX Chromium-Nickel Stainless Steels4XX Chromium-Stainless Steels5XX Low Chromium Heat Resisting Stainless Steels

Table 2

Alloy Steel

Classification and

Specification

Numbers

7

The power supply (or welding machine)connected to the torch is basically a bigtransformer/rectifier. Its purpose is to takehigh voltage (440v or 220v) and low current(20-50 amps/leg) AC power and transformit to low voltage (16-40v), high current(80-500 amp) DC power. To change ACto DC, a device called a rectifier is used.Direct current provides a much more stablearc. Most GMAW power supplies are set-upusing a reverse polarity connection. Reversepolarity is designated as DCEP, which meansDirect Current, Electrode Positive. An easyway to remember this is:

A.Power SupplyBasics

S E C T I O N 3

Electrical Characteristics

3

Figure 3

Typical GMAW

Power Supply

Congress is made of:

SENators And REPresentatives

Straight Electrode Negative

Reverse Electrode Positive

Almost all GMAW power supplies areconstant voltage machines, where StickElectrode (SMAW) machines areconstant current.

DCLow VoltageHigh Current

ACHigh VoltageLow Current

Work

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

+

8

All constant voltage power supplies haveat least two operator-adjustable settings:current and voltage. Current is set byadjusting the wire feed rate; voltage is setwith a voltage adjustment on the power

supply or on the remote control. Increasingwire feed increases current proportionally sothat enough current is available to melt thewire and deposit it in the weld pool. Voltageadjusts the length of the arc.

Some power supplies also provide the opt-ions of adjustable slope and inductance. Thepurpose of these controls will be discussedin the section on power supply characteris-tics. Figure 4 shows a power supply with thestandard voltage (arc length) and wire feedspeed (current) adjustments. This powersupply also allows the operator to changethe inductance and slope.

B.Constant VoltagePower SupplyControls

Figure 4

Power Supply

Adjustments

Electrical stick out (ESO) is the distancemeasured from the contact tip in the torch tothe workpiece as figure 5 shows. ESO is veryimportant, and can affect the following:

1. Electrode preheat2. Burning off of drawing lubricants3. Determination of current level

Figure 5

Measuring Electrical

Stick Out

C.ElectricalStick Out

V

V

A

I

A

+ Flat Steep

ESO

9

The wire in the GMAW process is calledan electrode because it conducts electricity.The current is transferred to the wire inthe contact tip. The energy resulting fromthe welding current is distributed to twodifferent places in the welding circuit;(1) resistance heating of the electrode, and(2) penetration into the base metal as figure6 shows. The electrode acts like the elementsin a home toaster. As current passes throughit, resistance heating occurs and its tempera-ture rises. The increased temperature burnsoff drawing lubricants used in the manufac-turing of the wire. The temperature rise alsohelps make it easier to melt the electrode.This is the reason that deposition rate incr-eases as ESO increases. As ESO increases,current is decreased. This also helps to keepthe contact tip cooler at higher deposition

rates. This is a big help in controlling pene-tration, because as current increases, so doesthe depth of penetration. By using a slightlylonger stickout, more weld metal can bedeposited without burning through thinnerparts. Increasing ESO makes the arc harderto start because less current is available atthe arc due to resistance heating. As moreresistance is put into the welding circuit(increased ESO), the effective slope of thesystem is also increased. This also tends toreduce the short-circuit current. ESO alsoaffects shielding gas coverage. As thedistance increases from the contact tip tothe work (also called TWD tip to workdistance), you reach a point where theshielding gas cannot effectively blanket themolten weld puddle. This will be coveredin more detail in the shielding gas section.

Figure 6

Current

Distribution

Current from the power supplyis distributed to:

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

10

D.Constant VoltagePower SupplyCharacteristics

1. Slope

The characteristics of a power supply aredetermined by the components used in itsdesign. The performance of a typical mach-ine is described by a graph such as figure 7.Most constant voltage power supplies with-out slope adjustment are factory preset atabout 2 volts/100 amps (flat slope).

This means that for each increase of100 amps, the power supply will produce2 volts less at the same voltage setting.The lower slope line is 6 volts/100 amps,and is about the maximum slope seen ina constant voltage power supply (steepslope). A few machines are still availablewith continuously adjustable slope; othershave external or internal taps to switchbetween slopes. Increasing the slope of apower supply to control short-arc welding atlow currents is necessary because the short-circuit current is limited. This reduces thetendency to burn-through on thinner materi-als and decreases spatter on arc starts. Thiswill be explained further in the section onmetal transfer.

Figure 8 shows a typical 2 volt/100 ampslope power supply characteristic curve.A review of this graph helps to explainwhy the arc changes during welding. Asan example, select a welding condition of27 volts and 250 amps. As welding contin-ues, if the stickout (ESO) is reduced thewelding conditions change. As that changeis made, the spatter level begins to increase.As ESO is decreased, less current goes intopreheating the wire and more goes into thearc. Suppose the current increases 50 amps,which is easily done with a small torchmovement of about 1/4". This moves theoperating point to the second point in figure8; here the voltage decreases to 26v whilethe current increases to 300 amps. Thisvoltage is at the minimum for spray transfer;this would account for the slight increasein spatter that is observed. This will beexplained in more detail in Section 6.

Figure 7 -

Power Supply

Characteristics

Curves

Figure 8

The Effect of Slope on

Current and Voltage

100 150 200 250 300 350

32

30

28

26

24

22

20

18

Current (Amps)

Volt

s CC-Drooping

CV-Flat Slope (2V/100A)CV-Steep Slope (6V/100A)

225 275250 300

22

21

20

19

18

17

Current (Amps)

Volt

s

Flat Slope2V/100A

11

Measuring Actual Welding VoltageMeasuring actual welding voltage is a use-ful way to be certain that the condition iswithin the range specified by the weldingprocedure. Hard starts can result from badconnections in the welding circuit. The vol-tage drop due to bad connections increasesthe slope of the system, and reduces theavailable short-circuit current. Comparingthe voltage at the power supply terminalsand between the feeder and the work (figure9) will give the voltage drop due to resis-tance. It can measure as high as 5 voltswhich makes starting the arc difficult.

12

2. Inductance

Inductance is an adjustment that is providedmore frequently than slope on CV powersupplies. Inductance is another method forcontrolling the arc. This is done by control-ling the rate at which the welding currentreaches the setting selected. Figure 10 showsa plot of inductance vs. time. The top curveshows what happens with no additional in-ductance and the current rises as quickly asthe power supply will allow it to rise. Thiscan result in very hot starts or even in thewire exploding at these very high currentlevels. Inductance should be kept low forspray transfer. This produces better arc start-ing and more stable arc at high currents.High inductance settings can make it hardto initiate the arc because it limits the maxi-mum short circuit current available for thispurpose.

Referring again to figure 7, as the electrodefirst touches the work to strike an arc, thevoltage falls from open circuit voltage(40-50v) to 0 arc volts. At 0 volts (no arclength or a dead short), the power supplyproduces the maximum short-circuit current.For a machine rated at 450 amps (at 100%duty cycle) this might be 550-600 amps. A600 amp machine might produce up to 800amps during this initial start. This is morethan enough current to explode the wire andmake the arc difficult to start. If the arc startis too hot, either the slope can be increasedor inductance added to reduce the current atarc initiation. Inductance can be beneficialwhen welding with low current short-arc, asit makes the puddle more fluid and allows itto better wet the base material.

Figure 9

Measuring Actual Welding Voltage

Figure 10

The Effect of Increasing Inductance

V

V

A

I

A

+ Flat Steep

+

27.5

Volt Meter

Work

Connect to+ terminal onfeeder and workpiece and readwhile welding

Least Inductance

More Inductance

Most Inductance

Time (milliseconds)

600

500

400

300Curr

ent

(am

ps)

12

3. Heat Input

A useful formula often used by weldingengineers is the heat input equation. Ifyou have been welding high strength orcorrosion resistant alloys, you may alreadybe familiar with this formula. It allows youto calculate the amount of heat deliveredto the workpiece.

The formula is:

Heat input is measured in units of energy/unit of length (joules/inch). A joule is a unitof energy equal to 1 watt of energy into theworkpiece per second. While a joule is not afamiliar unit of measurement, there are a lotof uses for the heat input formula itself.

For example, suppose that welding is doneat the second condition used as an examplein figure 8. That condition was 300 ampsand 26 volts. For this example, say the travelspeed is 10 in/min. This works out to a heatinput of 46,800 joules/in. If we know thatthe welding involves bridging a gap, ESOcan be increased to reduce current andincrease voltage while reaching the firstcondition indicated. That was 250 amps and27 volts, still at 10 in/min. The heat input isnow 40,500 joules/in. By increasing ESO(stickout), heat input has been reduced byalmost 15%.

The heat input formula is also used forwhen welding high alloy materials, but italso helps in understanding the uses of thepower supply characteristic curve.

The heat input formula can assist in pre-paring welding procedures where similarmaterial thicknesses and joint configurationsare welded. After determining the range ofheat inputs that produce an acceptable weld,the range of heat inputs can be calculated.This is really helpful in increasing speedsfor robotic welding. The required travelspeed can be calculated as wire feed speed(current) and voltage are increased. Theformula is also helpful in controlling dis-tortion. Conditions that put less energy intothe metal can be calculated, which reducesdistortion.

Heat Input = Amps X Volts X 60

Travel Speed (in/min)

13

At high temperature, all metals commonlyused for fabrication will oxidize in the pre-sence of the atmosphere. Every weldingprocess provides shielding from the atmo-sphere by some method. When weldingsteels, we want to exclude oxygen, nitro-gen, and moisture from the area above themolten puddle.

In the Oxy-fuel process, the weld poolis shielded from the atmosphere by thecombustion by-products of carbon mon-oxide (CO) and carbon dioxide (CO2). Instick welding (SMAW), CO and CO2 arealso the shielding gases. The 60XX type ofelectrode uses a cellulosic coating, which

has very high moisture content. The moistureproduces oxygen and hydrogen in the arcenvironment.

Submerged-arc welding shields the puddleby a different method. As the puddle pro-gresses, the intense heat melts the flux in thejoint area; this forms a slag that covers theweld and excludes the atmosphere.

GMAW (MIG) and GTAW (TIG) are bothgas shielded processes in which the shieldinggas is provided from an outside source. Nofluxing agents are included in the filler metalof solid wires.

S E C T I O N 4

Shielding Gases

4

A.Shielding GasFunctions

For the purpose of this discussion theGMAW process will be emphasized becauseit constitutes the greatest portion of weldingdone in industry. A good portion of thisinformation is applicable to GTAW too.

The major functions of a shieldinggas are to:

1. Protect the puddle from theatmosphere

2. Provide arc plasma3. Provide oxygen for wetting

(ferrous alloys)4. Control type of metal transfer5. Affect arc stability6. Control welding costs.

As was mentioned earlier, the atmospheremust be displaced while the puddle is cool-ing or oxidation will occur rapidly. Thisappears as a gray surface on the weld bead.One cause of porosity is the result of poorshielding when atmospheric oxygen com-bines with carbon in the puddle. As the weldmetal cools, porosity occurs as this carbonmonoxide escapes from the center of thebead. If air is aspirated into the shielding gasline through a leak, nitrogen and moisturewill also contaminate the shielding gas.Nitrogen, while very soluble in the puddle athigh temperatures, will cause porosity as itescapes during cooling of the weld bead.

14

The shielding gas also provides a portion ofthe arc plasma, which transfers the weldingcurrent across the gap between the electrodeand the work as figure 11 shows.

This is accomplished by ionizing the gas,which frees electrons to transfer the currentfrom the work to the electrode. Metallic andargon ions (atoms stripped of an electron)transfer the positive charge across the arc.This explains in part why the arc becomesvery unstable when a torch is hooked upusing straight polarity (DCEN) rather thanreverse polarity (DCEP). In DCEN, the posi-tive current is trying to remove iron atomsfrom the plate, which are much harder tomelt than a small diameter electrode.

In steels (carbon and stainless), oxygen sta-bilizes the arc and reduces the surface ten-sion of the weld metal. Oxygen is obtainedfrom direct additions of oxygen or carbondioxide to the shielding gas. Surface tension,the force that causes water to bead up ona waxed car surface, is not desirable whendepositing a weld bead. If pure argon is usedinstead of a mixed gas, the bead does notwet out and appears as though it is sittingon top of the part surface (convex bead).

Figure 12 shows the basic gases used inshielding GMAW. The ionization potentialis the amount of energy (in electron volts)required to establish an arc. The use of ahelium rich gas such as Praxairs HeliStar

A-1025 blend (for short-arc welding ofstainless steel), requires 3-5 volts more thanan Ar/CO2 mixture at the same current. Theshielding gas used also has a pronouncedeffect on the type of metal transfer obtained.

Figure 11

Transfer of Current

Across the Arc Plasma

Figure 12

Gases Used for

GMAW Shielding

Gas Characteristic Ionization

Potential (eV)

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

+

DCEP

eFe+

Ar+

15

There is a best gas for almost every app-lication, but there may be 2 or 3 gases thatwill do a very reasonable job. Gases areselected on the basis of performance, avai-lability, cost, and many other variables.Further discussion of gases and metaltransfer is found in section 6.

Stability of the arc plasma is another factorinfluenced by the shielding gas. Pure argonprovides a stable arc, and is used whenwelding reactive metals such as aluminum.

Argon/oxygen mixtures are also very stable,and are used in steel welding applications.Pure carbon dioxide provides a less stablearc plasma, but its additions to argon can bevery beneficial where depth and width ofpenetration need to be controlled. Someshielding gases also use additions of oxygenand carbon dioxide in one mixture (Praxair'sStargon gas blend).

Once the shielding gas is selected, it is cri-tical to make sure that the flow rate is with-in certain limits. For low current short-arcapplications, 25-35 scfh (standard cubic feetper hour) is adequate if ESO is held from3/8" to 1/2". For high current short-arc andthe spray transfer mode, flow rates need tobe increased to the 35-45 scfh range. Figure13 shows the best way to measure the flowrate using a torch flowmeter.

Regulator-flowmeters can vary the inletpressure to the flowmeter. As inlet pressurefalls when a cylinder gets low, the flow rateis actually skewed to a higher reading. Forexample, a regulator-flowmeter, installed ina low pressure line (20 psig), showed a flowreading of 70+ scfh, but the actual flow ratewas 15 scfh. The regulator had reduced thepressure in the flowmeter to about 5 psiginstead of the correct design pressure of 50psig. Flow rates must be kept in a controlledrange so that the shielding gas column doesnot become unstable and mixes with air atboth low and high flow rates or when forcedto flow past an obstruction in the nozzlesuch as spatter. This type of flow is calledturbulent (non-axial flow).

Figure 13

Measuring Actual

Flow Rate

B.Flow Rates

706050403020100

30 - 70 scfhmeasuredat the nozzle

16

Testing has shown that, the shielding gascolumn stays in laminar flow from 30 scfhup to about 70 scfh with a 400 amp gun(using a 5/8" diameter nozzle). Above70 scfh, the flow becomes turbulent andmixes with air. A cigarette in an ashtray canillustrate the difference between laminar andturbulent flow. The smoke initially leavesthe tip, in a tight orderly laminar flow. A fewinches above, the flow becomes turbulentand the smoke mixes with the air rapidly asshown in figure 14. This same thing happenswith a column of shielding gas.

The erratic quality of the shielding can pro-vide a weld that looks satisfactory, but cancontain subsurface (honeycomb) porosity.As deposition rates increase, a 35-45 scfhflow rate is still satisfactory unless there arebreezes or drafts. In high deposition MIGwelding, a .045 wire, can be run at 1300 ipm(35 lb/hr) at 50-60 scfh with no problems.Fans and drafts will displace a shielding gas,and may require increasing flow rates to50-70 scfh. Reducing cup-to-work distancecan also improve shielding.

Figure 14

Laminar and

Turbulent Flow

C.Gas Losses

Loss of shielding can cause problems inGMAW. Inadequate gas coverage can resultin an oxidized surface or porosity. The firststep in troubleshooting what you think maybe a gas loss is the use of a torch flowmeter.This flowmeter fits over the nozzle on atorch and measures the actual flow rate ofyour shielding gas (figure 13). Compare theactual reading with that of the station flow-meter (if used). The two readings should bevery close to each other. If not, there may besome potential problem areas.

Air

Air

LaminarFlow

TurbulentFlow

TurbulentFlow

LaminarFlow

17

Threaded Connections are notorious forleaking if not properly sealed. A pipelinewith shielding gas at 50 psi can leak a lotof expensive gas. It will also allow air todiffuse in, as figure 15 shows.

If the leak is large enough, no amount offlow can give a good quality weld. Use aliquid leak detector to find any leaks.

A few shops use quick disconnects fortheir shielding gasses. There are also primesuspects when investigating leaks. Use aliquid leak detector solution to check them.If the flow measurement at the torch indi-cates a much lower flow than the stationflowmeter, find the source of the problem

by disconnecting the supply hose at the wirefeeder. Use the torch flow meter to checkthe flow out of the hose. If the flow meter iscorrect, check if a leaking fitting or if theo-rings in the back of the liner have beendamaged (figure 16). These can be easilyreplaced, and should be lightly coated withsilicone grease to avoid damaging themduring reinstallation. Another place whereshielding gas flow can be disrupted is inthe diffuser. The gas diffuser is found at thepoint where the contact tip is mounted. Itspurpose is to distribute the gas evenly toproduce laminar flow out of the gas nozzle.If spatter builds up on the diffuser, it canclog it and reduce the gas flow enough toprovide poor shielding. If the diffuser is onlypartially blocked, the entire gas flow may tryto exit the holes still open and create unbal-anced turbulent flow. This in itself will as-pirate air into the shielding gas column andmay once again, cause porosity.

Figure 15

Back Diffusion Allows

Air to Leak into a

Pressurized Line

Figure 16

O-Rings Seal the Gas

Ports at the Feeder

Hose Wall

Hose Wall

ProcessGas

Air

N2, O2, H2O

CL

Wall

Wall

Distance

O2 C

oncenVelocity

N2, O2, H2O

Lube O-Ring with siliconegrease before installing

18

Holding the torch at too small an anglecan also create a venturi effect between theplate and the nozzle. This will also contami-nate the shielding gas stream with air andcause porosity (figure 17).

Some welders clean spatter from nozzles bylightly tapping the gas cup against somethingwhich knocks the spatter out. This can createa problem by eventually causing the cable togooseneck connection to loosen. When thishappens, it is possible to lose your gas cov-erage in the torch handle and aspirate air intothe shielding gas stream.

One final thing to check is incorrect inletpressure to the flowmeter. All flowmetersare calibrated for one specific inlet pressure,and the actual flow reading will be incorrectif the inlet pressure does not match the cali-bration pressure. Figure 18 shows what hap-pens when a 20 psig calibrated flowmeter isattached to a 50 psig line. The actual flow is26% higher than indicated on the flowmeter.The easiest way to check this is by using atorch flowmeter, because it is calibrated foratmospheric pressure at the outlet.

Penetration PatternsThe penetration pattern of a weld can be ex-amined by cutting and etching the weld asshown in figure 19. To examine a weld, cutthrough it at 90 to the face. The sample iseasiest to prepare when a saw is used. Thecut face is then sanded with progressivelyfiner sandpaper until a 320 grit paper isreached. An etchant reveals the penetrationpattern. For steels, there are many differentetchants available. For macro-etching, amixture of 10% nitric acid in methanolworks well. This etchant is called nital-10%.

Figure 17

Air Aspirated by the

Venturi Effect can

Contaminate the

Shielding Gas

Figure 18

Pressure Correction

Formula for Flowmeters

Figure 19

Weld Terminology

and Penetration

Measurement

Venturi effect pulls airinto the shielding gas

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

ActualFlow Rate

IndicatedFlow Rate

Actual Pressure (psia)

Calibration Pressure (psia)= x

ActualFlow Rate

40 scfh50 + 14.7

20 + 14.7= x

= 40 x 1.37 = 54.6 scfh

EffectiveThroat

Root

Toe

Face

19

When the etchant is swabbed on the weldarea, the different microstructures react atdifferent rates. The darkest area will be theheat-affected zone, or HAZ. This is basemetal that is adjacent to the weld metal. TheHAZ has been heated to near the meltingtemperature and then cooled very rapidly.

The cooling is caused by the quenchingeffect of the colder base metal. By measur-ing from the face of the weld to the root, wefind the effective throat of the fillet weld asshown in figure 19.

Different gases will change the shape anddepth of the penetration pattern. Oxygenadditions will reduce the diameter of theplasma, and lead to a deep and narrow pene-tration pattern as shown in figure 20. This issometimes called finger-like penetration.Additions of carbon dioxide and heliumincrease the diameter of the plasma becauseof their increased thermal conductivity. Thistends to decrease the effective throat andbroaden the penetration pattern.

Figure 21 shows the penetration patterns oftubular (flux-cored or metal cored) and solidwires at the same power (current and volt-age) level. The tubular wires are made with asolid sheath and filled with a powder; mostof the current is carried by the sheath. Thisincreases the diameter of the plasma, andmakes for a less penetrating or softer arc.Because of the axial metal transfer character-istics in spray transfer with solid wire, thepenetration pattern is narrower and deeperthan that of a tubular wire. Because the pow-er of the two arcs is the same in the example,the areas melted in the base metal are equal,but different in shape. In some welding app-lications where a solid wire produces burn-through on thinner parts, a tubular wire maybe a good choice because the larger diameterplasma spreads the heat over a larger areaand reduces the tendency to burn-through.

Figure 20

Different Gases

Provide Different

Penetration Patterns

Figure 21

Penetration Patterns

of Solid and Tubular

Wires in Spray Transfer

C-25 O-5 C-15/C-8

Equal Areas

Solid Tubular

20

21

Table 3

Shielding Gas Selection Guide for GMAW

Material Thickness Transfer Recommended DescriptionMode Shielding Gas

Carbon Steel Up to 14 gauge Short Circuiting StarGold C-8, Good penetration and distortion control to reduceC-15, C-25 potential burnthrough; good gap-bridging abilityStargon CS

14 gauge 1/8" Short Circuiting StarGold C-8, Higher deposition rates without burnthrough;C-15, C-25 minimum distortion and spatter; good puddleStargon CS control for out-of-position welding

Over 1/8" Short Circuiting StarGold C-15, High welding speeds; good penetration andC-25 puddle control; applicable for out-of-positionStargon CS weldsCO2

Globular StarGold C-8, Suitable for high current and high speed weldingC-25CO2

Short Circuiting StarGold C-50 Deep penetration; low spatter, high travel speeds;good out-of-position welding

Short Circuiting CO2 Deep penetration and fast travel speeds but withand Globular higher burnthrough potential; high current(Buried Arc) mechanized weldingSpray Arc StarGold O-1, Good arc stability; produces a more fluid puddle

O-2, O-5 as O2 increases; good coalescence and beadMig Mix Gold contour; good weld appearance and puddle controlStarGold C-10, C-15Stargon CSRoboStar CS

Short Circuiting StarGold C-5, Applicable to both short circuiting and sprayand Spray C-8, C-10 transfer modes; has wide welding current rangeTransfer Stargon CS and good arc performance; weld puddle has good

Mig Mix Gold control which results in improved weld contour

High Density Stargon CS Used for high deposition rate welding whereRotational StarGold C-8 15 to 30 lbs/hr is typical; special weldingTransfer HeliStar CS equipment and techniques are sometimes required

RoboStar CS to achieve these deposition levels

Gauge Pulsed Spray StarGold C-5 Used for both light gauge and out-of-positionStargon CS weldments; achieves good pulsed spray stabilityRobostar CS over a wide range of arc characteristics andHeliStar CS deposition ranges

Short Circuiting Stargon CS Good coalescence and bead contour withStarGold C-5 excellent mechanical propertiesHeliStar CS

22

Table 3

Shielding Gas Selection Guide for GMAW (continued)

Material Thickness Transfer Recommended DescriptionMode Shielding Gas

Alloy Steel Up to 3/32" Short Circuiting StarGold C-8, High welding speeds; good penetration andC-15 puddle control; applicable for out-of-positionStargon CS welds; suitable for high current and high speed

welding

Spray Arc StarGold O-5 Reduces undercutting; higher deposition rates(High Current Stargon CS and improved bead wetting; deep penetrationDensity and HeliStar CS and good mechanical propertiesRotational)

Over 3/32" Pulsed Spray StarGold O-2, Used for both light gauge and heavy out-of-C-5, C-8 position weldments; achieves good pulsed sprayStargon CS stability over a wide range of arc characteristics

and deposition ranges

Although a welder doesnt often get thechance to select a filler material, this sectionis included for information. Knowing howwires are alloyed and why, can sometimesbe helpful when a problem arises. We willconcentrate on a few carbon steel wires, be-cause they are used in the biggest portionof GMAW welding.

S E C T I O N 5

Electrodes

5

A.AlloyingAdditions

The list shown in figure 22 outlines theelements added to steel and the reasons fortheir addition. These are the same alloyingelements that are added to the base metals,proportions differ slightly for filler metals.

1. Carbon The addition of carbon to ironhas a very strong influence on its properties.Mild steels, 1010 and 1020 for instance,have low carbon contents (0.1 and 0.2%respectively.) Carbon is a very potentstrengthener, and when added above about0.3%, requires special welding proceduresto keep the material from cracking (preheat,interpass and post heat, etc.) Most commonwires are low in carbon content.

2. Manganese This element is added forthree reasons: (1) Deoxidation. Manganesecombines with oxygen in the weld metalbefore the carbon does so there is little orno oxidation of carbon in the weld puddleto produce carbon monoxide (and causeporosity. (2) Desulfurization. Manganesecombines with sulfur to form manganesesulfides before the sulfur can segregate tothe grain boundaries and form low meltingpoint iron sulfides. Iron sulfides can causehot cracking in steels. (3) Strengthening.Manganese remaining after these otherreactions combine with carbon to formmanganese carbides, which strengthen theweld deposit.

Figure 22

Steel Alloying

Additions

Figure 23

Deoxidation

Reactions

Steel is iron that is alloyed with

Carbon Strength

Manganese Deoxidation and Strength

Silicon Deoxidation

Aluminum Deoxidation

Zirconium Deoxidation

Titanium Deoxidation

Liquids

Gases

Si + 2O -> Si O2Mn + O -> MnO

C + O -> COC + 2O -> CO2

23

4. Aluminum The main function of thiselement is also deoxidation. It is a verystrong deoxidizer and forms aluminum oxide(Al2O3). A secondary function is that of agrain refinement, which produces a stronger,tougher deposit.

5. Zirconium This is also a deoxidizer,and is used in only a few wires.

6. Titanium This element is also adeoxidizer in low carbon steels.

3. Silicon This element is mainly added asa deoxidizer. Silicon combines vigorouslywith oxygen in the weld puddle and forms asilicon dioxide (see figure 23) slag. Beachsand is silicon dioxide. When the siliconcombines with the oxygen, heat is generatedbecause of this oxidation reaction. This isone reason why a wire higher in silicon willprovide a more fluid puddle. The brownglassy solid that forms on the weld depositis a combination of silicon dioxide andmanganese oxide.

B.1. Solid WireDesignationsand Chemistry

With some understanding of why alloyadditions are made to low carbon wires, thecompositions of some commonly used wiresare more meaningful. Figure 24 shows theAmerican Welding Society (AWS) nomen-clature used for solid wires. Table 4 showsthe chemistries of the available ER70S-electrodes.

Figure 24

AWS Solid Wire

Designation

Table 4

ER70S - Wire

Chemistries

AWS Carbon Manganese Silicon Phos. Sulfur OtherElectrode (max) (max)Class.

ER70S-2 .07 Max .9 - 1.4 .4 - .7 .025 .035 TI,ZR,AL

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 AL

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

ER70S-X

Electrode

70,000 UTS70 - 120 ksi

Chemistry2, 3, 4, 5, 6,7, 8, 9, 10, G

Rod Solid

24

These wire designations are set by the AWS,to standardize welding electrodes and fillermetals. The classification system is basedon chemical composition and strength of thedeposited weld metal. A typical solid wiredesignation would be ER70S-3. The E de-signates a wire can be used as an electrode,meaning it can carry current. The secondcharacter, R, indicates that this alloy isavailable as rod. Rods are usually the 36"straight lengths and are used for GTAW

(TIG). The third and fourth characters in-dicate the minimum tensile strength of theweld metal in thousand psis. An ER70S-Xwire would have a tensile strength of 70,000psi. The fifth character, S, indicates that thisis a solid wire. The number after the dashindicates the composition classification ofthe alloy. These numbers run from 2 to 7 andG. The G classification (stands for general)indicates a chemistry agreed upon by thesupplier and purchaser.

Since metal-cored wires perform like solidwires during welding, they have recentlybeen included for classification purposes inthe AWS specification for solid GMAWwires, but they follow a classification pro-cess more like a flux-cored wire than a solidwire. The two basic wire types are:

E70C-3X

E70C-6X

B.2. Metal-CoredWire Designationsand Chemistry

Where the C Indicates a cored wireand the X indicates the type of shieldinggas used in qualification of the wire(C=100% Carbon Dioxide, M=Mixed Gasof 75-80% Ar/balance CO2).

A second classification document coversflux cored and wires. An example of thedesignation for these wires is E70T-X(see figure 25). As with solid wires, Edesignates an electrode that carries current.

Notice there is no R, because straight lengthsof these wires are not usually practical.The second character is the tensile strengthmultiplied by 10,000 psi. The third characteris either a 0 or a 1. A 0 indicated thatthe electrode is for use only in the flat andhorizontal welding positions. A 1 indi-cated that the electrode is suitable for allposition work. The T in the fourth positiondesignates this as a tubular electrode. Thedigit after the dash indicates the type ofshielding required with this wire.

For Example, anE70T1 = 70,000 tensile, flat positionE71T1 = 70,000 tensile, all position

Figure 25

AWS Tubular Wire

Classification

C.Flux-CoredTubular WireDesignations

E70T-X

Electrode

70,000 UTS70 - 120 ksi70 = flat only71 = all position

Shielding Type1,4,5,6,7,8,11,G1,2,5 - with gas3,4,6,7,8, - w/o gas

Tubular

25

Deoxidation of the weld pool is very im-portant in metal joining. There are at least5 elements added for deoxidation, eachdoing a slightly different job. As a generalrule, the rustier or more mill-scaled a plateis, the more deoxidation required from theelectrode. The shielding gas is also a sourceof oxidation. If a rusty plate is welded witha gas of high oxidation potential, a cleanerdeposit would be obtained if a change from

D.Slag and GasFormation

an S-3 to an S-7, S-6 or even an S-2 solidelectrode is made. To determine the deoxi-dation potential of a flux-cored wire, themanufacturers literature must be consulted.Flux-cored wires contain increasing amountsof deoxidizers to remove the oxygen beingdeposited in the weld puddle by the shield-ing gas and by any mill scale or rust. Bothmill scale and rust are iron oxides (FeO-mill scale, and Fe2O3) .

Silicon and manganese are oxidized in themolten weld puddle. Silicon forms silicondioxide and manganese forms manganeseoxide. These reactions occur to remove theoxygen and keep the carbon and iron frombeing oxidized. Higher oxidizing potentialgases, such as pure carbon dioxide will oxi-dize more of the alloying additions and pro-duce a deposit of slightly lower strength andtoughness. The oxygen in the weld puddlewill also combine with the carbon to formcarbon monoxide if all of the manganeseand silicon have been oxidized. Porosity ina weld is usually caused by the evolution ofcarbon monoxide. The reactions that occurin the molten puddle are:

Si + O2 SiO22Mn + O2 2MnO2C + O2 2COC + O2 CO2

The SiO2 and the MnO are liquids that floatto the surface of the puddle. Upon cooling,they are commonly referred to as slag is-lands. Higher oxygen contents can removemuch of the silicon and manganese so thatthe oxygen will then begin to combine withcarbon. Carbon monoxide and carbon diox-ide result, and most of the gas formed iscarbon monoxide. The gases will evolvefrom the molten puddle, but if cooling israpid and the gas concentration is high gasbubbles will be trapped and create porosity.Nitrogen is also soluble in the puddle andwill cause porosity if shielding is inadequate.

26

Figure 26 shows how a weld puddle coolsand solidifies. At the weld puddle to basemetal interface, crystals begin to grow intothe molten weld pool. This is very similar tothe growth of ice crystals on a window seenin time-lapse photography. The crystals arecalled grains, and where they meet and stopgrowing is called the grain boundary. Asthe metal solidifies, the solubility of gasesdecreases greatly. If there is just slightlymore oxygen in the puddle than manganeseand silicon available for deoxidization

(and also possibly nitrogen), these gaseswill be pushed to the centerline as the puddlefreezes. This causes porosity along the soli-dification line and is known as centerlineporosity. Larger amounts of contaminationcan cause gross porosity in the weld andlead to the condition shown in the bottomillustration in figure 26.

As the weld is finished, the cooling rate atthe crater increases because the weld islosing heat in all directions. This rapidcooling rate leaves less time for the gasesto leave the puddle, and a hollow gas cavitycan form at the crater.

Calculation of Deposition RatesA very useful piece of information neededwhen calculating the cost of welding is thedeposition rate. The deposition rate is us-ually stated in pounds of wire per hour ofweld time. Figure 27 shows the multipliersthat can be used to determine depositionrate for different diameters of solid wires.The multiplier is a factor that takes intoaccount the cubic inches of wire per hourconsumed and the density of steel, to arriveat the rate in pounds per hour. To calculatethe deposition rate of an .045" diameterwire at 500 ipm wire feed speed, multiplythe 500 ipm times the .027 multiplier, anddetermine a rate of 13.5 lbs/hr of arc-ontime. To determine the actual amount ofmetal deposited, multiply this weight bythe duty cycle (% of an hour that the arc isactually on) and the deposition efficiencyof the process.

Figure 27

Calculating Deposition

Rates for Solid Electrodes

Wire Diameter Multiplier

.030 .012

.035 .0163

.045 .027

.052 .0361

.0625 .0521

Example: an .045 wire at 500 ipm500 ipm x .027 = 13.5 lbs/hour

Figure 26

Solidification of

a weld

E.Solidificationof the WeldPuddle

27

S E C T I O N 6

Metal Transfer

6

An understanding of metal transfer is veryhelpful when trying to solve a weldingproblem such as why is there so muchspatter?, or how can more penetration be

obtained. In this section, electrical charac-teristics, wires and shielding gases all cometogether. There are four major types of metaltransfer that will be discussed. They are:

1. Short-Arc

2. Globular Transfer

3. Spray Transfer

4. Pulsed Spray Transfer

Figure 28 shows the pinch effect. The pincheffect is a function of current, and tries topinch off the molten tip of the electrode.Higher currents and smaller areas increasethe pinch effect and give cleaner metaltransfer with less spatter.

A.Short Arc

Short-arc, or short-circuit transfer, is basi-cally a low heat input, low penetrationprocess. Currents range from 40-50 amps(.023" wire) up to 250-275 amps (0.052"diameter wire). Voltage ranges from 14-21v.

This process is a good choice on thinmaterial and sheet metal and has been usedextensively for out-of-position MIG weld-ing. Short-arc is also a good choice wherebridging gaps is a problem.

This form of metal transfer is called short-arc because the wire does electrically shortto the workpiece. When the wire touchesthe base material, the arc goes out, and thecurrent flowing through the wire begins torapidly raise the temperature of the wire.As seen in the power supply characteristiccurve, at 0 volts the power supply tries toproduce a maximum current output. Whenthe wire reaches its melting point, it flowsinto the puddle and the arc reignites. Thisshorting takes place very rapidly, from 60to 120 times per second. Figure 29 showsthe droplet of molten weld metal pinchingoff just before the arc reignites in the thirdillustration.

Figure 28

The Pinch Effect

Tries to Pinch Off

the End of the

Electrode

Figure 29

In Short-Arc Transfer,

the Electrode Shorts

60-120 Times Per

Second

P PP P

Short-Arc Globular Spray

The pinch effect is a function of current

Time (milliseconds)

Cu

rren

t Volt

age

28

After adjusting the wire feed to do the job,the voltage can be fine-tuned to where thesound from the arc becomes smooth andvery regular. Electrical stick-out must beclosely controlled as it has a great impacton current levels. At the low-end conditionof 40-50 amps and 14-15 volts with a .023"electrode, stick-out should be about 1/4".With a .035" electrode at 80-90 amps and15-16 volts, ESO should be about 3/8". Atthe high end of short-arc transfer with a.045" electrode, current will be at 225-235amps and 20-22 volts. Because of the highcurrent levels, we increase the ESO to pre-heat the electrode and reduce the current.The ESO with these conditions would be inthe 5/8"-3/4" range. This method gives avery controllable arc. If the arc is unstable,the usual cause is that the ESO is too longor voltage is too high. There should be verylittle spatter with this process, regardlessof shielding gas. Argon mixtures, however,provide smaller droplets with better gapbridging and arc stability as a result.

To adjust the power supply for short-arctransfer, two variables can help, if they areavailable. The slope on some machines isadjustable, either externally, or with internaltaps. Figure 7 shows that the steeper curvewill limit the maximum current that thepower supply can deliver, which is a realbenefit when short-arc welding at low cur-rents on thin materials. If the majority ofyour work is in the short-arc range, it isprobably worthwhile to change an internaltap if the machine has one. An internal tapisnt something you change for each job.

A power supply that has an inductancecontrol is easier to use for short-arc transfer.As shown in figure 10 adding inductanceslows the rate of current rise. With thisavailable, after wire feed and voltage areadjusted, inductance should be increased tothe point where the metal begins to transfersmoothly. By increasing the inductance,the current rises at a slower rate, and is at alower level when the cycle begins again thanit would be without the inductance in thecircuit. Limiting short current reduces ex-plosive, harsh metal transfer. Most short arcwelding is done with .023", .035" or .045"electrodes. Larger wires require too muchcurrent for most applications.

Gases that work well in short-arc spanfrom C-8 through C-25 mixtures (Praxair'sStarGold blends), straight CO2 andPraxair's Stargon CS gas blend. For lowcurrent applications carbon dioxide is some-times a good choice. The arc is hotter thanwith an argon mixture, and at low currentlevels, there is not much spatter. For in-creased deposition rates and travel speeds,C-25, C-15, or C-8 mixtures will usuallyprovide better results with decreased spatterlevels. Reducing the amount of carbondioxide makes the puddle less fluid andeasier to control. Burn-through is alsoreduced. On thinner materials, gases lowerin carbon dioxide content work best byminimizing burn-through and permittinghigher currents and travel speeds. Forwelding higher alloys, like stainless steel,helium is sometimes added to the shieldinggas to increase heat input at lowerdeposition rates.

29

Globular transfer is usually not the recom-mended way to deposit weld metal becauseof the inefficiency of the process. Thistype of transfer produces the most spatter.Depending on the current range, shieldinggas, and power supply settings, globulartransfer can waste 10-15% of the weld metalas spatter. Because of the inefficiency of theprocess, slower travel speeds or smaller beadsizes result at wire feeds comparable tospray or short-arc transfer.

When the tip of the wire begins to melt inglobular transfer as shown in figure 30, itonly shorts to the workpiece occasionallydue to higher voltages. The inconsistentcracks and pops you hear are the breakingof the short circuits. Unlike short-circuittransfer, an arc is present most of the time,and the metal begins to form a ball on theend of the wire. This ball is held by thesurface tension and the force of the arc.

The arc is continuously moving to the placewhere the glob of metal is closest to thework, where the minimum voltage is re-quired to sustain the arc. This creates theinstability that you see and hear in the arc.When the surface tension and the force ofthe arc are finally overcome by gravity theglob transfers. As the glob of metal hits thework, it tends to splash, throwing spatterout of the puddle onto the work.

Globular transfer occurs when voltages andcurrents exceed that of the short-arc range.Other than short-arc, this is the only type oftransfer you get with carbon dioxide in thecurrent range used in industry. If you areusing gas blends like Praxair's Stargon CSand StarGold (Ar/O2 or Ar/CO2), globulartransfer is what you get when voltage orcurrent falls below the spray transfer range.This is where spatter develops when a sprayarc mixture is used improperly.

B.GlobularTransfer

Figure 30

Globular Transfer

Produces High

Levels of Spatter

Unstable ArcHigh Spatter Levels

30

Spray transfer is a very clean, high efficiencyprocess. All wire diameters can be used. Formost applications in the 175 amp to 500 amprange, .035" to 1/16" wires work well. Whenthe welding equipment is set up properly,there is almost no spatter and 97-98% of thefiller weld is deposited in the weld puddle(deposition efficiency).

In spray transfer, the tip of the electrode be-comes pointed as figure 31 shows. Becausethe tip is so small, the current density (amps/square inch) and the pinch force are veryhigh. This pinches off metal droplets that aresmaller than the diameter of the wire. Thedroplets are accelerated by the magneticfield, around the arc instead of transferringby gravity as in globular transfer. The smalldroplets are absorbed into the weld poolrather than splashing.

Spray transfer can be used on materials asthin as 14 and 16 gauge metals with theright wire diameter (.023"). Thicker sectionwelding is where spray really gains an ad-vantage, especially in the flat and horizontalpositions. This type of metal transfer canbe used out of position but wire diametershould be smaller and the operating condi-tions less than in the flat position. All steels(carbon and stainless), and most other mater-ials, can be GMAW welded in spray transfer.

The gases used for spray are lower in activegases (CO2 and O2) than gases for short-arcand globular transfer. Most contain from85-90% argon, and some blends containboth carbon dioxide and oxygen. Some ofthe newer gases also contain small additionsof helium (Praxair's HeliStar gas blends)to increase the energy in the arc.

C.SprayTransfer

Figure 31

Spray Transfer

is a Very High

Efficiency Process

Droplets smallerthan diameter ofelectrode

Very low spatter

Minimum voltageand currentrequired

31

Transition Currents(Steel and Stainless Steel)To set a welding system for spray, there areminimum voltages and currents required.Voltages range from 24 v (small diameterwith Ar/O2) to 30 v (hi-deposition with He).A good place to start is around 26-27 volts.To estimate what the minimum transitioncurrent for spray transfer would be, multiplythe wire diameter (in thousandths of an inch)by 10,000 and divide by two as figure 32shows.

This approximation is accurate for a 95%Ar/5% O2 shielding gas. If you are using agas with 10% CO2, a closer approximationis made by adding the % CO2 to the transi-tion current calculated above and tabulatedin figure 33. For example, a .045" wirewith C-10 would produce spray transfer atapproximately 225 + 10 = 235 amps andabout 27 arc volts. Figures 34 and 35 showthe spray transfer ranges for 95% Ar/5% O2(figure 34) and the short-arc and spraytransfer regions for Ar/8% CO2 (figure 35).

Figure 32

Transition Currents

for 95% Ar/5% O2Shielding Gas

Figure 33

Transition Currents

with Various

Shielding Gases

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

.035 x 10,000 = 350/2 = 175 amps

.045 x 10,000 = 450/2 = 225 amps

.052 x 10,000 = 520/2 = 260 amps

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

32

Figure 34

Spray Transfer

Ranges for

95% Ar/5% O2.035" and .045"

Electrodes

Figure 35

Short-Arc and Spray

Transfer Ranges for

Ar/8% CO2 (C-8)

with 0.35" and 0.45"

Electrodes

100 200 300 400

36

32

28

24

22

18

.035

.045

Spat

ter

Spatter

Hiss

.035" and .045"wire withO-5 gas

Current

Volt

age

100 200 300 400

40

35

30

25

20

15

.035

.045

Spat

ter

Spatter

Hiss

.045" wirewith C-8 gas

Current

Volt

age

Short-arc

33

Pulsed spray transfer is a process that com-bines the lower heat inputs associated withshort-arc with the clean metal transfer andgood penetration associated with spray trans-fer. A graph of current vs. time (figure 36)shows the shape to be a square wave. Thecurrent at the top of the square wave is call-ed the peak current, and the current at thebottom of the square wave is called the back-ground current. The background currentkeeps the arc lit, but at very low currents typically 20-40 amp. When the current risesto the peak current, one droplet is transferredin spray transfer. Because of the small sizeof the droplet, spatter is minimized andpenetration is maximized due to the spraytransfer.

Due to its low heat input, pulsed spray isbeneficial for out of position work and forfilling gaps. Since it can produce high peakcurrents, a larger wire can usually be used atlower deposition rates. A larger wire (.045instead of .035) will usually reduce wirecosts and reduce wire feeding problems,especially for materials such as aluminum.

Recent research has shown that inverter pul-sed power supplies with very rapid currentrise can reduce the fume associated withhigher current GMAW welding. The fumingis caused by superheating the molten tip ofthe wire and causing the metal to boil. Thevery rapid current rise reduces the super-heating, leading to the reduced fume gene-ration rates.

D.Pulsed SprayTransfer

Figure 36

Pulsed Spray

Transfer Produces

Low Heat Inputs

With Very Clean

Transfer

140

275

20

Peak Current

Background Current

Time (milliseconds)

Cu

rren

t

Average Current

34

S E C T I O N 7

Welding of High Strength Steels

7

Higher tensile and yield strengths can beachieved by increasing carbon content, add-ing alloys, or a combination of both. In thesection on materials, it was seen that thereare hundreds of different steels availabletoday. US Steels T1 construction alloywill be used as an example of an alloy steel.Most of the major steel producers now makesimilar High Strength Low Alloy (HSLA)steels, which are designated A514 and A517grades B, Q, H, and F by the ASTM.

The ASME (American Society of Mech-anical Engineers) grades for these alloysteels are ASME SA517, grades B and F.

A comparison will be made between T1steel and a carbon steel of comparablestrength (SAE 1080) in table 5 to look atthe procedures required to weld the twodifferent materials. It is usually easier toweld an alloy steel than a carbon steel ofequivalent strength.

1080 T1 T1A T1B T1C

Tensile 112,000 110,000 110,000 110,000 110,000

Yield 61,000 100,000 100,000 100,000 100,000

Elongation 10% 18% 18% 18% 18%

% RA* 25% 40% 40% 40% 40%

Table 5

Mechanical Properties

Comparison of 1080

High Carbon Steel and

4 T1 Construction

Alloy Steels

The tensile strengths of the four materialsare all in the 110,000 to 112,000 psi range.Most products are designed using the mater-ials yield strength; in this case there is adramatic difference in the yield strengthsof the materials. Yield strength is wherethe material begins to plastically deform

(stretch), and the structural members wouldtake a permanent set instead of returning totheir original shape. Elongation and reduc-tion in area are measures of the ductility ofthe material. A ductile material will deforminstead of fracture under severe loading.

* Reduction of Area

35

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 .04Sulfur (max) .04 .04 .04 .04 .05Silicon .15 - .35 .2 - .35 .2 - .35 .15 - .35

Nickel .7 - 1.0 1.2 - 1.5

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

Titanium .01 - .03

Table 6

Comparison

of Chemical

Compositions of

1080 and T1 Alloys

of Equal Tensile

Strengths

Welding of high strength steels is differentthan welding low carbon steels. The weldermust pay a lot more attention to detail whenwelding high strength steels.

A.Select theProperFiller Metal

If standard T1 steel were to be welded thetensile strength of the material should beconsidered. From table 5 it is seen that thetensile strength is 110,000 psi. For theGMAW process, an ER110S-1 electrodecould be used. The 110 indicates a tensilestrength of 110,000 psi. Some manufacturersalso develop electrodes that do not exactly fitin the AWS classifications. These electrodescan also be used, but a welding engineershould look at the mechanical properties toensure compatibility. If the FCAW processwere to be used, an E110T5-K3 (Mn-Ni-Cr)or an E110T5-K4 (Mn-Ni-Cr-Mo) wouldbe selected according to the mechanicalproperties required.

36

Molten weld metal is capable of absorbingconsiderable amounts of hydrogen. Hydro-gen in weld metal causes two problems,porosity and cracking. Hydrogen can comefrom a variety of sources, and all of themcan cause problems. Hydrogen is normallyfound in nature as a diatomic molecule, H2.

In the arc, the hydrogen immediately disso-ciates to monatomic hydrogen (H), which isthe smallest atom known. A single hydrogenatom is about 1/100,000 the diameter of aniron atom. The hydrogen atoms are very sol-uble in the weld metal. Some of the hydro-gen sources are:

1. Grease, Oil, Oxidation or Paint on thePart or Electrode

Grease and oil are both hydrocarbons, andin the heat of the arc (about 10,000 oF), willrapidly dissociate to produce hydrogen, car-bon, and other contaminants in the puddle.The carbon can over-harden the weld metalwith possible carbon increases of .1 to .25%.Hydrogen goes into solution in the weldmetal. Some paints are also hydrocarbonbased, leading to the same problems as oiland grease. Mill scale and rust can also con-tain moisture. When the arc heats these ma-terials, the water molecules are dissociatedinto hydrogen and oxygen. Both of thesegases go into solution in the weld metal andcan cause weld problems such as porosityand cracking.

2. Excess Drawing Lubricantson the Electrode

Drawing lubricants on the wire electrode canalso contain hydrocarbons. When they areexposed to the arc heat, they dissociate andcontaminate the puddle.

3. MoistureMoisture can come from surface contami-nants such as rust and mill scale, and it canbe adsorbed on the surface of a clean sheetof steel. Bringing a piece of high strengthsteel inside in the winter will sometimescause moisture to condense on the surface.For T1, it is recommended that the materialtemperature be at least 70 oF before weldingbegins.

Moisture can also come from the shieldinggas if there is a leak in the supply line asdiscussed in the shielding gas section. Aircontains oxygen, nitrogen, and moisture, allof which are detrimental to the integrity ofthe weld metal.

As the puddle freezes, the solubility ofhydrogen in the puddle decreases. Thehydrogen tries to escape from the weldmetal by two different mechanisms.

Porosity FormationIf the concentration of hydrogen inthe matrix exceeds its solubility in thesolidified weld metal, the excess hydro-gen can form bubbles of hydrogen gas.These bubbles can show up as porosityin an x-ray. At very high concentrationsof hydrogen they can cause visible poro-sity at the face of the weld.

B.MinimizeHydrogenContamination

37

Slowly Diffuses Out of the Weld MetalAfter welding is completed, hydrogencontinues to rapidly diffuse out of theweld metal. In lab tests for measuringdiffusible hydrogen, the welded couponis quickly inserted in a bath of liquidnitrogen. The sample is then placed inmercury, and the quantity of hydrogenreleased is measured over time. If the

hydrogen content is high enough, thereare visible streams of bubbles rising fromthe coupon. The rest of the diffusible hy-drogen will escape within 20 to 30 days.The remaining hydrogen in the weld metalis called residual hydrogen, and it cancause cracking problems after welding.

During the welding process, the base metalin contact with the molten puddle was sub-jected to temperatures very close to themelting temperature of the material. In thatperiod, hydrogen atoms also diffuse into thebase metal in what is referred to as the heataffected zone (HAZ - see figure 37). Thesolubility of hydrogen increases with tem-perature even when the metal remains asolid.

The diffusion of the hydrogen into the basemetal in conjunction with the very rapidcooling rate can lead to underbead crackingor delayed brittle fracture.

C.ControlHeat Input

To understand the welding of the higherstrength steels, it is necessary to know a littleabout what happens when steels solidify. Asmolten metal cools, it undergoes a transfor-mation from a phase called austenite to anumber of different structures. A low carbonsteel like a 1008 grade changes from aus-tenite to ferrite, which is a very soft andductile microstructure. Adding slightly morecarbon and cooling more slowly forms amicrostructure called pearlite, which is fer-rite and cementite (Fe3C). When carbon andother alloying elements are added, the trans-formation from austenite is modified to formharder and sometimes more brittle micro-structures. The cooling rate of the weld

metal and the HAZ is very important indetermining the properties of the material.A familiar microstructure found in metalfiles is called martensite. Martensite is verystrong, with a tensile strength of 200,000 psior more, but the microstructure is very brit-tle. Martensite can be formed by very rapidcooling of the material. If a file is heated to1300 oF - 1400 oF and then cooled very slow-ly, a hole can be drilled in it. If it is then re-heated and quenched in oil, martensite willreform and the file can be used again. Thereis also an intermediate microstructure calledbainite, between pearlite and martensite,which has properties that fall between thetwo.

HAZ

38

Figure 37

Hydrogen is Driven Into the

HAZ by Stress Gradients

Preheat and Interpass Temperature (oF) 70 150 200 300 400

Thickness 3/16" 27 23 21 17 13

1/4" 36 32 29 24 19

1/2" 70 62 56 47 40

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

Metallurgists use charts called continuouscooling transformation (CCT) diagrams todetermine the cooling rates required to ob-tain certain microstructures. A CCT diagramtells what microstructure to expect whencooling occurs at different rates. At the bot-tom of the chart are three different scalesfor air, oil, and water quenching. Air cool-ing produces the slowest cooling rate witha water quench being the fastest. Weldingon a piece of base metal that has just beenbrought in the shop from the yard on a verycold day can produce some very rapidcooling rates.

The reason why a CCT diagram is mention-ed is that weld metal and the heat affectedzone (HAZ) also go through the same cool-ing process after welding. The HAZ is the

base metal right next to the weld metal thatalmost reached its melting temperature. Thecooling rate of the HAZ is much faster thanthe weld metal because the base metal isacting like a quenching medium. This is thereason that the heat input must be controlledduring the welding of the high strengthsteels. For T1, higher heat inputs lead tograin growth in the HAZ, and the strengthof the material is reduced.

To control the cooling rate, minimumpreheat and interpass temperatures arespecified to avoid a brittle crack sensitiveHAZ and weld metal.

Table 7 below shows the maximum heatinputs for T1 for different thicknesses andpreheat and interpass temperatures.

Table 7

Maximum Allowable

Heat Input for T1

Steel in Kilojoules

Per Inch of Weld

39

The thicker the material is the higher theallowable heat input. Some of the thickermaterials have no maximum heat input be-cause there is sufficient material to ensurea rapid cooling rate with currently availablewelding processes. To check interpass tem-perature, use a temperature indicating crayon1/2" to 1" away from the joint. It is possibleto contaminate the weld with the temperaturestick, and it is the temperature of the basemetal, not the previous pass, that isimportant.

If the preheat and interpass temperatures forT1 steel are compared with those for 1080, areal difference is seen. The 1080 steel has acarbon equivalent of up to 1.03%. The steelwith 1% carbon would require preheat andinterpass temperatures of between 600 oFand 800 oF depending on thickness. A postweld heat treatment at high temperaturewould also probably be required to preventcracking. Compared to the preheat require-ments (70 oF) in table 7, it is easy to seethat T1 steel is much easier to weld than theequivalent strength carbon steel. The alloy-ing additions in T1 steel undergo transforma-tion to carbides at higher temperatures thanthe carbon steels, which allows the greatlyreduced preheat and interpass temperatures.

For materials that require a slow cooling rateto avoid cracking, it is also very important tocontrol preheat and interpass temperature. Ahighly strained microstructure, such as thatcreated by rapid cooling, is very sensitive tohydrogen.

Any hydrogen in the weld metal will diffuseinto the highly strained microstructure andcould lead to underbead cracking. Hydrogenin a strained microstructure can also lead toa problem called delayed brittle fracture.Delayed brittle fracture is a weld defect thatdoes not show up immediately. The productcan be in service for a period of time, andthen fail at loads well below the yield str-ength of the material. Since the hydrogenatom is about 1/100,000 the size of an ironatom, it can diffuse through the metal easily.Stress gradients in the material cause the hy-drogen atom to migrate and concentrate inareas of high triaxial (3 dimensional) stress.In the matrix, these areas would be at the tipof a microcrack, a grain boundary imperfec-tion, or at the base of a surface imperfection,such as undercut. When the hydrogen con-centration reaches a critical level, the crackgrows slightly to relieve the stress. The dif-fusion mechanism begins again at the tip ofthe new crack, and the process repeats itself.As the crack continues to grow, the materialresisting the load decreases. At some point,rapid crack growth occurs, and there iscatastrophic failure.

40

The correct technique incorporates all of thetopics just mentioned, such as controllinghydrogen and watching heat input. With highstrength materials, it is also very importantto watch torch angle. The ideal fillet has aflat face and the toes blend into the side wallsmoothly. No undercut can be tolerated inhigh strength materials, because undercutacts just like a notch to greatly reduce thestress at which the component fails. A boltthat fails in the first thread illustrates howundercut can affect weld properties. Thethread, just like undercut, (called a stressriser) is basically the beginning of a crack.Undercut will also increase the strain in thematerial directly below it and make it moresensitive to any residual hydrogen that maybe in the material.

When beginning to weld, it is very goodpractice to use the back step technique. Theback step technique involves starting to weld1/4"-3/8" ahead of where the beginning ofthe weld is required, and then backing upto the beginning. The beginning of a weldusually cools too rapidly because the basemetal has not been preheated from the heatof the arc. Back stepping begins to preheatthe material, and greatly reduces the possi-bility of lack of fusion because the materialwhere you strike the arc will be re-melted.

Weaving is also not recommended on higherstrength materials. A stringer bead techniquegives better results, and helps control heatinput. If heat input were calculated using aweaving technique, it would probably bedifficult to control the maximum heat inputdue to the slow travel speeds associated withweaving.

D.Use theCorrectTechnique

41

Technique is very important when weldingany type of material, and gets more impor-tant as the material strength increases. Torchangle, feed roll tension, burnback, arc and

S E C T I O N 8

Technique and Equipment Set-Up

8

A.TorchAngle

There is a specific amount of energy avai-lable from the arc to heat and melt the basemetal. Torch angle plays a very importantrole in the shape of the bead and the depth ofpenetration into the base metal as figure 38shows. A leading (or push) angle will usesome of the arc energy to preheat the basemetal before it is welded. Because of the

elevated temperature of the base metal, thebead will cool more slowly. This allows theface of the weld to come to equilibrium andwill give a relatively flat face. If a lagging(or drag) angle is used, very little of the arcenergy goes into preheating the base metal,and deeper penetration is the result. Becauseof the lack of preheat, the bead will tend tobe convex (humped) because the weld willcool more quickly. If a line were drawn bet-ween the toes of the weld, the weld metalabove that line is wasted. If a fracture wereto occur, it would start at the toe of the weld,not through the thick section. In fatigue ser-vice, a humped bead actually reduces theservice life of the component. The decreasedreentry angle (angle that the bead face makeswith the base metal) tends to raise the stresslevel at the toe of the weld. A flat bead facedistributes the stress more evenly across thejoint. A slight drag angle does work well ina deep groove, and also in the first pass of amulti-pass weld. As a first pass, the benefitof increased penetration is obtained, but it iscritical to be sure to melt out the toes of thefirst pass when the cap passes are put in. Alack of fusion at the toe of the first pass is adefect that will reduce the service life of thecomponent.

puddle position, vertical down welds,gaps, crater filling, and arc starting willbe discussed here.

Figure 38

Torch Angle Affects

Penetration and

Bead Shape

Travel

Lead Angle (Push) Lag Angle (Pull)

42

Time

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

Wir

e f

eed

sp

eed

Motor upto speed

Motoroff

Gas oncontactorclosed feedmotor on

Gas off

Contactoroff

the wire to deform it between the feed rolls.Figure 39 shows the shape of the wire asit leaves a two U-grooved roller setup withexcessive pressure. The fin will be scrap-ed off as it feeds through the liner, and willmake it more difficult to feed the wire asthe excess material begins to clog the liner.Excessive pressure on the feed roll will wearout the feed rolls, plug up the liners, andusually lead to a burnbacks at the tip.

The best method of adjusting feed roll ten-sion involves running about a foot of wireout of the gun. Bend the wire 180o to forma curved end and run the wire into a glovedhand. Pull the trigger and slowly adjust thefeed roll pressure until the wire will makethe turn and feed smoothly. At this feed rollpressure, the wire will feed without deform-ing and the rolls will slip if you get a burn-back instead of bird nesting.

Setting feed roll tension is important toimproving consumable life and reducingdowntime due to feeding problems. Howmany times have you seen a welder have afeeding problem corrected by increasingfeed roll pressure? There are two main typesof feed roll designs used for solid wires, onegrooved and one flat roll, or two groovedrolls. Increasing feed roll tension on eitherdesign can actually put enough pressure on

B.Feed RollTension

Figure 39

Feed Roll Tension Adjustment

C.Burnback

The burnback control is designed to reducethe stick-out at the end of a weld and keepthe wire from sticking in the puddle. Whenwelding begins, the trigger on the gun clos-es the contacts in the welder, opens the gassolenoid, and starts the wire feed motor asshown in figure 40. When you release thetrigger at the end of the weld, the gas sole-noid and the contactor can react very quick-ly. The motor has the inertia of the armature,the gearbox, and the feed rolls to overcome,so it does not stop instantly. What a burn-back control does is put a timer in the circuitthat delays the opening of the contactor andthe closing of the gas solenoid. This allowsthe motor to spin down and the wire to con-tinue to burn off so you dont have to try tostart the next weld with too much stick-out.A short stick-out greatly improves the start-ing of the arc because more current isavailable.

Figure 40

The Burnback

Control Is Essentially

a Timer

Too much feed roll pressure deforms the wireand causes slivers to form

Slivers clog the liner and make the wireharder to feed

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

43

It is very important to watch the position ofyour puddle in relation to the arc. In orderto get good fusion into both pieces of basemetal, the heat from the arc must be direct-ed onto the base metal (figure 41). If a verylar