A Look at Heat Input

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A Look atInputWhat is Heat Input?In arc welding, energy istransferredfrom the welding electrode tothe basemetal by an electric arc.When thewelder starts the arc, both thebasemetal and the filler metal aremelted tocreate the weld. This meltingis possiblebecause a sufficient amount

ofpower(energy transferredper unittime)and energy density issupplied totheelectrode.Heat input is a relativemeasure of theenergy transferred per unitlength of

weld. It is an importantcharacteristicbecause, like preheat andinterpasstemperature, it influences thecoolingrate, which may affect themechanicalproperties and metallurgicalstructureof the weld and the HAZ (seeFigure1). Heat input is typicallycalculated

as the ratio of the power (i.e.,voltagex current) to the velocity ofthe heatsource (i.e., the arc) asfollows:

EI60

H S1000

where,H = heat input (kJ/in orkJ/mm)E = arc voltage (volts)I = current (amps)S = travel speed (in/min ormm/min)This equation is useful forcomparingdifferent welding proceduresfor agiven welding process.However, heatinput is not necessarilyapplicable forcomparing differentprocesses (e.g.,SMAW and GMAW), unlessadditionaldata are available such as

the heattransfer efficiency (Linnert,1994).Figure 1. Heat inputinfluences cooling rate.

How is Heat InputMeasured?Heat input can not bemeasureddirectly. It can, however, becalculatedfrom the measured values of

arc voltage,current and travelspeed.Arc VoltageIn determining the arc voltage(E), thevoltage should be measuredas closeto the arc as possible, asopposed tothe value displayed on theweldingmachine voltmeter.Measuring thevoltage across the arcprovides theactual voltage drop acrossthe weldingarc. The welding machinevoltmeterreading is always higher thanthe arc

voltage due to the resistanceof theWelding Innovation Vol. XVI, No.1, 1999welding cables (see Figure2). Themachine voltage, therefore,

can beused only for approximatecalculations

Heat input is arelative measure ofthe energytransferredduring weldingand, in the case of significantvoltagedrops, may lead to heat input

calculationerrors.CurrentThe welding current (I) ismeasuredwith either an inductancemeter (tongmeter) or a shunt withappropriatemetering equipment. Thecurrent isnever fixed with respect totime, especially

on a microsecond level.WithSMAW, the current is also afunctionof the arc length, which isdependenton the welder's skill.Therefore, thecurrent used in the heat inputcalculationsshould be the averagevalue.Travel SpeedThe travel speed (S) is theforwardvelocity of the arc measuredin eitherinches per minute ormillimeters perminute. Only the forwardprogress contributesto the travelspeed.


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If a weavingtechnique is used, only theforwardspeed counts, not theoscillation rate.For vertical welding, theupward ordownward speed of the arc isused. Thetravel speed must be in termsof minutesand not seconds for thedimensions tobalance in the heat inputequation.When the travel speed ismeasured,the arc should be establishedfor anamount of time that will

produce anaccurate average speed. Acontinuousweldingtime of 30 seconds issuggested.If this is not possible forthe production joint (e.g.,short welds),a test weld should be run ona mockup

joint that will providea sufficient

lengthto determinethe travelspeed.Thetravelspeed accuracywith manualor semi-automatic weldingis dependenton the welder.However, withautomatic welding, the speed

is set onthe motor controlled travelcarriage.Transient ValuesFor processes in which thevoltageand current vary significantlywith time,such as short-circuitingGMAW, the

average values of thesevariables areused in calculating the heatinput. Forexample, with GMAW-pulsedarc, thecurrent is pulsed at aspecified frequencyfrom a minimumvalue(backgroundcurrent) to the maximumvalue(peakcurrent).The average valuebetween the maximum andminimumcurrent and voltage willprovide an

approximate heat input valuefor thesewelding processes.With SMAW, the resistanceof theelectrode changes as it ismelted,which results in a voltagechange.The temperature of theelectrode also

The current

is never fixedwith respect to timeincreases while its length isreducedduring welding, both of whichinfluencethe overall resistance.Average valuesare used in this case as well.The transient nature of thesefactors isusually not considered whencalculating

heat input, and the averagesareadequateforprocedure qualification orsimplecomparisonof weldingprocedures.However, for scientific experi-

Figure 2. The arc voltage isalways lower than themachine voltage due to theresistance of the weldingcables.Welding Innovation Vol. XVI, No.1, 1999mentation of cooling rate andheatinput a more accurateanalysis proceduremaybe required, includinginstantaneouslymonitoringthe voltage,current and travelspeed to calculatethe actual heat input.

Weld Size is Relatedto Heat InputThe cross-sectional area of aweld isgenerally proportional to theamount ofheat input. This intuitivelymakessense, because as moreenergy issupplied to the arc, more fillermetaland base metal will be meltedper unitlength, resulting in a largerweld bead.If a welder makes one weldwith a fasttravel speed and another witha slowtravel speed, keeping currentand voltagethe same forboth, then the weldmadeat the slowertravel

speed willbelarger than the fasterone.The followingequation is an approximationforthe fillet weldleg size


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based onheatinput (Miller,1998):

500H

where,

= fillet weld leg size (in)H = heat input (kJ/in)Although the preciserelationshipbetween heat input and filletweld sizealso depends on othervariables,including the process andpolarity, this

equation is a helpful tool,especially increating and reviewingwelding procedures.For example, if a minimumfilletweld size is specified, thenthe correspondingminimumheat input can bedeterminedand controlled.

Cooling Rate is a

Function of HeatInputThe effect of heat input oncooling rateis similar to that of thepreheat temperature.As either the heat input or thepreheat temperatureincreases, the rateof cooling decreases for agiven basemetal thickness. These two

variablesinteract with others such asmaterialthickness, specific heat,density, andthermal conductivity toinfluence thecooling rate. The followingproportionalityfunction shows

this relationshipbetweenpreheat temperature,heatinputand cooling rate:where,R = cooling rate (F/sec orC/sec)T= preheat temperature (F orC)H = heat input (kJ/in orkJ/mm)o

R

HT

1o

The cooling rate is a primaryfactorthat determines the finalmetallurgicalstructure of the weld and heataffectedzone (HAZ), and is especiallyimportantwith heat-treated steels.Whenwelding quenched andtemperedsteels, for example, slow

cooling rates(resulting from high heatinputs) cansoften the material adjacentto theweld, reducing the load-carryingcapacity of the connection.

How Does HeatInput

Affect MechanicalProperties?Varying the heat inputtypically willaffect the material propertiesin theweld. The following tableshows howthe listed properties changewith

increasing heat input. Anarrow pointedup,, designates that thepropertyincreasesas heat input increases.Anarrow pointed down, ,designates thatthe property decreases asheat inputincreases. Next to the arrowis theapproximate amount thatpropertychanged from the minimumto maximumvalueof heat input tested.

Other than notch toughness,all of themechanical properties showa monotonicrelationship to heat input, thatis,themechanical propertyonly increasesor decreases with increasingheatinput.Notch toughness, however,

increases slightly and thendrops significantlyas heat input increases.Thechange in notch toughness isnot justTable 1. How MaterialProperties are Affected byIncreasing Heat Input forSMAWProperty* ChangeYield Strength 30%Tensile Strength 10%

Percent Elongation 10%Notch Toughness (CVN)10%, for 15 < H < 50 kJ/in50%, for 50 < H < 110 kJ/inHardness 10%* SMAW with a heat inputrange of 15 to 110 kJ/in.tied to the heat input, but isalso significantlyinfluenced by


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the weldbeadsize.As the bead size increases,which corresponds to ahigher heatinput, the notch toughnesstends todecrease. In multiple-passwelds, aportion of the previous weldpass isrefined, and the toughnessimproved,as the heat from each passtempersthe weld metal below it. If thebeadsare smaller, more grainrefinement

occurs, resulting in betternotch toughness,all other factorsbeing even.Tests have been conductedwithSMAW electrodes andprocedures thatprovided heat inputs varyingfrom 15kJ/in (0.6 kJ/mm) to 110 kJ/in(4.3kJ/mm) (Evans, 1997). This

representsa verylarge heat input range,whichencompasses mostapplicationsofSMAW.If the changes in heat inputare relativelysmall, as opposed to those oftheprevious

table,then the mechanicalpropertiesmaynot be significantlychanged.In another study, nosignificantcorrelation betweenheat input

andmechanical propertieswasestablishedforsubmerged arc welding(SAW)with typical highwaybridge

fabricationheat input levelsof 50 to 90kJ/in(Medlock,1998).In this case,the tests results did showvaryingproperties; however, no

discernabletrends were established.Welding Innovation Vol. XVI, No.1, 1999

Welding CodesAs discussed previously, heat

inputcan affect the mechanicalpropertiesand metallurgical structure inthe weldand HAZ of weldments. TheAWSWelding Codes have specificprovisionsrelated to heat input forthis veryreason.Below are the requirements

for heat input from AWS D1.1andD1.5.AWS D1.1 Structural WeldingCode SteelThe AWS D1.1 StructuralWeldingCode Steel controls heatinput in

three areas: (1) qualifiedWeldingProcedure Specifications, (2)minimumfillet weld sizes and (3)quenched andtempered steels.Qualified Welding ProcedureSpecifications (WPSs)When heat input control is acontractrequirement, and if theprocedureused in production has acorrespondingheat input that is 10% orgreaterthanthat recorded in theProcedure

QualificationRecord (PQR), then thequalifiedWPSmustbe requalified(AWSD1.1-98, Table4.5, item 18).Thisis primarilydue to concernsregarding

the potential alterationoftheweldmetal and HAZ mechanicalproperties.Minimum Fillet Weld SizesThe code also controls theheat inputby limiting the minimum sizeof filletwelds (AWS D1.1-98, Table5.8).

According to theCommentary, Fornon-low-hydrogen processes,theminimum size specified isintended toensure sufficient heat input toreducethe possibility of cracking ineither the


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heat-affected zone or weldmetal(AWS D1.1-98, para. C5.14).For multiple-passfillet welds,the Commentaryincludesthe following:Should fillet weld sizesgreater thanthe minimum sizes berequired forthese thicknesses, then eachindividualpass of multiple-passweldsmustrepresent the same heatinputper

inch of weldlength as providedbythe minimumfillet sizerequiredbyTable5.8.(AWS D1.1-98, para.C5.14).Quenched and TemperedSteels

When quenched andtempered steels(e.g., A514 and A517) are tobe welded,the heat input, as wellas minimumpreheat and maximuminterpasstemperatures,mustconformto thesteel

producer's specific writtenrecommendations(AWSD1.1-98, para.5.7).If high heat input welding isused, the HAZ can besignificantlyweakened due to hightemperatures

and slower cooling rates.However,the requirement does notuniversallyapply to all quenched andtemperedsteels. For example, withASTMA913 Grades 60 or 65, whicharequenched and self-tempered,theheat input limitations of AWSD1.1paragraph 5.7 do not apply(AWSD1.1-98, Table 3.1 and 3.2,footnote 9and 4, respectively).AWS D1.5 Bridge Welding

CodeThe AWS D1.5-96 BridgeWeldingCode has provisions for heatinput intwo areas: procedurequalification andfracture critical nonredundantmembers.Procedure QualificationThere are three differentmethods forqualifying procedures in

D1.5: theMaximum Heat Input Method,theMaximum-Minimum HeatInputMethod, and the ProductionProcedureMethod. For the MaximumHeat InputMethod, the heat input mustbebetween 60% and 100% ofthe value

from the ProcedureQualificationRecord (PQR) used to qualifytheWPS (AWS D1.5-96, para.5.12.1).With the Maximum-MinimumHeat

D1.1-98 controlsheatinput in three areasInput Method, the heat inputmust fall

between that of the tworequired qualificationtests.If the ProductionProcedure Method is used,the heatinput can only deviate fromthe PQRby the following: an increaseof up to10% or a decrease notgreater than30% (AWS D1.5, Table 5.3,item 17).

Fracture CriticalNonredundantMembersChapter 12 of D1.5 applies tofracturecritical nonredundantmembers(FCMs). The minimumpreheat temperaturefora FCM is selected basedonthe heat input, material

gradeandthickness,and filler metal diffusiblehydrogencontent (AWSD1.5, Tables12.3,12.4 and12.5).Although thefocus in chapter 12 of D1.5 isthe minimumpreheat temperature,

the heatinputvalueis an equally controllingvariable.Welding Innovation Vol. XVI, No.


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SummaryHeat input is a relativemeasure of theenergy transferred duringwelding. Itis a useful tool in evaluating

weldingprocedures within a givenprocess.The cooling rate, weld sizeand materialpropertiesmayall be influenced bytheheat input.Some welding codesplace specific controls on theheat

input. To ensure high qualityin weldedconstruction, it is important tounderstandand apply these principleswhennotchtoughness and HAZpropertiesareto be controlled and whenweldinghighalloysteels.

ReferencesANSI/AASHTO/AWS D1.5-96 BridgeWeldingCode. American Welding Society,1996.ANSI/AWS D1.1-98 StructuralWelding Code -Steel. American Welding Society,1998.Evans, G. M. and Bailey, N.Metallurgy of BasicWeld Metal. Abington Publishing,1997.Graville, B.A. The Principles of Cold

Cracking

Control in Welds. Dominion Bridge Company,1975.Linnert, G.E.Welding Metallurgy, V

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A Look at Heat Input