Materials Science and Technology 34 (2018) 519-528
Residualstresscontrolofmultipassweldsusinglow
transformationtemperaturefillers
R.J.Moat1,2,S.Ooi3,A.A.Shirzadi2,H.Dai1,A.F.Mark1,H.K.D.H.Bhadeshia3andP.J.
Withers4
1formallyatSchoolofMaterials,UniversityofManchester,Manchester,M139PL,UK2MaterialsEngineering,OpenUniversity,MiltonKeynes,MK126AA,UK3Departmentofmaterials,UniversityofCambridge,Cambridge,CB24MS,UK4SchoolofMaterials,UniversityofManchester,Manchester,M139PL,UK
Abstract
Ithasbeenshownpreviouslythatsmartlowtransformationtemperature(LTT)
weldfillerscanbeusedtocontrolresidualstressesanddistortioninsinglepass
weldsinausteniticplates,leavingtheweldinastateoflow,orevencompressive,
residualstress.Howeverinasinglepassweldthefillerisexposedtojustone
thermalexcursion.Bycontrast,weldfillersinmultipassweldsexperiencea
numberofthermalexcursionsassuccessivepassesarelaiddownwhichmeans
thatthebenefitofthesmartLTTfillerisnotrealised.Hereneutrondiffraction
andthecontourmethodareusedtomeasuretheresidualstressinan8pass
grooveweldofa304LstainlesssteelplateusingtheexperimentalLTTfiller
Camalloy4.Thestressmeasurementsshowthatthestressmitigatingeffectof
Camalloy4observedforsinglepassweldsisdiminishedbysuccessivethermal
excursionsduringmultipasswelding.Consequently,theuseofanelevated
temperatureinterpassholdtemperatureisproposedtoensuretheweld
transformationcansuccessfullymitigateresidualtensilestresses.Interpasshold
temperaturesof50°Cand200°Careinvestigatedexperimentallyalongsidefinite
elementmodellingtoidentifytheoptimumholdtemperature.Theseresults
showthatwhiletheinterpasstemperaturedoesn’taffectthefinaltransformed
fraction,aninterpasstemperatureof200°Cissuccessfulinintroducing
compressiveresidualstressesthroughoutthefusionzoneifsomewhathigher
tensilestressesthanariseforlowinterpasstemperatures,orwhenusingnon
transformingfillers.
Materials Science and Technology 34 (2018) 519-528
Martensitictransformation,gasmetalarcwelding,displacivetransformation,
lowstresslowdistortionwelding.
Introduction
Tensileresidualstressesthatformasaresultofweldingcanhaveadeleterious
effectonthestructuralintegrity,leadingtocomponentdistortion,mechanical
underperformance,orevencatastrophicfailures[1].Suchweldresidualstresses
canbeespeciallyproblematicforthenuclearpowerindustry[2],becausethere
aremanythickandlargeweldedsectionswhichcannotbeheattreatedto
reduceresidualstresses.Further,inpressurisedwaterreactor(PWR)
assembliessuchweldscanexperienceoperatingconditionswhichincreasetheir
susceptibilityto“environmentallyassistedcracking”(EAC)orcreepcavitation
[3]bothofwhicharestronglyaffectedbyresidualstresses.Thismakescontrol
andmitigationofresidualstressinweldedjointsanimportantfeatureofmodern
welddesign.
Anelegantandefficientmethodofresidualstressmitigation,exploiting
transformationplasticityassociatedwiththemartensitictransformationin
ferriticsteel,wasshownbyWangetal.[4]andOhtaetal.[5]tobehighly
effective.Thisapproachrequiresasufficientlylowmartensitestarttemperature,
Ms,ofthefillermaterial..Howevertheseearlystudiestendedtorelyonthe
martensitictransformationresultinginweldswithimpracticallylowtoughness.
Bhadeshia,Withersandco-workersfocusedonthedesignoflowtransformation
temperature(Ms)weldfillerswithhighertoughness,bothforstainlesssteels[6]
andferriticsteels[7–9]Inthesestudies,alloysspeciallydesignedtohavelowMs
andgoodmechanicalpropertieswereselected,forwhichsuitablealloydesign
processesarereportedin[6].Theabilitytocontrolthestateofresidualstress
usingsuchalloyshasbeendemonstratedbyDaieta.l[10],Murakawaetal[11],
Moatetal.[12],Thibaultetal.[13]andRamjaunetal.[8,9,14]whousedneutron
diffractionand/orthecontourmethodtoquantifytheresidualstresses.Further,
thesestresseshavebeenwellexplainedbyfiniteelementmodelsthatcapture
thetransformationstrainalongwithassociatedtransformationplasticity
[15,16].
Materials Science and Technology 34 (2018) 519-528
ThelowtransformationtemperaturefillerreportedbyShirzadietal.in[6]
namedCamalloy4,isamartensiticstainlesssteel.AsisevidentfromtheSatoh
testshowninFigure1ithasamartensitestarttemperature,Ms,of
approximately200°C.This,alongsidegoodmechanicalproperties,makesitwell
suitedasastressmitigatingfillermaterialfortheweldingofausteniticstainless
steel;givinghighstrength,hightoughessandlowdistortionwelds.Todateonly
singlepassweldsofCamalloy4havebeenexaminedwhichhasbeenreportedin
[12].
Sincesinglepassweldsundergoonlyonethermalexcursionthetransformation
uponcoolingisrelativelyeasytocontrol.Bycontrasttheweldfillerinmultipass
weldsexperiencesseveralthermalexcursionsofdecreasingintensityas
subsequentweldpassesareapplied.Thisopensupthepossibilityoftensile
residualstressesbeinggeneratedinamultipasswelduponsubsequent
reheatingcyclesthataresufficienttocausere-austenisationofthealready
depositedlayersbutarenotcooledsufficientlytoregeneratemartensitic
structure.Inthispapercontrolofinterpasstemperatureisinvestigatedasa
meansofcontrollingthetimingandextentofmartensitictransformationsoasto
generatetherequiredcompressiveweldresidualstresses.Weldingwas
performedusingtwodifferentinterpasstemperatures,50°Cand200°C(closeto
Ms)todeterminetheeffectofinterpasstemperatureonthefinalstressstate.
Previousmultipassweldmodelling[16]andresidualstressmeasurementshave
suggestedthataninterpasstemperatureof200°Cshouldbeenoughto
significantlydelaythetransformationtogivesignificantcompressiveweld
stresses.Asabenchmarktodemonstratetheeffectofphasetransformation,a
thirdweldwasproducedusingastandard,commerciallyavailablenon-
transforming,austeniticfillerAutorod308LSi.
Experimental
Materialandweldpreparation
Fouridenticalplates(200×150×20mm)werepreparedfromconventional
austeniticstainlesssteel(304L).Ahalf-thicknessdeep,V-groovewasmachined
alongthecentrelineofeachplate,withanenclosedangleof60°.Thethermal
Materials Science and Technology 34 (2018) 519-528
historywasfollowedduringweldingbythermocouplesspot-weldedontothe
surfaceoftheplatenearthegroove.Thepreparationoftheplateforwelding,the
orderofweldbeadsandit’srepresentationinFEareshowninFigure2.The
platewasrestrainedbyfilletweldstotheworkbench(tackwelded)onallsides
oftheplate.
Twofillermetalswerechosen;thefirst,CamAlloy4,designedtohavecorrosion
resistanceaswellasalowtransformationtemperature[6];thesecond,Autorod
308LSi,whichisacommerciallyavailablenon-transformingausteniticfiller
metalcommonlyusedforwelding304Lsteel.Thechemicalcompositionsof
CamAlloy4,Autorod308LSiand304LbasemetalarelistedinTable1.
Theweldtrialswerecarriedoutusinggasmetalarcweldingwithashieldinggas
of98%Ar-2%CO2.Theweldingcurrentandvoltagewere230-295Aand26-30V,
respectively,andtheweldingspeedwasbetween65-78mmmin-1.Eightlayers
offillermetalweredepositedoneachplateusingtheinterpasshold
temperaturesgiveninTable1.PlateT50andplateT200areproducedto
investigatetheeffectofinter-passtemperature;whileplateA50servesasa
referencetocurrentindustrypractice.
Metallographicandmicrohardnessexamination
A10mmthick,crosssectionalslicewastakenfromthecentreoftheweld,after
thecross-weldcutforthecontourmethodwasmadeandthesurfacehadbeen
profiled.Thecrosssectionalsurfacewasgroundflatandpolishedusingstandard
metallographictechniques.Thepolishedsurfacewaseletrolyticallyetchedusing
Fry’sreagentandimagedusingopticalmicroscopy.Becausethebasemetalis
anodicwithrespecttotheweld,itwasfirstcoveredwithnailpolish,whichwas
removedusingacetoneafteretching.Theinitialexaminationsshowedtheweld
dilutionextendedto5mmoutsidetheoriginalweldgrooveinalldirections.
Vickershardnessmappingwasalsocarriedoutonthetransversesectionofthe
asweldedplateswith1mmby2mmgridspacingusinganappliedloadof2kg.
Metallographyandhardnessmeasurementsareonlypresentedformartensitic
welds(plateT50andplateT200).
Materials Science and Technology 34 (2018) 519-528
Neutrondiffractionstressmeasurement
Residualstressesineachsampleplatewerefirstcharacterised,non
destructively,byneutrondiffractionmethodusingtheEngin-Xbeamlineatthe
ISISfacility,Oxfordshire,UK.Thelatticeparametervariationsineachofthe
phasesweremeasuredinthreeorthogonaldirectionsusingthe“timeofflight
method”withfullReitveldstylerefinement[17].Asaresultofthedual-detector
systememployedatEngin-X,thethreeorthogonaldirectionscouldbeobtained
using2sampleorientations.Asetof3mmradialcollimatorswereemployedto
giveanominalgaugevolumeof3x3x3mm[18].Whenthesamplewasoriented
suchthattheweldingdirectionwasvertical,theverticalslitswereopenedto
10mmtoreducecountingtimes,butwithoutreducingthespatialresolutionin
thenormalandtransversedirections.Strainwascalculatedusing:
ε = !!"#!!!!"#
!!!"# (1)
whereaxyzistherefinedlatticeparameterataparticulargaugelocation(x,y,z)
anda0x,y,zisthelatticeparametercorrespondingtoastress-freeconditionatthe
samplelocation(x,y,z).Stress-freesampleswerecreatedusingcomb-like
samplecutfromaregiontowardstheendsoftheweldedplates.Thecomb-like
samplesallowedstressfreelatticeparametermeasurementsatthesame
distancesfromtheweldcentrewheretheresidualstressesaremeasured(i.e.
eachtoothonthecomb-likesamplecorrespondstoacertaingaugelocation)
[19,20].Giventhesizeofeachsampleplate,cuttingathincomb-likesamplefrom
theendofeachplatedoesnotsignificantlyaltertheresidualstressprofileinthe
centreoftheplates.Stresswascalculatedfromtheelasticstrainmeasurements
using:
σ!! =! !!! !!!!! !!!!!!!
!!! !!!" (2)
whereσiiandεiiarethenormalstressandstrainrespectivelyfortherespectivei-
direction.Inviewofthefactthatthewholediffractionprofileisrefinedthebulk
Young’smodulus,E,andPossion’sratio,ν,areemployedratherthandiffraction
elasticconstants[21].
Materials Science and Technology 34 (2018) 519-528
Contourmethodofstressmeasurement
Uponcompletionoftheneutrondiffractionmeasurements,theplateswere
sectionedalongthenormal-transverseplaneatthemid-pointalongtheweld
beads.ThesectioningwasperformedusingWire-Electro-Discharge-Machining
(WEDM),completedinasinglepassusingskimcutsettings.Duringcuttingthe
plateswerekeptclampedtotheworktabletopreventanyrigidbodymovement.
ThesurfacedisplacementsweremappedusingaNanofocus,Microscan,confocal
laserprofilometer.Thedatawerecleanedandsmoothenedtoremoveoutliers
andnoiseandthenfedintoanFEmodel.Thestressrequiredtocausesuch
surfacedisplacementswasdeterminedandextractedacrossthesurfaceofthe
cuttorevealthestresspresentedinthesamplepriortosectioning.
Thecontourmethodcanbeaffectedbyartefactsresultingfromthecutting
process[22,23].Theseareoftenonlysignificantinthefirstmillimetreorsofrom
afreesurface.Becausethebulkstressesclosetothewelddeposit(farfromthe
plateedges)areourprimaryinterest,noattemptwasmadetocorrectforthe
surfacemeasurementsandthesedatawereremovedfromthecalculation.
Finiteelementmodelling
Inthiswork,SYSWELDFEcode[25]isusedasitisoneofafewspecifically
designedcodestoincludetransformationplasticity(GreenwoodandJohnson
mechanism).A2Dmodelwasbuilttoinvestigatetheeffectofinterpass
temperaturewhichwasverifiedbysome3Dmodellingpreviouslyreported[16].
ThemechanicalpropertiesofCamalloy4weredeterminedasafunctionof
temperatureusinganelectro-thermomechanicaltesting(ETMT)machine
coupledwithadigitalimagecorrelation(DIC)systemforsurfacestrain
measurement.Moredetailsofthemechanicalpropertiesofthetestedalloycan
befoundin[15,24].
Definingthethermalmodel
Eachweldbead(pass)isconsideredtobedepositedasablock(sometimescalled
block-dumping),withathermalcycleimposedonthewholepass[25].Theheat
powerdensityfunction,asshownineq.(3),isdescribedmathematicallyby
Materials Science and Technology 34 (2018) 519-528
empiricalparametersthatneedtobecalibratedbeforetheheatsourcecanbe
applied.Inallcasesanarcefficiencyof0.8wasassumedforeachweldingpass.
Thecalibrationwascarriedoutbyseekingthebestpossiblereproductionofthe
fusionzone(FZ)andheataffectedzone(HAZ)inthefirstpass.TheFZ/HAZ
boundariesarenormallyestimatedbyplottingthemaximumtemperature
envelopeonacontourplotfocusingonacontourvalueof1500°CfortheFZand
aminimumcontourvalueof850°CfortheHAZ.
(3)
Anaveragedthermalcycleforthewholeweldbeadwasthenexportedand
compiledfortheuseofthemulti-passweldingsimulations.Theweldingspeed
forPass8was1.08mms-1comparedto1.3mms-1forallotherpasses.
ConsequentlyadifferentsetofparameterswerefittedforPass8usingaseparate
2Dtransientweldinganalysis.
Thethermalpropertiesusedinthisworkaregivenelsewhere[16].The
predicted1500°Cpeaktemperaturecontourcorrespondingtothefusionzoneis
comparedwiththemacrostructureoftheweldcross-sectioninFigure2.
Considerablewelddilutionisobservedinthefusionzoneextendingwellbeyond
thefootprintoftheoriginalgroove,butstillcomparablewiththeoptical
micrographs.
Inordertoidentifytheoptimuminterpassholdtemperature,fourdifferent
interpasstemperatures,50,100,150and200°C,aremodelled(denotedasModel
T50,ModelT100,ModelT150andModelT200).
ModellingtheMartensiticTransformation
TheextentofthemartensitetransformationcanbedescribedbytheKoistinen-
Marburgerrelationship[26]:
forT<Ms (4)
whereMs=214°Candbisafitparameter(=0.014K-1inthisstudy).
( )
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛++−=
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⎟⎟⎠
⎞⎜⎜⎝
⎛++−=
2
2
2
2
2
2
2
2
2
2
2
2
exp,,
exp,,
cz
by
axQzyxQ
cz
by
axQzyxQ
rr
ff
( ) 1 exp( ( ))sX T b M T= − − −
Materials Science and Technology 34 (2018) 519-528
ThedetailsofFEmeshingusedinthe2Danalysisofmultipasswelding,isshown
inFig.2c.Thestainless318Lplatecannottransformthroughouttheentire
weldingprocessanditisassumedthatthefirstbeadstaysmostlyausteniticinall
casesbecauseofdilution.Tomodelthedilutioneffect,asimpleapproximationis
appliedinwhichtheinitialphaseofthefirstbeadisassumedtobe50%
austeniticplus50%martensitic,whilethesecondtotheeighthbeadsareableto
fullytransformmartensitically.Iftheamountoftheretainedaustenitecanbe
measuredoraccuratelyestimated,abetterapproximationcouldbemadeby
calibratingtheK-McoefficientsMsandb,orthroughtheuseofaprogressive
transitionzone.Nevertheless,thecurrentassumptionshouldnotaffectourmain
focusontheeffectoftheinterpassholdingtemperatureofresidualstresses.
Results
VickersMicrohardness
Thehardness(HV2)mapsareshowninFigure3fromwhichthepresenceof
martensiticweldmetalisevidentforbothplatesT50andT200fromthehigh
hardnessvalues(>250HV2)comparedtothatoftheparentausteniticregions
(<200HV2).Itisalsoobservedthatthehardnessofthefirstweldbeadofboth
testedplatesisbetweenof200-250HV2similartothehardnessobtainedinthe
heataffectedzone(HAZ)forbothplatessuggestingthemicrostructureofthe
firstbeadissignificantlyausteniticratherthan100%martensitic.Itisalso
evidentthatthehardnessplateauoftheHAZforthehigherinterpass
temperature(200°C)iswiderthanforthelowerinterpasstemperature(50°C).
Fortheplateweldedusinga200°Cinterpasstemperature(plateT200),the
resultshowsthatexceptforthefirstweldbead,thereisnomajorvariationinthe
hardnessvaluesofthedifferentpasses.However,forplateT50,ahigher
hardnesswasobtainedinthecentralpasses(beads2,3,4and5).Thisis
presumablyduetoaccumulatedplasticstraininthemartensitecausedby
subsequentpasses,althoughfurtherworkwouldberequiredtoinvestigatethis
relationship.
Materials Science and Technology 34 (2018) 519-528
Phasefraction
Thevariationinphasefractionofmartensitemeasuredattheweldcentrelineis
presentedinFigure4.TheseresultsarecalculatedusingASTMstandardfor
retainedaustenitedeterminationfromtheneutrondiffractionprofiles.As
expectednomartensiteisobservedforthenon-transformingausteniticfiller
(AutorodOK308LSi)nortowardsthebottom(~8mm)oftheplates.Thisis
despitethepresenceofthefusionzoneextendingbeyondthislocation.
Martensiteisfirstdetectedintheneutronmeasurementswiththegaugecentre
10mmfromthetopsurface,steadilyincreasingtoamaximumof70-80%6mm
fromthetopsurface.Theseresultsbroadlyagreewiththehardnessmapsand
arewithinthetoleranceoftheassumptionsmadefortheFEmodelling.
Residualstressmeasurements
Theresidualstresses,measuredbyneutrondiffractioninthelongitudinaland
transversedirections,areshowninFigure5(versusdepth)andFigure6(versus
lateraldistancefromtheweldcentreline).Forbothweldsmadewiththe
transformingfillermaterial(Camalloy4),significantregionsofcompressive
stressareobservedintheweldzone.Unsurprisingly,thestressesinthe
longitudinaldirectionareofthegreatestmagnitude.Fortheausteniticweld,the
residualstressislargestattheweldcentrereachingaround+400MPa.By
contrastthestressintheweldzonefortheLTTweldfillerislargelycompressive
reachingapeakofabout-600MPawhenusingthehigherinterpasstemperature
(T200).Fortheweldwiththelowerinterpasstemperature(plateT50),
significantcompressivestressesareonlyobservedforthelaterpasseswhile
tensilestressessimilartothoseintheausteniticweldareseenfortheearlier
passes(~+400MPa).IntheHAZ,bothtothesideandbelowthefusionzone,the
stressesaresimilartothosefortheausteniticweld.Thecontourmeasurements,
conductedatthesamelocationsasoftheneutrondiffractionmeasurements,
showvaluesoflongitudinalresidualstress(seeFigure5&6)thatareinvery
goodagreementwiththeneutrondiffractionscans.
Fullcontourmapsoflongitudinalresidualstressacrosstheentirenormal-
transverseplane,calculatedfromcontourmethodmeasurementsareplottedin
Materials Science and Technology 34 (2018) 519-528
Figure7forthethreeplates.Theseconfirmthatthecompressivestressinplate
T200extendsacrossthemajorityofthefusionzone,whereasforplateT50thisis
confinedtothelast3passesonly.Inaddition,thesecontourmapsrevealthat
despitethelargeregionsofcompressivestress,regionsofhightensilestressare
presentintheHAZforplateT200andintheHAZandfusionzoneofplateT50.
ThesetensilestressesareofgreatermagnitudethanthosefoundinplateA50,
albeitoversmallerregions.Itshouldalsobenotedthattensilestressesare
observedinthebottomcornersoftheplates.Thesearisefromthetackwelds
usedtoattachtheplatestotheweldingtable.
Discussion
InordertorationalisethemeasurementsitisusefultoconsidertheFE
predictions,whichenableustoexaminethetemporalandspatialvariationinthe
extentofthetransformation(Figure8).FromtheFEpredictions,itisevident
thatforthelowinterpasstemperaturemodel(T50)theweldfillertransformsto
around50%martensiteasitislaiddown.Thepercentageofmartensiteremains
almostconstantasmorelayersaredeposited.However,afterthefinal(8th)pass
theweldiscooledtoroomtemperaturecausingfurthertransformationsuchthat
thefinalmartensitefractionisaround70%.Themartensitefractionpredicted
bytheFEmodelisingoodaccordancewiththephasefractiondetermined
experimentally(Figure4).
Bycontrasttheelevatedholdingtemperaturebetweenweldpasses(model
T200)preventssignificanttransformationfromtakingplaceduringthewelding
procedure.Consequently,onlyaround40%martensiteformsneartheboundary
withthebaseplateandalmostnotransformationispredictedtooccurinthe
beadslaiddownawayfromthebaseplate.Itshouldalsobenotedthatdilution
maydecreasetheleveloftransformationobservednearthesidesoftheoriginal
groove.Asaresultinthiscase,mostofthetransformationispredictedtooccur
uponfinalcoolingoftheweld,givingapproximatelythesamefractionsof
martensite(~70%)asforthelowerinterpasstemperatureweld.
Theobserveddifferencesarereflectedinthefinalresidualstressesasafunction
ofinterpasstemperatureshowninFigure9.Whatevertheinterpass
Materials Science and Technology 34 (2018) 519-528
temperature,thereisacompressiveresidualstresszonetowardsthetopofthe
fusionzoneassociatedwiththemartensitictransformationofthefinalpasses.
Similarlyinallcasesthestressesintheparentmetalbeneaththeweldaretensile
justbelowthegroovefallingalmosttozerotowardstheundersideoftheplate.
Similarlyinallcasesthebaseplatefarfromtheweldisinlowleveltension.
Consequentlythemajordifferenceslieintheregionwheretheearlyweldbeads
arelaiddown.Astheinterpasstemperatureisloweredthecompressivestressin
thisregionislost.
ThiseffectcanbeseenevenmoreclearlyinthelineprofilesshowninFigure10.
Indeed,theFEpredictsahighermagnitudeoftensilestressinthelowerpartof
theweldgroove(~1000MPa)thanismeasured(~400MPa).Bycontrastthe
magnitudesofresidualstresspredictedforthemodelT200caseisingood
agreementwiththemeasurementsforplateT200.
Itisnoteworthythatforboththemodelsandtheexperimentalplatesthe
stressesinthelastfewbeadsarethesameindependentofinterpass
temperature.
Theoverpredictionoftensileresidualstressintheregionwheretheearlybeads
werelaiddownisclearfromFigure10.Furtherworkisrequiredtoidentifythe
sourceoftheinconstancyinpredictedandmeasuredtensileresidualstress..
Clearlyanumberoffactorsneedtobetakenintoaccountincludingannealing
effectofsubsequentpassesaswellastheeffectoftransformationplasticityand
theworkhardenabilityofthiszone.
Neverthelessthesimulationsdoindicatethatatemperaturearound200°Cis
necessaryifthetransformationstrain,accompanyingthemartensitic
transformation,istobeexploitedtogeneratecompressiveweldstresses
throughoutthewholemultipassweldment.
AnotherimportanteffectisthedilutionwhichisevidentformFigure2.Inthe
modelthecompositionofthe1stbeadhasbeenassumedtobe50%austenitic
plus50%martensitictoaccountfordilution.FortheT200case,somefurther
studiesontheeffectofdilutionhavebeencarriedoutbymodifyingthe
untransformedandtransformedphasefractionsandtheresultsareplottedin
Materials Science and Technology 34 (2018) 519-528
Figure11.Itcanbeseenthatthecompressivezoneispredictedtomovefromthe
bottomofthe1stbeadtowardthe2ndbeadwhenthelevelofretainedaustenite
increases.Thisisnotsurprisingsincetransformationplasticitycannotoccur
withoutthemartensitephasetransformationleadingtohighertensileresidual
stressesthere.Itisthusveryimportanttoaccuratelymeasureorestimatethe
retainedausteniteintheweld.Iftheretainedaustenitecannotbecontrolled
belowacertainlevel(~70%inourcase)asignificantincreaseinthetensilezone
shouldbeexpected..
Conclusions
Inthisstudythreeweldsampleshavebeenproduced,twousingthenovelfiller
material,Camalloy4butwithdifferentinterpasstemperatures(~50°Cand
~200°C)andthethirdweldwasabenchmarksamplemadeusinganon-
transformingausteniticweldfillerwithaninterpasstemperatureof~50°C.As
expected,thestressfieldinthenon-transformingweldfilleristensile
everywherereachingapeakstressofaround~+400MPa.Forthelow
transformationtemperaturefiller,70%transformationtomartensitewas
measuredthroughouttheweldzoneirrespectiveoftheinterpasshold
temperatureinexcellentagreementwithpredictions.Thefinalresidualstresses
howeverwerefoundtobeheavilydependentontheinterpasstemperature.The
simulationssuggestthattheinterpassholdtemperatureneedstobearound
200°Cinordertodelaythephasetransformationofthedepositedlayersuntil
thewholeweldingsequenceiscomplete(i.e.ideallyalllayersshouldbecooled
belowthemartensitestarttemperatureandtransformtomartensiteinthesame
time).Inthiscasethewholeweldzoneisplacedinresidualcompression
reachingaround-600MPainthecentreoftheweldzone.Witha50°Cinterpass
temperaturemostofthetransformationoccursimmediatelyasthebeadislaid
down.ThestressesintheHAZareslightlymoretensilethanarisewhennon-
transformingfilleroralowerinterpasstemperatureareemployed.
WhilethisstudyhasusedanLTTfilerwithatransformationanda
transformationtemperaturesuchthatsignificantcompressiveresidualstresses
aregenerated,inprincipleitwouldalsobepossibletodesignanLTTfillersuch
Materials Science and Technology 34 (2018) 519-528
thattheweldstressesareapproximatelyzeroaccordingtothestructural
integrityrequirementsinagivenapplication.
Themodelledtensilestressesappeartobemuchlargerintheearlyweldpasses
thanaremeasuredinpractice–thisdifferencewillbethefocusoffurtherstudy.
Neverthelessthisstudyshowsthattheinterpassholdtemperatureneedstobe
carefullyselectedforlowtransformationtemperatureweldfillersiftheyareto
mitigatetensileweldstressesinthesamemannerashadbeendemonstrated
previouslyforsinglepasswelds.
Acknowledgements
ThefundingofEPSRCthroughgrantsGR/S62840/01andGR/S62857/01is
acknowledged.Complementaryfundingaswellashelpwiththeweldingis
gratefullyacknowledgedfromRolls-Royceplcalongwithhelpfulsuggestions
fromDrPaulHurrellregardingtheuseofinterpassholdtemperature.
Materials Science and Technology 34 (2018) 519-528
Table1:Chemicalcompositions(wt%).
Alloys C Si Cr Ni Mo N Mn
CamAlloy4 0.01 0.73 13.00 6.00 0.06 0.026 1.50
AutorodOK308LSi 0.01 0.90 19.70 10.70 0.00 0.01 1.90
304L(base) 0.03 0.75 18.00 8.00 0.00 0.10 2.00
Table2:Summaryofweldingconditions.
WeldPlateNo. Interpass/°C FillerMetal PlateCondition
PlateT50 50-70 CamAlloy4 Constrained
PlateT200 190-210 CamAlloy4 Constrained
PlateA50 50-70 Autorod308LSi Constrained
Materials Science and Technology 34 (2018) 519-528
Figures
Figure1:a)Stressrecordedduringconstrainedcooling(Satohtest)ofthetwofillermetalsusedinthisstudy[12]
Materials Science and Technology 34 (2018) 519-528
a) b) c)
Figure2:a)Aphotographofthegroovedplatespreppedforweldingb)opticalmacrographshowingthefusionzoneandtheweldlay-upsequenceforthegasmetalarcwelds,c)finiteelementmeshusedinthemultipassweldmodelshowingeachpassandthe1500°Cthermalcontourthatapproximatestothefusionzoneboundary.
Materials Science and Technology 34 (2018) 519-528
Figure3:a)andb)weldmicrostructuresandmicrohardnessmaps(inHV2)fortheplateT50(50°Cinterpasstemperature)andplateT200(200°Cinterpasstemperature)weldsrespectively
Materials Science and Technology 34 (2018) 519-528
Figure4:Variationinmartensitefractionfromtopsurfacetoweldrootasdeterminedbyneutrondiffraction.
weld baseplate
Materials Science and Technology 34 (2018) 519-528
a) b)
Figure5:a)Longitudinalandb)transverseresidualstressprofileswithdepththroughthethicknessoftheplatesattheweld-centreline.Themarkersdenoteneutrondiffractionmeasurementswhilein(a)thecorrespondingcontinuouslinesdenotethecontourmethodresultsforthesameplate.
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Figure6:Longitudinal(top)andtransverse(bottom)residualstressprofilesasafunctionofdistancefromtheweldcentrelinesatadepthof4mmbelowtheplatesurface.Markersdenoteneutrondiffractionmeasurementsandthecorrespondingsolidlinesdenotethecontourmethodresultsforthesameplate.
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Figure7:MapsoflongitudinalresidualstresscalculatedusingthecontourmethodforplatesT50,T200(bothCamalloy4)andA50(308L).
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Figure8:Thepredictedmartensitefractionforafterthelayingdownofeachpassforinterpassholdtemperaturesof50°C(left)and200°C(right).Afterthe8thpasstheplatehasbeenallowedtocooltoroomtemperature.
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Figure9:Predictedlongitudinalresidualstressesfor4interpasstemperaturesplottedalongsidemeasuredlongitudinalstressesforplateT50andT200usingthesamestresscontourslevels.
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Figure10:Predictedlongitudinalresidualstresses(continuouslines)plottedalongsidethecontourmethodmeasurementsforsimilarconditions(modelleddatadenotedMandcontourdatadenotedP)
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Figure11:Effectoffinalaustenitefractiononthepredictedstresswhentheinterpasstemperatureiskeptat50°C.
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