residual stress control of multipass welds using low …€¦ · weld fillers can be used to...

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.


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].


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


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).


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].


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


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= − − −


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.


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


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


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


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


thattheweldstressesareapproximatelyzeroaccordingtothestructural

integrityrequirementsinagivenapplication.

Themodelledtensilestressesappeartobemuchlargerintheearlyweldpasses

thanaremeasuredinpractice–thisdifferencewillbethefocusoffurtherstudy.

Neverthelessthisstudyshowsthattheinterpassholdtemperatureneedstobe

carefullyselectedforlowtransformationtemperatureweldfillersiftheyareto

mitigatetensileweldstressesinthesamemannerashadbeendemonstrated

previouslyforsinglepasswelds.

Acknowledgements

ThefundingofEPSRCthroughgrantsGR/S62840/01andGR/S62857/01is

acknowledged.Complementaryfundingaswellashelpwiththeweldingis

gratefullyacknowledgedfromRolls-Royceplcalongwithhelpfulsuggestions

fromDrPaulHurrellregardingtheuseofinterpassholdtemperature.


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


Figures

Figure1:a)Stressrecordedduringconstrainedcooling(Satohtest)ofthetwofillermetalsusedinthisstudy[12]


a) b) c)

Figure2:a)Aphotographofthegroovedplatespreppedforweldingb)opticalmacrographshowingthefusionzoneandtheweldlay-upsequenceforthegasmetalarcwelds,c)finiteelementmeshusedinthemultipassweldmodelshowingeachpassandthe1500°Cthermalcontourthatapproximatestothefusionzoneboundary.


Figure3:a)andb)weldmicrostructuresandmicrohardnessmaps(inHV2)fortheplateT50(50°Cinterpasstemperature)andplateT200(200°Cinterpasstemperature)weldsrespectively


Figure4:Variationinmartensitefractionfromtopsurfacetoweldrootasdeterminedbyneutrondiffraction.

weld baseplate


a) b)

Figure5:a)Longitudinalandb)transverseresidualstressprofileswithdepththroughthethicknessoftheplatesattheweld-centreline.Themarkersdenoteneutrondiffractionmeasurementswhilein(a)thecorrespondingcontinuouslinesdenotethecontourmethodresultsforthesameplate.


Figure6:Longitudinal(top)andtransverse(bottom)residualstressprofilesasafunctionofdistancefromtheweldcentrelinesatadepthof4mmbelowtheplatesurface.Markersdenoteneutrondiffractionmeasurementsandthecorrespondingsolidlinesdenotethecontourmethodresultsforthesameplate.


Figure7:MapsoflongitudinalresidualstresscalculatedusingthecontourmethodforplatesT50,T200(bothCamalloy4)andA50(308L).


Figure8:Thepredictedmartensitefractionforafterthelayingdownofeachpassforinterpassholdtemperaturesof50°C(left)and200°C(right).Afterthe8thpasstheplatehasbeenallowedtocooltoroomtemperature.


Figure9:Predictedlongitudinalresidualstressesfor4interpasstemperaturesplottedalongsidemeasuredlongitudinalstressesforplateT50andT200usingthesamestresscontourslevels.


Figure10:Predictedlongitudinalresidualstresses(continuouslines)plottedalongsidethecontourmethodmeasurementsforsimilarconditions(modelleddatadenotedMandcontourdatadenotedP)


Figure11:Effectoffinalaustenitefractiononthepredictedstresswhentheinterpasstemperatureiskeptat50°C.

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residual stress control of multipass welds using low …€¦ · weld fillers can be used to...

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