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英文文献翻译

翻译

英文原文

Post-weldheattreatmentcrackingsusceptibilityofT23weldmetalsforfossilfuelapplications

ABSTRACT

Thepost-weldheattreatment（PWHT）crackingsusceptibilitiesofT23steelweldedwithfourtypesoffillermetalswereevaluatedbymeasuringthestress-ruptureparameters（SRP）,whicharebasedonboththestress-rupturestrengthandthestress-ruptureductility.Post-weldheattreatmentcrackingtestswereperformedonT23weldmetalsusingaGleeblesystemattemperaturesof650-750℃andatstresslevelsof100-500MPa.TheresultsshowedthattheweldmetalswithahigherSRPexhibitedimprovedrupturestressandductility.InadditiontomeasuringtheSRPvalues,thePWHTcrackingsusceptibilityofeachweldmetalwascomparedanddiscussedwithrespecttothefracturemorphology,thesolutesegregationatthegrainboundary,theprecipitationbehavior,andthedenudedzoneformedduringtreatment（whichcanaffectthestress-ruptureparameter）.Itwasfoundthatsolutesegregationwasseverelydeleteriousandresultedinlowerstress-ruptureparameters.Inaddition,MowasfoundtointerruptW-depletionadjacenttoM3CorM23C6particles,retardingandweakeningofthedenudedzonealongthegrainboundaries.

Keywords:

Heattreatments;Welding;Failureanalysis

1.Introduction

T23（2.25Cr-1.6W）steelwasdevelopedforhightemperatureapplicationssuchaschemicalandfossilfuelpowerplants[1,2].IthasbeenreportedthatT23exhibitsimprovedweldabilityandcreeprupturestrengthcomparedtothoseofconventionalT22（2.25Cr-1.0Mo）steel.Suchimprovementsareachievedbyreducingthecarboncontenttoamaximumof0.1%andaddingbothasolidsolutionelement（W）andprecipitationstrengtheningelements（V,Nb）[1-3].WhilethedemandforT23hasgraduallyincreased,reportsonthePWHTcrackingproblemsofthismaterialhavealsoincreased[4-13].Post-weldheattreatmentcracking,alsoknownasreheatcrackingorstressreliefcracking,isgenerallydefinedasintergranularcrackingwithintheweldmetalorintheheat-affectedzonethattakesplaceduringheattreatmentorhightemperatureservice.Thistypeofcrackingisknowntobeasaresultoftheresidualstressproducedafterwelding.ThephenomenonofPWHTcrackinghasbeenexplainedby

（1）precipitationstrengtheningofthematrixandtheformationofasoftdenudedzoneadjacenttothegrainboundaryand

（2）trampelementsegregation（P,S,Sb,Sn,As,Al）ataprioraustenitegrainboundary[5–13].Intheformermechanism,theprecipitationofcarbides（suchasM3CandM23C6）alongaprioraustenitegrainboundaryleadstotheformationofaC-orCr-denudedzone.Assuch,thematrixadjacenttothegrainboundarybecomessofterthanthatoftheactualgrainboundary.Inaddition,theaustenitegraininteriorcanbestrengthenedbytheprecipitationofafinecarbidesuchasMC.Hence,mostofthestrainthatarisesfromstressrelaxationduringthepost-weldheattreatmentcanbeconcentratedinthesoftdenudedzone,causingintergranularcracking.ThesegregationofimpuritiesisalsoknowntocausePWHTcrackingduetotheloweringofthecohesivestrengthalongthegrainboundaries.InastudybyNawrocki,T23wasfoundtopossessahighersusceptibilitytoPWHTcrackingthanT22,duetotheexistenceofafine,dispersive,andstable（V,Nb）CcarbideinthematrixoftheT23[10].

PreviousworksregardingPWHTcrackinginT23havebeenlimitedtoanalysesofthecoarsegrainheat-affectedzones[9,10].Thus,PWHTcrackingbehaviorinweldmetalsisrelativelyunknown.However,suchcrackingfrequentlyoccursinweldmetalsduringtheiruseinpowerplantapplications,resultinginoperationalproblemsandrevenueloss.Assuch,theneedforresearchregardingPWHTcrackinginT23weldmetalshasbeenincreased.Thereby,theaimofthisworkistoobservethePWHTcrackingbehaviorofT23weldmetals.

2.Experimental

Twoferriticsteels,T12（1Cr-0.5Mo）andT23,innormalizedandtemperedconditionswereusedasbasemetalsforthepreparationofdissimilarwelds.ThechemicalcompositionsofthebasemetalsandtheT23fillermetalsareshowninTable1.

Table1ChemicalcompositionsofT23,T12andfillermetals.（wt%）

CMnSiPSCrWMoVNbAl

T230.04–0.10.1–0.60.5<0.03<0.012.251.60.10.2–0.30.02–0.03–

T120.04–0.15<0.5<0.3<0.03<0.021.1–0.5–––

Fillermetal

A0.070.5–0.90.380.01<0.0052.091.420.020.2–0.30.02–0.030.02

B0.070.5–0.90.380.01<0.0052.11.430.090.2–0.30.02–0.030.005

C0.070.5–0.90.380.01<0.0052.391.350.510.2–0.30.02–0.030.007

D0.070.5–0.90.380.01<0.0052.311.320.120.2–0.30.02–0.030.006

GastungstenarcweldingprocessusingfourdifferentT23fillermetalsandaheatinputof8-10kJ/cmwasemployed,andtheweldingparametersaregiveninTable2.TheT23basemetalandthefourT23fillermetalshavesimilarCr,W,V,andNbcontents,butdifferintheircontentsofotheralloyingelementssuchasMoandAl.The“A”and“C”fillermetalscontainmoreAlandMo,respectively.

Table2Weldingconditionsappliedinthisstudy.

WeldingprocessWeldingcurrent（mA）Voltage（V）Travelspeed（cm/min）Heatinput（kJ/cm）

GTAW100–16010–179–148–10

ThetensiletestcapacityofGleebleallowsittosimultaneouslyimposethestressandtemperatureconditionsthatareexperiencedduringweldingandpost-weldheattreatments.ThePWHTcrackingtestswereperformedunderseveralstressstatesat650,700,750℃usingaGleeble1500.ThedimensionsandshapesofthetestspecimensaregiveninFig.1,andaschematicillustrationofthePWHTcrackingtestcycleisshowninFig.2.

Fig.1.GleeblesampleemployedinthePWHTcrackingtest.

Fig.2.SchematicillustrationofthePWHTcrackingtest

Testtemperatureswereselectedforevery50℃incrementbelowtheAe1temperature,basedonaThermo-calccalculation.Mechanicalpropertiesweremeasuredfortheas-weldedspecimensusingaVickershardnesstester,andthefracturesurfacesofthespecimenswereexaminedusingscanningelectronmicroscopy（SEM,JEOL630f）.Boththinfoilsamplesandreplicaswereusedforobtainingtransmissionelectronmicroscopy（TEM,JEOL2010）images.Athincarbonfilmwasdepositedontothesamplesandextractedat2Vinthesameetchant.ThecollectedcarbonreplicaswereobservedusingTEM,andtheprecipitateswereidentifiedusingenergydispersivespectroscopy（EDS）andselectedareaelectrondiffraction（SAED）patterns.Thinfoilswerepreparedbymechanicalpolishing,followedbyjetelectropolishingusinganelectrolyteconsistingofamixtureof95%methanoland5%perchloricacid,maintainedatatemperatureof-40℃.Anexaminationofthegrainboundarysegregationwascarriedoutusinganelectronprobemicroanalyzer（EPMA,JEOLJXA-8200）.NanoindentationexperimentswereperformedwithNanoindenter-XP（MTSCorp,Oakridge,TN）andacommonBerkovichindenter.Inthenanoindentationexperiments,apeakloadof10mNandaconstantstrainrateof0.2s－1wereemployed.TheequilibriumWandMopartitioningbetweenthecarbidesandthematrixwerecalculatedusingthethermodynamicsoftwareTHERMO-CALC.

3.Resultsanddiscussion

3.1.Microstructuresandrupturetests

AsrevealedbytheTEMimagesinFig.3,themicrostructuresoftheas-weldedspecimenswitheachfillermetalarecomprisedoflathmartensiteandFe-richM3Ccarbideswithasimilarprioraustenitegrainsize（~50um）.TheFe-richM3Ccarbideprecipitatesalongthelathboundaryandtheprioraustenitegrainboundary.Suchprecipitateslikelyformduringthecoolingstageoftheweldthermalcycle.AsshowninFig.4,allweldmetalshavesimilarVickershardnessvaluesduetothefactthattheirmicrostructuresaresimilar.Intheas-weldedcondition,thereisnonoticeabledifferenceinthemicrostructuresofthefillermetals.

Fig.3.TEMmicrographsshowingthemicrostructuresoftheas-weldedspecimens（a）A-weldmetal,（b）B-weldmetal,（c）C-weldmetaland（d）D-weldmetal;（e）SAEDpatternand（f）EDSanalysisofFe-richM3Ccarbide.

Fig.4.Vickershardnessesoftheweldmetals.

Inordertocomparethestress-rupturestrengthandthestress-ruptureductilityofweldmetals,thetimetoruptureandthereductioninarea（%）wereplottedasfunctionsoftheappliedstress,inFig.5,inwhichthetimetofractureincreasedwhenlowerappliedstresswasemployed.

Fig.5.ResultsofPWHTcrackingtestsat（a）650℃,（b）700℃and（c）750℃.

Inaddition,theruptureductilitydecreasedasthetimetoruptureincreased.Generally,theapplicationofahigherstresscanproduceagreaterinitialstrain,andthus,causeaductilityincrease.Inaddition,adecreaseintheruptureductilitywithanincreaseinthetimetorupturemayresultfromcarbideprecipitation（M3CandMC）duringtesting.The“C”weldmetalisfoundtopossessrelativelygoodrupturestressandductility.Incontrast,the“A”fillermetalhastheworstruptureductilityandrupturestressatthetemperaturestestedinthiswork.Inthisstudy,TheSRP（stress-ruptureparameter）wasmeasuredbasedonthestress-rupturestrengthandductility[14].ThismethodofcalculatingthePWHTcrackingsusceptibilityhasbeensuccessfullyemployedinSA508,SA533,andCr-Mosteels[15,16].TheSRPistheproductofthestressatarupturetimeof10minandthecorrespondingreductioninarea.AhighvalueofSRPindicatesgoodstress-rupturestrengthandductility,andconsequently,alowsusceptibilitytoPWHTcrackingwouldbeexpected.TheSRPatdifferenttemperaturescanbemeasuredbyextrapolatingthedatafromFig.5.TheSRPsoftheweldmetalsatdifferenttemperaturesaresummarizedinTable3.

Table3Stress-ruptureparametersoftheweldmetalsatdifferenttemperatures.

T