LiquationPhenomenainthe SimulatedHeat-AffectedZone of Alloy 71 8after MultiplePostweldHeIatTreatmentCyclesDelta-phaseformation increases liquation cracking susceptibilityBYM. QIANANDJ. C.LIPPOLDABSTRACT.Long-termisothermalsolu-tionheattreatmentswereconductedtosimulatemultipleweldrepair/postweldheattreatmentcyclesinAlloy718wroughtplate.Theseheattreatmentsre-sulted in extensive precipitationof needle-andplate-shaped8 phaseinthey-nickelmatrix.8-phaseaccumulationrepresentstheprincipalmetallurgicaldamagefromsimulatedmultiplerepair/postweldheattreatmentcyclesinAlloy718.Grainsizedid notincreaseduring thisexposuredueto the grain boundary pinning effectof the8phase.Simulatedweldheat-affectedzonethermal cycles resultedin a varietyofmicrostructuralchangestotheheattreatedmaterial,including5-phasedisso-lution-promotedliquation,boroncarbideconstitutionalliquation,andsegregation-induced grainboundary liquation.Theef-fect of these liquationphenomenaontheweldability degradationof Alloy 718 is dis-cussed.IntroductionWeldrepair of aircraftgasturbineen-ginecomponentshasbecome increasinglyprevalentasa meansof extendingenginelifeand reducing the costsassociatedwithcomponentreplacement.Aspartof therepairweldingprocess,the precipitation-hardenedsuperalloysmustundergopost-weldheattreatment(PWI-IT)torestoretheirmechanicalproperties.Becausecomponentsaresubjecttomultiplere-pairs over their lifetimes,they willalso beexposedtomultiplecyclesof PWHT.Ithas beenobservedthat theweldabilityofsomesuperalloysdegradesafteranaccu-mulationof repair/PWHTcycles(Refs.1-5),makingfurtherrepairdifficult.Heat-affectedzone(HAZ)liquationcrackingis the rootcause of thisdifficulty,which occursin the partially melted regionoftheHAZ.SuchcrackinghasbeenM QlANandJ. C LIPPOLD arewviththleWeld-ingand Joining MetallurgGroup atTheOhlioState UniversityColuimbus, Oltio.shownto beassociatedwithlocalorpar-tial melting of grainboundaries,causing ashort time hightemperaturegrain bound-ary(GB) weakening(Ref.6). Preliminaryinvestigationsontheeffectof multiplePWHTcyclesonrepairweldabilityhavebeenconductedfor Alloy 718(Refs.2-5,7).Adirectresultof themultiple PWHTistheaccumulated,abundant8-phase(Ni3Nb,orthorhombic)precipitationinthenickelmatrix,constitutingthemetal-lurgical "damage"that degrades the weld-abilityof Alloy718.The8-phasedissolu-tionduringweldthermalcycleswasreportedto beafactorresultingingrainboundaryliquation(Ref7) by promotingGB segregationof Nb,a melting point de-pressant(Ref1).Thepurposeofthispaper is to elucidate the effectof liquationphenomenaontheweldabilitydegrada-tioninthe Alloy718duetomultiplere-pair/PWHTcycles.ExperimentalProcedureThematerialusedinthisstudywasAlloy718intheformof wroughtplate.Thecompositionofthismaterial,baseduponanindependentanalysis, is shown inTable1. Theplatemicrostructureintheas-receivedconditionisshowninFig.1.There arefinedeformedgrains surround-ing the "normal"grainsand some 8 phaseispresentinthey-nickelmatrix.Simula-tion of multiplePWHT cycles was accom-plishedthroughmetallurgical-equivalentlong-termisothermalheattreatments.This techniquehad previouslybeen showntoyieldmicrostructuresandpropertiessimilar to those achievedthrough multiplethermalcyclesforequivalenttimes(Ref6).A normalPWHT for Alloy718 wouldbe954Cfor0.5-2h.wvoisothermaltreatmentswereperformedtosimulatemultiplePWHTcycles:954C/40hand954C/100h.The isothermaltreatmentswere conductedin air in a box furnace withaircooling.Subsequently,Gleeblehotductilityspecimensweremachinedfromthebulk materialsafter heat treatment.Gleeblehotductilitytestingwascon-ductedto evaluateandcomparethesus-ceptibilityof Alloy 718toHAZ liquationcrackinginthreedifferentconditions:1)as-received,2)954C/40h,and3)954C/100h.The specimensare6.35mmindiameter,100 mm inlength,takenlon-gitudinallyalongtherollingdirectioninthe plate.The hotductilitytesting followsconventionalproceduresashavebeenre-portedpreviously(Refs.2-7).Aheatingrate of 111C/s,hold time attest tempera-ture of 0.5s, coolingrate of 43C/s (for on-coolingtests),andstroke rateof 25mm/swereused.Alltestingwasconductedunderanargonatmosphere.Figure2schematicallyshows the procedure for de-terminingcriticalvaluesof nil-strengthtemperature(NST), nil-ductilitytempera-ture(NDT),andductility-recoverytem-perature(DRT).Initially,thenil-strengthtemperature(NST)wasdeterminedbyheatingthespecimenundera small staticload(approximate10kg)untilfailureoc-curred.On-heating hot ductility tests wereconductedbyheatingsamplestovarioustesttemperatures,andpullingthemtofailureatthestrokeratereported,untilNDT was achieved.On-coolingtests wereperformedafterheatingtoa peaktem-perature(Tp)betweenthe NDT and NST,and then cooling to a desired temperature,and pulling to failure to identifythe DRTMetallographicsampleswereexam-inedusing bothan optical microscopeanda Phillips XL-30 scanningelectronmicro-scope(SEM)equippedwithanenergy-dispersivespectrometer(EDS).Elec-trolyticetchingwith10%aqueouschromic acidwas generallyusedto revealIWELDINGJOURNALKEYWORDSAlloy 718WeldRepair,5-PhasePrecipitationBoronCarbideConstitutionalLiquationLiquationCrackingIntergranularFractureFig. I -Microstructure of as-received Alloy718 base metal:A -distributionoffine deformed grains at grain boundaries;B-short bar-shlapedintra- andiutergranzularSphlase.lhble 1-ChemicalCompositionof Alloy718WroughtPlateElementCrMnCoVAlCTiBMoSiNbSFepWMgCuTaNiwt-%18.420.090.20.0220.50.0331.030.00183.030.15.040.000517.580.0120.0290.0110.150.006BalanceNSTTpNDTDRT'M.-Pull tofailureG)l.DeterminingNST2.On-heating test3.On-coolingtestTimeFig. 2-Schematic of Gleeble hot ductilioy test procedutre.Table 2-The Effectof HeatrreatmentonGrainSizeof Alloy718HT ConditionGrainSize(df)As-received954CC(1750F)/40h954C(1750F)/100h7940.6 lim8327 1tm8331pmmicrostructuresof interest. Grain size wasmeasuredin Feretdiameter(df),ondigi-tally recordedoptical images usingimage-Tool 2.0(Ref.8).Fracturemorphologywasevaluatedonselectedhotductilitysamples by usingSEM.Resultsand DiscussionsThe principalmicrostructuralfeatureinlong-termisothermallytreatedAlloy718isthehighfractionof precipitated8 phaserelativeto thatof theas-receivedmaterial.Theas-receivedbasemetalhadrelativelylittle5 phaseandthesizeofintra-andintergranularbar-shapedophase is comparable-Fig.1. In contrast,thefractionof bothintra-and intergranu-lar(IG)o phase inthe long-term,isother-mally treatedmaterialincreaseddramnati-cally,as shown in Fig. 3. Theintergranularo phase becomesessentiallycontinuousasa resultof the long-termisothermaltreat-ment,and somebar-shapedo phase alongtwin boundariesalso becamecontinuous.Themorphologyoftheintragranularo phase appearsas fine needles that inter-crossedcompactly- Fig.3B.Therewasno evidenceof gammadouble-prime(Y')observedinthisstructuresincetheheattreatmenttemperatureisabovethepre-cipitationrangefor Y' and none wouldbeexpectedtoformuponrapidcoolingtoroomtemperature.Inaddition,therapidformationof the ophase depletesthe ma-trixof niobium,furtherreducingthepos-sibilityof Y'.Thegrainsizeremainedessentiallyconstantafterthelong-termisothermalheat treatmentat 954C, as shown in Table2. Thisis duetothepinning effectof theo phaseongrainboundaries.Thebulkyangularphasesinthenickelmatrix(Fig.3B)arecarbidesthataredistributedasstringersintherollingdirectionof thewrought plate.GleeblehotductilitytestingrevealedHAZliquationcrackingsusceptibilityin-creaseswiththeincreaseinholdtimeat954C,asdeterminedbytheliquationtemperaturerange (LTR), which is the dif-ferencebetween NST and DRT (Table 3).Noticeably,the increaseof LTR is consis-=JUNE 2003- --->beTXtFig. 3-Higl fraction of needle-shiaped intragranularand bulky continuous, intergramnara phasefrom long-temnisothiemial treatment.A-9540C/40 h;B - 954C/100 hi.Fig. 4-8-phase dissolution during simulated HAZtihennal cycles in Alloy718.A- 8-phase "thickening" at NDT of thle as-received material; B-dis-solving s-phase nenvork atNDT of 954'C140 h treated material; C - 8phlase"rounding"at NST of thie as-received material (liquatedboron carbide is indi-cated);D - interconnected cluster of B- and Nb-rich eutectic constituentsfromdissolved 8-phase along grain bounidaries atNST of 954C/100 1h-treatedmaterial.tent with the densityincrease of 8 phase asafunctionof hold timeat 954C.Microstructuralevaluationrevealedthat8-phasedissolution-promotedliqua-tion occursduring the on-heatingthermalcycleandpersists tothe NST8phase hasanorthorhombicstructurewithcomposi-tionof Ni3Nb.In Alloy718, itsprecipita-tiontemperaturerangeisfrom650to1050C(Refs.9,10).6 phase willdissolveabovethe uppertemperaturelimit.Upondissolution,thesurroundingy-nickelma-trix isenrichedinNb,bothintra-andin-tergranularlydependingonthe6-phasedistribution. Since Nb forms two eutecticswithNias(Ni+Ni3Nb)at12820C(23 wt-% Nb)and(Ni3Nb+Ni6Nb7)at1175C(52 wt-% Nb),Nbactsas amelting pointdepressantelementin=Ni(Refs.11,12).WithregardtoAlloy718,itis expectedthatthe dissolutionof5 phase willresultIWELDINGJOURNALAXLJDJItLt7lt;ffi~~Ni 4I__________________Fig. 5-5-phase dissolution gradientin DRTspecimens.A -As-received;B - 954'C1100 h:treated condition (arrowvs indicateeutectic constituentsformedin the cores of original Spitase).Fig.6-Fractograplzyof NDTsamples of as-receivedAlloy718.A -Intergranularfracture;B-detail of Fig.6Ashoving evidence of liquid films and li-quated boron carbides.TIble 3-Resultsof Gleeble HotDuctilityTestingfor Alloy718HT ConditionNDTNSTDRTPeak temp(Tp)LTRT;DRTAs-received1199C1274C1171C1240'C103C- 69C954C/40hl191'C12720C1158C1240*C114C82'C954C/IOh1190'C1276C1149C1240C127C91CinformationofNb-richeutecticcon-stituents,as has beenobservedin thecur-rent research.Figure4showsmicrostructuralevi-denceof8-phasedissolutionandassoci-atedliquation.Thedissolutionof theophaseresultsinNbenrichmentatthegrainboundarythatgivesrisetothedif-ferentetching characteristicof the bound-ary.TheNb-enrichedboundaryvariedasafunctionofthepeaktemperaturereached during the hot ductilitytest and ismanifestedmicrostructurallybydifferentmorphologies.AttheNDT,themi-crostructureappearsas5-phaseneedle"thickening,"asshownintheas-receivedmaterial- Fig.4A. Notethat the "holes"alongtheGBsaresitesof eutectic,5-phasedissolutionproductsthathaveetchedout of the structure.Fracturesur-faceanalysisintheSEMrevealedaddi-tionaldetailsof theliquationprocess,in-cludingapparentconstitutionalliquationof boroncarbides.TypicalintergranularfeaturesareshowninFig.6,whereliquatedparticlesandevidenceofliquidfilmsonthefrac-ture surface canbe seen. Note the angularbutslightlyroundedparticlesandholeswhereparticleshavedroppedoutofthefracturesurface- Fig.6AandB.EDSanalysisrevealedthattheparticlesarecomplex boron carbidescontaining Cr, Ti,andsomeNb.Thepresenceofroundholesaccommodatingtheroundedboroncarbidessuggeststhatboroncarbide-relatedconstitutionalliquationhasoc-curredalongtheGBs.Heatingofthedenselydistributed8phasein954C-treatedsamplestotheNDTproducedmoreextensive,dissolving8 phasenet-works- Fig.4B.As thetesttemperature,wasraised to the NST,5-phasedissolutionbecamemorepronounced.Theonce-thickened,intragranular8-phaseneedlesbegan"rounding"(Fig.4Cand D),indi-catingamoreacceleratedNbdissolutionrate.Someadjacentdissolved5phasenear or alongGBs interconnectedto formB-andNb-richeutecticconstituents-Fig.4D.Theselow-meltingeutecticliq-uidsalongGBswillresultinGBweaken-':JUNE2003lIAT4~~~~k-k'Xtw:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~tCl;~~~~'Fig.7-Fractograplzyof954C140 1:-treated specimensA-IGfeatulrc at NDT;B -detailof Fig. 7Ashlowing terraced nipple pattemnwitht particles dis-persed at NDT;C-detailoft/ice liquated bomntcarbide; D-terracedripplepattern of liquiadflowv trace atDRT:ing and potentialfailure (cracking)duringthe weldthermalcycle.At the DRT,the microstructuresof as-received718(Fig.5A)didnotchangemuchcomparedwiththatoftheNDTsample(Fig. 4A). However,the Nb disso-lution is more extensive(Fig. 5B) than thatattheNDTsincethissamplehasbeenheatedthroughthepeaktemperature(Tp) during the on-coolingtest. It can alsobeobservedthattinybutdistinguishableeutecticconstituentsformin the center ofthe original 5 phase - Fig. 5B. This is dueto the Nb enrichmentassociatedwiththedissolved5 phase.The center has ahigherconcentrationof Nb,whiletheperipheryhasalowerconcentrationof Nb.Asthehighconcentrationof Nbcorrespondstotheeutecticwithalowermeltingpoint(1175C),liquationstartedpreferentiallyatthecenterwhentemperaturesbecamefavorable.Figure7showsintergranularfractureofthe954C-treatedsamples,butwithmore liquid presentthan seenin Fig. 6fortheas-receivedmaterial.Therearemoreparticlesonthesefracturesurfacesandthe flow pattern of the liquid films is muchmoreevident,appearingasterracedrip-plepatterns.Atthecenterofconcentricripples,aliquatedparticleinaholewasidentifiedasaNb-richboroncarbide,asshowninFig.7BandC.Similarbutsmaller particlescan also be differentiatedinthe samemicrograph.The angularpar-ticleatthe upperrighthasnotliquatedandwasidentifiedasTiC.MoreGBli-quationassociatedwithripplepatternscanbe seen inFig. 7D.Figure8presentssecondaryelectron(SE)andback-scattered-electron(BSE)micrographsthatclearlyshowthe mor-phologyof boroncarbides(lighterbulkyphase)andMCcarbides(darkphases),whereboroncarbidehasapparentlyde-composedfromitsperiphery.Asimilarcasecan beseen in Fig.4C(arrow)andisprobably due to constitutionalliquation.Basedontheaboveresults,itcanbeconcludedthat8-phasedissolution-promotedliquationplaysamajorroleininfluencingthesusceptibilityto liquationcrackinginAlloy718subjectedtolong-termheattreatmentsat954C.Segrega-tion of Band Nb as well asother melting-pointdepressantsalsocontributetothelocalizedmelting of theGB.Interstitialelements,suchasboron,arewellknownto havethepropensitytosegregate to GBs. Nb fromthe dissolutionof 8 phases couldpreferentiallysegregatetoGBthrougheitherdiffusion(Refs.13,14)ora segregationmechanism(Ref.7),by which Nb couldbe swept by mobile GBsoncethe pinning effectof5 phase wasre-ducedbydissolution.Nb-containingboroncarbide liquationalsocontributedto the Nb segregationtoGBs,thoughthisisexpectedtoaccountforonly a small fractionof the liquidpre-sent,sincethefractionof boroncarbidesis much less thanthe 8 phase.In addition,boroncarbideliquationproducesalow-meltingeutectic,anextralowmeltingpointconstituentthatfurtheraggravatestheliquation.Insummary,theeffectofliquationphenomenaon the susceptibilityto liqua-tioncrackingof Alloy718thathasbeenexposedto multiple PWHT cycles that re-sult in a high fractionof 5 phase can be ex-plainedas follows.As thenumber of weldrepaircycles accumulates,8-phase precip-itationbecomes more extensive.Uponre-IWELDINGJOURNAL=YA4L DkkFig. 8-Morphology of boron carbidesand MC carbidesin a DRTspecimen.A-SEMsho)ving rounded 8 phase and associated liquation; B-BSE imagedifferentiatingphasesindle y-nickelmatrix.heatingintheweldHAZ,rapid5-phasedissolutionoccurs.8-phasedissolution-promotedliquation,possiblycombinedwithboroncarbideconstitutionalliqua-tion,results in extensivegrain boundary li-quationthatthenleadstocrackingif re-straintlevelsaresufficient.Thisexplainswhy the buildup of 8 phase in Alloy 718 in-creasesthesusceptibilitytoHIAZliqua-tion as the number of repair/PWHTcyclesincreases.Conclusions1)SimulatedmultiplePWHTcyclesusingequivalentlong-termisothermalheattreatments(954C/40-100h)re-sultedin extensive precipitationof needle-andplate-shapedophaseinthey-nickelmatrix,whichisthemajormetallurgicalchangerelativeto thestarting plate mate-rial.Grain sizedid not change appreciablyduring these heat treatmentsdueto grainboundarypinning by the8phase.2)Gleeblehot ductilitytesting showedweldability(resistancetoI-IAZliquationcracking)of Alloy718 degradedasacon-sequenceofthesimulatedmultiplePWI-ITcycles.3)Thedegradationof weldabilityre-sultsfromgrainboundaryliquationre-sulting primarilyfromthe 8-phase dissolu-tion andassociated Nbenrichmentof thegrainboundary.Constitutionalliquationof boron-richcarbideswasalsoobservedand maycontributeto the grain boundaryliquation.AcknowledgmentTheauthorswishtothankEdisonWeldingInstituteforsupportingthisre-search.References1. 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Vol.33,pp.1205-16.MJUNE2003IREPRINTSREPRINTSToorder custom reprintsofarticlesinWelding JournalContact DenisMulliganat(800)259-0470FAX:(717)481-7677or viae-mailatinfo @reprintdept.comREPRINTSREPRINTSCOPYRIGHTINFORMATIONTITLE: LiquationPhenomenaintheSimulatedHeat-AffectedZoneofAlloy718afterMultiplePostweldHeatTreatmentCyclesSOURCE: WeldJ82no6Je2003WN: 0315200146016Themagazinepublisheristhecopyrightholderofthisarticleanditisreproducedwithpermission.Furtherreproductionofthisarticleinviolationofthecopyrightisprohibited.Copyright1982-2003TheH.W.WilsonCompany.Allrightsreserved.