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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