The Metallurgy Of Alloy 625 - TMS
The MetallurgyStephenFloreen,Gerhardof AlloyE. Fuchs,625and WalterJ. YangKnollsAtomic Power LaboratoryP. 0. Box 1072Schenectady,New York12301-1072AbstractThe objectiveis to describethe effectsof alloycompositionandprocessinghistoryon the microstructureand propertiesof Alloy 625.Thisincludesdiscussionof the solidificationbehavior,the precipitationofdifferentcarbides,and Delta,Laves and y" phases.The paper willshowhow variouspropertiescan be optimizedby tightercontrolof alloychemistryand processingsteps.Superalloys718,625,706 and Various DerivativesEdited by E.A. LoriaThe Minerals, Metals&MaterialsSociety, 199413
INTRODUCTIONAlloy 625 is a highlyalloyednickel-basealloythat can providehighstrength,corrosionresistancein a varietyof environments,and goodBecause of this attractivecombinationfabricabilityand weldability.properties,Alloy 625 has found wide-spreadapplications.ofTable I gives the nominal compositionrange for Alloy 625.The high levelsof Cr and MO providegood corrosionresistanceplus strength,while Fe andNb providefurthersolidsolutionstrengthening.The Al and Ti additionsare principallyfor refiningpurposes and are kept low compared to AlloysHowever, as discussedbelow, withlike 718 to enhance weldability.sufficient(Nb t Ti t Al) contentprecipitationhardeningby y" can beachievedin Alloy 625.This review willfirstlook at the solidificationbehaviorof Alloy 625compositions,since this directlyimpacts the propertiesof castingsandweldments,and can significantlyinfluencethe microstructurein wroughtproducts.This sectionwillbe followedby a descriptionof the variousphase transformationsthat can take place as a functionof time,temperature,and composition.One of the principalmessages that willcome out of this review is that thecompositionrange shown in Table I is overlybroad.Depending upon theproductform, and the desiredproperties,tightercontrolof thecompositionoften can be used to help ensure more consistentpropertiesbetween differentheats of Alloy 625.As is usuallythe case whencompositionadjustmentsare made, however, a change that is helpfulfor onepropertymay be deleteriousto some other property.Some of these tradeoffs in compositionversus propertieswillbe discussedin the FeMO5.0max8.0 10.0I625 ComoositionNb(t .40maxBEHAVIORAlloy 625 can beAlloy718.Thisunderstandingthehas been done ongeneralfeaturesconsideredto a usefuldegree to beapproximationis especiallyhelpfulsolidificationbehavior,since thethat alloycan be used to advantageobserved in Alloy 625.a modificationofin terms ofextensivework thatto help describetheThe dominatingsolidificationreactionin both alloysis the enrichmentofthe remaininginterdendriticliquidin niobium,and the consequentformationof niobium-richLaves phase and/or niobium carbideduring thefinalstages of solidification.Figure 1 shows that psuedo-binaryphasediagram for Alloy718 originallyproposed by Eiselstein(Reference1) toexplainthe enrichmentof the remainingliquidin Nb (and Ta) and theresultantformationof A,B Laves phase.Detailedversionsof this diagramhave been developedby more recent investigators,but the principalfeaturein all cases is the increasedNb contentin the finalliquidto solidify.14
Figure1. Psuedo-EquilibriumShowing Formation(Reference1)SolidificationDiagram for Alloy718of A,B Laves Phase During SolidificationFor present purposes,a betterway to view the solidificationis shown inFigure 2 in a psuedo ternaryequilibriumdiagram.This is a schematicrepresentationbased on the Alloy718 diagram proposed by RadhakrishmanandThompson (Reference2).As shown in Figure 2, the C/Nb ratiodictatesthesolidificationpath and the resultantmicrostructures.Three differentpaths can be followed.Path 1, at high C/Nb ratios,leads to the formationof y t NbC with no Laves phase.Path 2, at intermediateC/Nb ratios,leadsfirstto y t NbC, followedby Laves phase formationat the end ofsolidification.Path 3, at low C/Nb ratiosleads to y t Laves with no NbC.Nb CONTENTFigure2. SchematicView of Solidification15LAVESPathsin A625
Examinationsof the microstructuresin Alloy 625 heats generallyappearconsistentwith the compositionaleffectsshown in Figure 2.In typicalheats Path 1 or Path 2 microstructuresare observed,i.e.,with NbC orwith NbC t Laves particles.However, a well-definedC/Nb value at whichthe solidificationchanges from Path 1 to Path 2 is not apparentfrom theseThis doubtlessreflectsthe fact that the solidificationexaminations.rate and other alloychemistryvariationsalso affectthe microstructureinadditionto the C/Nb ratio.Path 3 microstructures,withoutNbC particles,are not common but have been seen in Alloy 625 heats containingless than0.01% C (Reference3).The iron contentalso affectsthe solidificationstructure.In cast heatsof Alloy718, loweringthe Fe contentreduced the amount of Laves phaseReducing the iron and siliconcontentsin Alloy 625 welding(Reference4).fillerwire has also been helpfulto the ductilityin weldments because theformationof Laves phase is minimized.Table II compares the propertiesofheavy sectiongas tungstenarc weldments made with a standardchemistrywire and with a low iron and siliconwire.Significantquantitiesof Lavesphase and NbC particleswere observed in the interdendriticregionsof theweld metal made with the standardfillerwire,but not in the weld madewith the low Fe and Si fillerwire.The significantimprovementintoughnessappears to be due primarilyto this change in microstructure.TableIIComparison of Impact Propertiesand Weld Metal CompositionsStandard Allov 625 Wire and Low Fe and Low Si WireFillerWireTestCharpy V-NotchImpact EnergyTemp.-3kF42.337.2RT-320 F152.7127.7StandardLow Fe & Low SiWeld .230.140.280.0221.58.643.49Low Fe &SiThe influenceof iron on the solidificationbehaviorcan be shownschematicallyby another pseudo-ternarydiagram,adapted from studiesofAlloy 718 (Reference5).Figure 3 shows this diagram,in terms of thetrace of the solidificationpath moving from the y to the y t A,B Lavesparticles.Higher levelsof Fe (or Cr) shiftthe solidificationpathdownward and to the leftin Figure 3, and thus increasethe amounts ofLaves phase formed duringsolidification.16
NiNb MO SiFe CrFigure3. Psuedo-EquilibriumShowing SolidificationLaves Phase (AfterTernary SolidificationPath DirectionReference 5)Diagram for A718from y to y t A,BFigure 3 also indicatesthat higher levelsof Nb, MO, and Si promote theformationof Laves phase.This is consistentwith chemicalanalysesofLaves phase particlesin heats of Alloy 625.Laves phases are hexagonalInclose packed A B compounds that are found in a number of binaryalloys.more complex alloys,however, Laves particlescan containsignificantamounts of other alloyingor impurityelements,and depart considerablyincompositionfrom a simple A,B type chemistry.Table III gives thecompositionsof the Laves particlesfound in three differentproductformsof Alloy625, banded plate stock, weld metal,and originallyLaves-freeplate stock that was heat treatedfor 48 hours at 1600 F.The resultsaregiven in atomic percentunits.There are noticeablecompositionalvariationsbetween the three materials,that likelyreflectthe differencesin processinghistoryof the materialsin which the particleswere formed.In all cases, however, the Laves phase was significantlyenrichedin Nb,MO, and Si, while the Fe, Cr, and Ni levelswere not noticeablyenrichedversus the nominal Alloy 625 composition.Also includedin Table III are the compositionsof Laves particlesfound invariousAlloy 625 GTA welds by Cieslak(Reference6) and in Alloy625 weldmetal made by three differentweld processesby Wilson et al (Reference7).The ranges of compositionsfound by these two investigationsare shown inTable III,and furtherillustratethe considerablevariabilityincompositionfound in Laves particles.Cieslak,et al, (Reference8) have reportedsimilarcompositiontrendsinthe Laves particlesin heats of Alloys718, 909, and a precipitationhardened versionof Alloy 625 calledCustom Aged Alloy 625.Additionalcompositionalstudiesreportedin References9-11 indicatethat B and TiminimizeLaves formation.also promotes Laves phase, while Mg additions17
TableChemicalCompositionsIIIof Laves Phase Particlesin AtomicDifferentAlloy 625 MaterialsPercentsThreefromWeldThe compositionsof the NbC particlesin variousAlloy 625 weldments alsohave been reported(Reference8).These are given in Table IV.Note thatboth blocky carbidesand a dendriticChinese scriptmorphologyat the grainAlso includedin Table IV are the chemistriesofboundarieswere found.blocky NbC particlesfound in wrought Alloy 625 samples by two differentinvestigations(References12 and 13).The carbidesare primarily(Nb,Mo)C, with minor amounts of Ni and Cr present.TableCompositionIn commonand Lavespracticesandetc.,willhaveAlloy 625,appears toof NbC ParticlesIV(wt %) Reportedin Literaturewith other similarnickel-basealloys,the tendency to form NbCphases duringsolidificationcauses inherentlimitationson meltfor Alloy 625.The meltingprocedure,i.e.,AOD vs. VIM vs. ESR,the ingot size taken togetherlimitthe maximum heat size thatan acceptablestructure.This limitis not well definedforbut for currentESR melts the maximum practicalingot sizebe about 40,000 pounds.Even in much smallerheats, or in weldments,indicatesthat using compositionswith lowerFe, and Si could be advantageous.To further18the precedingdiscussionlevelsof elements such as Nb,explorethe effectsof
differentialcompositionon the solidificationbehavior,(DTA) has been used to determinethe liquidusand solidusduring coolingfor a series of Alloy 625 with emistryTable V gives the compositionsof a set of 100 pound vacuum inductionmeltsThe DTA tests were conductedonthat were used for this investigation.samples weighingapproximatelytwo grams that were heated and cooled at anominal rate of 20 C per minute.Tungsten was used as a referencematerial.The test method generallyfollowsthat used by Cieslakand coworkers (References3, 6 and 8) and the resultsgenerallyappear consistentwith the compositionaleffectsthey see in Alloy 625 and other 0.026ComoositionsVof A625 Heats Used For DTA 2.14-26.025-011.1078.903.64.0014.053in terms of the solidificationrange (liquidusFigure 4 shows the resultstemperatureminus solidustemperature)during coolingversus the carbonThe curve in the figureis drawn thru the data forcontentof the heats.Increasingcarbon significantlythe heats with differentcarbon contents.Very similareffectsof C on theincreasedthe solidificationrange.solidificationrange of Alloy 625 were observed by Cieslak(Reference6).The effectsof other individualalloyingelement variationsare also shown.Increasingboron to 0.018% markedly increasedthe solidificationrange,because of the commonly observed effectof boron reducingthe solidustemperature.A higher nitrogencontent(0.10%) had surprisinglylittleto values approachingtheReducing the Ti, Nb, MO, and Fe contentseffect.lower limitsfor these elements in Table I tended to reduce theCieslak(Reference6) also found that ed the solidificationrange.Cieslakfound that loweringthe solidificationrange minimizedthe tendencyfor hot crackingduring welding(Reference6).Reducing the solidificationrange should minimizethe amount of segregationthat would take placeimprove the hot workability.Thus,duringsolidificationand, therefore,reducingthe C and Nb contentswould be beneficialin reducingthe amountof materialavailableto form NbC or Laves phase, and also in reducingthe19
Decreasingthesegregationof these elements duringsolidification.solidificationrange also may be part of the beneficialeffectof reducingthe iron contentin minimizingLaves formationin weldments.3200II3000.53Fe10.020II0.04II0.06WEIGHT PERCENT CARBONFigure4. SolidificationRange DuringCoolingof A625 HeatsIf an ingot containingexcessiveLaves and/or NbC particlesis hot workedto plate these particlescan become strung out in planes parallelto theplate surfaceto give a banded microstructure.Figure 5 shows an exampleof a badly banded microstructurewith high localconcentrationsof Lavesphase particlesfound in some Alloy625 plate stock.Layeredmicrostructuresof this type may have acceptablepropertieswhen deformedin directionsparallelto the bands, but can displayvery poor ductilitywhen strainedperpendicularto bands.The materialshown in Figure 5, forexample, showed extensivecrackingalong the banded regionsafterthe platehad been deformed to about 20% outer fiberstrainby bending.Figure5. Photographof Banded Microstructures20in A625 Plate(425X)
As discussedbelow, Laves phase particlecan be eliminatedby solutionThus, suitablehot workingand annealingannealingat high temperatures.practicescan get rid of Laves particlesin wrought productsand the ratherflagrantexample describedabove should not be consideredtypical.However, NbC particlesare much more stableand once formed are probablyHere alsoimpossibleto eliminateby conventionalprocessingsteps.suitablehot workingpracticescan usuallyproduce wrought tionsof NbC. However, high localconcentrationsof NbC in banded microstructurecan be found in wroughtBands of carbideparticlesalso willdegrade the ductilityinproducts.directionstransverseto the bands, althoughthe loss in ductilitydue tocarbidesdoes not appear nearlyas severe as that resultingwhen Lavesparticlesare present.Bands of carbidesalso can cause problems during weldingbecause ofliquationof the carbidesin the heat affectedzone of the weldment.Thishas been examined by using a thermo-mechanicaltestingdevice(Gleeble)toproduce five second thermalexposuresat varioustemperaturesin Alloy625plate stock containingbands of carbideparticles.Following1600 F.particlesis highlycooling,precipitateacicularthat isliquated.specimenthe samples were heat treatedatthis short time thermalpulse,This latterheat treatmentwas used to delineatewhere the carbideWhen NbC particlesliquate,the resultantliquidhad dissolved.concentratedin Nb and MO. When this liquidsolidifiesonit forms a solidsolutionvery rich in Nb and MO that willthenDelta phase particleswhen heat treatedat 1600 F.Theshape of the Delta providesa distinctiveneedle-likemorphologyeasy to identify,and marks the regionswhere NbC particleshadFigure 6 shows examples of these localizedpatches of Delta in aexposed at 2300 F for five seconds.Figure6. Delta Phase ParticlesParticlesMarkingLocationsof LiquatedNbCa set of samples were examined to look forUsing this marking technique,signs of liquationafterfive second thermalexposuresat variousThe resultsindicatedthat liquationof the NbC particlestemperatures.began at temperaturesin the 2175-2200 Frange, and became much moreIt was also evidentthat liquationcouldextensiveat highertemperatures.be very extensivein banded,regionsthat containedhigh concentrationsof21
NbC particles.Hence banded regions would be much more likelyto formcracks during weldingthan materialsin which the NbC particleswere moreuniformlydistributed.It is also clear that appliedor thermalstressesperpendicularto the bands would have much more severe effectsthanstressesparallelto the bands.This constitutionalliquationof the NbC particlestakes place attemperatureson the order of 150 F below the bulk solidustemperaturesobserved during heatingin the DTA tests.The localizedmeltingcan beexplainedin terms of the Path 1 and Path 2 solidificationpaths shown inFigure 2.If one plotteda verticalsectionthrough the firstleg ofeitherpath, it would look schematicallyas shown in Figure 7; i.e.,as apsuedo-binaryeutecticbetween the y matrixand NbC. Upon heating,whenthe NbC particleslocallycome into equilibriumwith the (Nb t L) field,they willbegin to melt.The present resultsindicatethat this eutectictemperatureis in the range of 2175-2200 F.ALLOY CoMPOSITlONPERCENT CARBONFigure7. Pseudo Phase Diagram ShowingEquilibriumWith MC PhaseLower TemperatureEutecticinIn summary, the formationof Laves phase and/or NbC particlesplays a majorThere are no obvious benefitstorole in the solidificationof Alloy 625.the presence of eitherof these phases in the finalmicrostructure,andthere are clearlydetrimentaleffectsif excessiveor highlylocalizedconcentrationsof these particlesare present.Meltingto lower levelsofNb and C, as well as lower levelsof Fe, MO and Si, would be helpfulinminimizingthe formationof these phases.The resultantbenefitswouldprobablybe greatestin castingsand weldments.In wrought productsLavesparticlescould be eliminatedand NbC particlesat least leftin reasonablynon-localizeddistributionsby suitablehot working and heat treatingprocedures.The need to prevent the formationof these phases duringingotsolidificationis correspondinglyless critical.22
PHASE TRANSFORMATIONSIn this section,the phase changes that occur in Alloy625, as a functionof time and temperature,willbe discussed.The startingmaterialissolutionannealed so that all phases, except the primary NbC particlesdiscussedabove, have been put into solidsolution.Several time-temperature-transformation(T-T-T)diagrams have beenThese differsomewhat from each other,and alsopresentedfor Alloy 625.from the proposed T-T-T diagram that willbe describedbelow.There areseveralpossiblereasons for these discrepancies,and it is not possibleatthis stage to clearlyidentifythe reasons for the differencesin thevariousdiagrams.However, two factorswillbe discussedthat illustratewhy there can be noticeablevariationsin T-T-T diagrams for what isostensiblythe same alloy.The firstis simply that variationsincompositionwithinthe ranges given in Table I can significantlyaffecttheT-T-T behaviorand thus two differentheats of Alloy 625 can givesurprisinglydifferentresults.Examples of some of these compositionaleffectswillbe described.A second source of variabilityis experimental,and involvesthe techniquesused to identifythe phases.One example describedbelow is that Lavesparticlescan form at grain boundariesthat may be very difficulttodistinguishfrom grain boundary carbidesby opticalmetallographicexaminations.With these caveatsin mind, Figure 8 presentsthe T-T-TNominal in this case is definedheats of wrought A625.ranges given in Table VI,found in commercialheatsyears.2000diagram for nominalby the chemistrywhich are typicalof the compositionscommonlyof Alloy 625 made in this countryin recent180016001FigureTlME (HOURS)108. hases Forming at High Temperaturesin A62523for
anges for8.18.9CommercialNb( Ta)MO-VI-3.43.7-C0.010.04Allov625 1 0.25As indicatedin Figure 8, a number of differentcarbidesand intermetalliccompounds can precipitatein A625 after thermal exposures,for times on theorder of 0.1 to 100 hours.Stillfurtherchanges, as discussedbelow, willoccur with more prolongedexposures.Table VII summarizes the crystalstructuresand typicalcompositionsinNote that the compositiondata areatomic percent values of these phases.partitionedin this table by crystallographicposition.Thus, in the caseof the A,B Laves phase, the Cr, Fe, and Ni are assumed to occupy the Aand Si, P, Nb, and MO occupy the Bpositionsin the crystalstructure,positions.As discussedbefore,there are two differentNbC morphologies:the blocky shape that forms duringsolidification,and the dendriticformreportedin we
The Metallurgy of Alloy 625 Stephen Floreen, Gerhard E. Fuchs, and Walter J. Yang Knolls Atomic Power Laboratory P. 0. Box 1072 Schenectady, New York 12301-1072 Abstract The objective is to describe the
HAYNES HR-224 alloy HAYNES HR-235 alloy HAYNES NS-163 alloy HAYNES R-41 alloy HAYNES Waspaloy alloy HAYNES X-750 alloy STELLITE 6B alloy HAYNES 25 alloy HAYNES 75 alloy HAYNES 80A alloy HAYNES 188 alloy HAYNES 214 alloy HAYNES 230 alloy HAYNES 242 alloy HAYNES 244 alloy HAYNES 263 alloy HAYNES .
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HASTELLOY G-3 alloy 0.935 HASTELLOY X alloy 0.925 HAYNES HR-160alloy 0.910 HAYNES 214 alloy 0.906 HAYNES 230 alloy 1.010 HAYNES 242 alloy 1.019 HAYNES 263 alloy 0.941 HAYNES 625 alloy 0.950 HAYNES 718 alloy 0.925 HAYNES 188 alloy 1.009 HAYNES 25 alloy 1.028
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Precipitation hardened (PH) Alloy 625 is a modification of Alloy 625 that permits strengthening by aging in significantly shorter times than can be achieved with Alloy 625. The strengthening occurs by the precipitation of 7" (Ni,{Nb,Ti,Al)
