ISOTHERMAL M2C CARBIDE GROWTH IN ULTRAHIGH STRENGTH HIGH Co-Ni STEELS

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Hyuck Mo Lee, Hyunchul Sohn* and Choong Hwa Yoo** Departmentof MaterialsScience& Engineering,KoreaAdvancedInstituteof Scienceand Technology, Kusung-Dong373-1,Yusung-Gu,Taejon,Korea305-701 *MemoryProcessTeam,HyundaiElectronicsIndustriesCompany,Ichon,Korea467-860 **ColdRolling Tern,Winks SteelDepartment,PohangSteelCompany,Pohang,Korea790-360 (ReceivedJanuary27, 1997) (AcceptedSeptember, 1997) Introduction The precipitationof free-scaleMZCalloycarbidesgivesrise to secondaryhardeningbehaviorwhenthe strongcarbide-formingelementsMo and Cr are addedto high Co-Nisteels[1]. However,this reaction is precededby the formationof metastablecementiteprecipitates.The cementitesare quite coarseand thereforelimitthe Iiacturetoughnessof manyultrahigh-strengthmartensiticsteels.A prolongedtempering treatmentis requiredto dissolvethe cementite.The stabilityand resistanceof the MZCdispersion with respectto Ostwaldripeningare importantsincecoarsenedparticlescausea drop in strength and can even cause embrittlementthroughgrain boundaryprecipitation.A modelwas introducedfor the coarseningresistanceof multicomponentcarbides[2]. Whilethe model treated the coarseningof shape-preservingparticles,it was applicableto non-spherical,in particular,rod-likeMZCparticlesin AF141Osteel [3]. Quite recently,CarpenterTechnologyCorporation[4] developedand patentedan ultrahigh-strength martensiticsteel,namedAerMet 100,whichis strongerbut has lowerfracturetoughnessthan AF141O. As the material itself is a recent development,the publishedstudies on it are quite limited and they primarilyinvestigatedthe mechanicalpropertiesafter isochronalaging for only about 5 hours [5,6].In another studyby these authors[7], this alloywas tested and analyzedin terms of the secondaryhardening reactionin temperingat severaldifferenttemperatures.In this work, the growth and coarsening behaviorof the MzCphasemainly at 482°Cwill be exploredalongwith measurementsof latticeconstantsand orientations. ExperimentalProcedures The compositionof the alloyused in this work is Fe-13.4Co-ll.lNi-3.2Cr-l.2Mo-0.23C (in mass%). Specimenswere homogenizedat 1200”Cfor 8 hours,fbrnacecooled then austenitizedat 885°Cfor 1 hour, quenchedin oil to room temperature,and immediatelytransferredto a cryogenicbath of liquid nitrogenfor 15 minutes.The temperingtemperaturewas 482°C and the temperingtimes spannedthe 1931

1932

range horn 5 to 200 hours.All of the sampleswere furnacetreatedafter beingsealedin silica capsules under an Ar atmospherein orderto preventoxidationand decarburization. TEM was used to observeprecipitatedMzCcarbidesand to determinetheir sizeand morphology.To prepare thin foils for TEM, specimenswere cut horn heat-treatedsamples,groundto a thicknessof 50-60 ~m, and electro-polishedin a perchloricacid-ethanolsolutionat -35*C.Extractionreplicaswere also taken horn samplestemperedfor 200hours.To producethese,specimenswere mounted,polished conventionallyand etched with 5 nital. After coatingwith carbon,replicaswere extracted in 10°/0 nital. In this study,a PhillipsEM300(operatingat 160kV), a TopconEMO02B(operatingat 200 kV: high-resolutionTEM) and a HitachiNAR 9000 (operatingat 300 kV) were used. The Iatticespacing was measuredusingthe 0.27run spacingofthe {002}siliconplanesin the high-resolutionTEM image of a singlecrystallinesiliconspecimenas a magnificationstandard.The same position and the same currentswere used. Results Severalhundredparticleswere examinedfor each heat treatmentto obtainreliabledata. The result is shown in Table 1 alongwith the data of measuredaspectratios.They were measuredusing the beam directionof c1OO>of the matrixand dot-shapedparticleswere excludedin measurementbecausethey were seeminglyalignednormal to the plane. They are close to 3 in all cases, which means that the aspectratio is not affectedby temperingtime. The averagelengthsof MzCcarbideprecipitatesmeasured in this studyare in reasonableagreementwith thoseof Novotny[5] and Ayer and Machmeier[6]. The samplestempered at 482°C for 5 hours had an average length of 9.5 nm, compared to 9.1 mu reportedby Novotny [5] and 8.5 nm measuredby Ayer and Machmeier[6]. The average diameterof MIC carbideprecipitatesobtainedin this study,3.1 nm fromsamplestemperedat 482°Cfor 5 hours,is comparableto the 2.5 runreportedby Ayer and Machmeier[6].Novotny[5] did not reportthe carbide rod diameters. In the case of carbidesextractedfrom the sampletemperedfor 200 hours as shown in Fig. l(a), the original alignmentis lost. To determinewhetherthe precipitatedcarbides are indeed MzC, selected area diffractionpatterns(Fig. l(b)) were obtainedfrom a numberof particles.To measurethe correct latticeconstants,specimenswere coatedseparatelywith a 5 rim-thicklayer of Al as a calibrationstandard. The lines are indexedfrom the center (100), (002), (101), (102), (110), (200), and (112). It is indexedas hexagonalas in the pioneeringwork of Nagakuraand Oketani [8], and the Iatiicepammeters a and c are measuredto be 0.298and 0.473run,respectively. Discussion The selected-areadiffractionpatternfromthe MzCcmbideswasthe sameas that frommeasurementby Lee and Allen on AF141O[3] and quite similarto that fromthe work by Ayer and Machmeier[6] on TABLE1 AverageSize(innm)andAspectRatioofRod-shaped MzCCarbides I T

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b Figure 1.(a) TEMmicrographshowingrod-shapedM,C carbidesextractedfromAerMet100temperedat 482°Cfor 200hrs. (b) is a selectedareadiffractionpatternfromthe samespecimenof (a).

The latticeconstantsof hexagonalMzCphasemeasuredfromthe selectedarea diffraction pattern in this study are somewhatlarger than that from AF141Oas measured by Montgomeryand Olson [9]. This discrepancyis probably due to the compositionaldifferenceof Cr and Mo in MzC phase,althoughthis was not quantitativelydeterminedin this study. Using Vegard’srule and citingthe latticeparametersof variousMzCcarbidesin the work of Olson [10] and Montgomeryand Olson [9], Allen et al. [11] reported on the empiricalrelationshipin the latticeparameters,a and c, of (Cr,Mo)zC.Adoptingthe sameequationsof Allenet al. [11],the composition of MZCin this studyis calculatedas (CrO165Mo0835)2C based on the me=ured latticeconst@ a, and ( basedon anotherlatticeconst@ c. This discrepancyin the compositionof MX c phaseremainsto be resolvedby scanningTEMor atomprobe/fieldionmicroscopy(APFIM). Accordingto Ayer and Machmeier[6], at least some of the precipitateswere crystallineMzCcarbides with the well-knownhexagonalstructurein the early temperingstage of 5 hours at 482”C.This

was based on their [101]diffractionspots.In this study,a high-resolutionTEM image of the carbide, Fig. 2(a), was obtainedfroma cmbonreplicaof a specimentreatedat the sametemperingcondition,5 hours.Thusthis particleis crystalline,not a cluster.In the figure,latticeplanesrepresentedas A, B and C are foundto cross one anotherat 64°, 52°and 116°.Moreover,their spacingis calculatedas 0.259, 0.227 and 0.227 —— nm. This result impliesthat the zone axis of the carbide is [11~~]. The pyramidal plane of(O111 ) meets the prism plane of ( ITOO)at 64° and anotherpyramidalplane of (1011) at 52°. From this analysis,the lattice constantsof hexagonalMzCphase, a and c, are measured to be 0.300 and 0.473nm, respectively.The sameis true for the specimentemperedfor 200 hours as seen in Fig. 2(b). Lattice constantsof the carbidephasehas not changedat all after 5 hours of tempering at 482°C,even thoughthis measurementcamefrom only one particleat a time. This also meansthat the MzCphasemaintainsan incoherentstatewith thematrix.TheWorkfor MzC in AF141Omeas~ed by Montgomeryand Olson [9] showedthat they increasedwith time and the temperingstage at 51O”C includedboththe coherent(before8 hours)and incoherent(after 100hours)states. If coarseningis dominatedby volumediffusionof solutes,the size is expectedto increasewith cube root of time accordingto the classicalLifshitz-Slyozov-Wagner(LSW) coarseningtheory [12,13]. Based on this model, there has been a body of work on coarseningin ternary or higher order alloys [2,14-16].However,most of the thermodynamicand kineticparametersin the models describingthe multicomponentcoarseningkineticsare unknownand hard to measure.Amongthem,the modelwhich was introducedfor the coarseningkineticsof multicomponentcarbideprecipitates[2] and appliedto explain the coarsening of shape-preservingnon-spherical,in particular, rod-like MzC particles in AF1410steel [3], is used in this study.

(a) Figure2. HREMimageof rod-shapedM carbideextractedfromthe specimentemperedat 482°Cfor (a) 5 hrs and (b) 200hrs. Thecarbideis orientedin the [ 11~] direction.(FigureContinued.)

37, No. 12

CARBIDE GROWTH IN Co-Ni STEELS

(b) Figure2. (FigureContinued.)

1935

Co-Ni STEELS

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Accordingto the modelby Lee et al. [2],the coarseningrate equationin the formof cubed diameter vs time is givenby the followingform: -3 -3 d -do= Kdt

(1)

And, the rate constantKdis definedas (2) where ~ the MzCphase, ct the matrix phase, o, the interracialenergy, V~ the molar volume of the MzCphase, A, the aspect ratio, ki a partitioningcoefficientof elementi, LXithe difision coefficient of M in cx,Xti,. the molarIiactionof M in the matrixphaseat equilibriumwith the particleof infinite radius. From the measuredcoarseningrate constantof the MzCphasein Eq. (l), 1.662x 10-31m3/secas in Table 2, the interracialenergywill be inferredand its implicationswill be discussed.The term in the bracket of Eq. (2) can be calculatedusing the Fe databaseof the Thermo-Calcprogram [17] and diffusivity data shown in Table 3. Now this term is to be multiplied by the preceding term of 646, V~/9RTln(2A,)in Eq. (2). Here, the knownparameteris RT, and the rest are unknownyet. First, the aspect ratio, A,, is not known clearly but was observedduring the experimentsand found to be comparativelya constant.Therefore,an averageA, was used. The molar volumeof the MzCphase, V%, originated from the normalized Gibbs-Duhemequationwhich deals with a mole fraction [2], which meansthat the molar vohunefOrthe hcp MzCphase is that for the mcdecularform of M wherethe total numberof molesof all speciesbecomesone.Thus,it is calculatedas follows:

(3) = 7.3859x IOAm3/molar unit where the lattice constantsof a andc weremeasuredinthe studyand A n Using all these parameters,the interracialenergyis calculatedas 0.6 J/m*.As this study is concentrating on incoherentMzC p J/mz whichis tYPicalOfthe ceTABLE 3 Diffusion Coefficient Data Used in the Calculation of the Rate Constant Defined by Eq. (2) D

R

- 8 =1 w

c u

( J

mentitephase in the iron-richmatrix [18].If all of the circumstancessurroundingcoarseningbehavior are true for assumptionsmade and all of the parametersexceptinterracialenergy are correct and experimentaldata are flee fromany kind of experimentalerror,then the calculatedinterracialenergywill be a correctvalue.However,suchidealresultsare unlikely.Nonetheless,overall,the coarseningmodel appearsreasonable. Acknowledgments Many discussionswith Prof. J.Y. Lee of KAIST are acknowledged.This work has been financially supportedin Koreaby KoreaScience& EngineeringFoundation(KOSEF)underGrantNo. 961-0801009-2. Specimensused in this studywere suppliedfrom CarpenterTechnologyCorporationand U. S. Naval Air Warfare DevelopmentCenter.Part of this studywas performedat the Lawrence Berkeley Laboratorywhere one of the authors(HML) was on a sabbaticalleave which was supportedby the Center for Interface Scienceand Engineeringof Materialsat KAIST. Their hospitalityis gratefully acknowledged. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

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G.R.Speich,D.S.DabkowskiandL.F.Porter,Metall.Trans.,4,303 (1973). H.M.Lee, S.M AllenandM. Grujicic,Metall.Trans.A, 22A,2863(1991). H.M.Lee andS.M.Allen,Metall.Trans.A, 22A,2877(1991). U.S. PatentNo. 5087415,CarpenterTechnologyCorporation,Reading,PA(1992). P.M. Novotny, Proceedings of G.R. Speich Symposium,eds. G. Krauss and P.E. R I S S PA,215(1992). W R. AyerandP.M.Machmeier,Metall.Trans.A, 24A, 1943(1993). C.H.Yoo,H.M.Lee,J. ChrmandJ.W.Morris,Jr., Metall.Trans.A, 27A,3466(1996). S. Nagakoraand S. Oketani,Trans.IronandSteelInst.Japan,8,265 (1968). J.S. Montgomeryand G.B.Olson,Proceedingsof 34thSagamoreArmyMaterirdsResearchConference,eds. G.B.Olson, M. AzrinandE.S.Wright,U. S. ArmyMat.Tech.Lab.,Watertown,MA, 147(1990). G.B. Olson, Proceedingsof 34th SagamoreArmyMaterialsResearchConference,eds. G.B. Olson, M. Azrin and E.S. Wright,U. S. ArmyMat.Tech.Lab.,Watertown,MA,3 (1990). A.J.Allen,D. GavilletandJ.R. Weertman,ActaMetrdL,41, 1869(1993). I.M.LifshitzandV.V.Slyozov,J. Phys.Chem.Solids,19,35 (1961). C. Wagner,Z. Electrochem.,65,581 (1961). A, UmantsevandG.B.Olson,ScriptaMetalI.,29,1135(1993). J.E. MorrrdandG.R.Purdy,J. AlloysandCompounds,220, 132(1995). C.J.KuehmannandP.W.Voorhees,Metrdl.Trans.A, 27A,937(1996). B. Sundman,B. JanssonandJ.O. Andersson,CALPHAD,9,153 (1985). S. Bjorklund,L.F. DonagheyandM. Hillert,ActaMetalI.,20,867 (1972). A.W.BowenandG.M.Leak,Metall.Trans.,1, 1695(1970). V.T.Borisov,V.M.GolikovandG.V.Sherbedinskiy,Phys.Met.Metallogr.,22, 175(1966).