THERMODYNAMIC AND KINETIC MODELING OF PRECIPITATION PHENOMENA IN P9 STEELS(4)

precipitates.

P92steel:Figure5showsthevariationofthephasefrac-tionofprecipitatesduringtheheattreatmentandthethermalex-posureat600 CofP92steel.Fourprecipitates,MX,M23C6andZ-phaseandLavesphaseappearinthemicrostructure.Z-phasestartsnucleatingrightafterthetempering.TheamountofZ-phasestartstoincreaseslightlyafter10,000hofthermalexpo-sureandcontinuestoincreaseuntiltheserviceterminates,whichisincontrastwithMXandM23C6exhibitingthesaturationofprecipitationinashorttime[19].Lavesphase,thoughsmallinquantity,formsduringthethermalexposureandkeepsoncoars-ening(Figure6).ThisisattributedtohighamountofWandMointhissteel.TheaveragesizeoftheLavesphaseprecipitatesreaches～1µmatabout100,000h.TheprecipitationofLavesphasecanimprovecreepstrengthifitscoarseningdoesnotpro-ceedtoofast.ThispositivebehavioroftheLavesphaseisseeninNF616(similartoP92)steel.However,thepresenceoflargeM23C6andLavesphaseparticlesaboveapproximately0.5µmaregenerallyconsideredtobedeleterious[20].

E9016steel:Figure7showsthevariationofthephasefractionofprecipitatesinthecaseofE9016steel.LikeinthecaseofP91steel,fourkindsofprecipitates,MX,M23C6,LavesandZ-phase,appearduringthecourseofthermalexposure.PhasefractionofM23C6precipitatesremainconstantafterattainingtheequilibriumvalueduringthethermalexposure.Signi cantamountofLavesisseenfromabout100honwards.TheamountofZ-phasestartstoincreasesigni cantlyafter10,000hofther-malexposureandcontinuestoincreaseuntiltheservicetermi-nates.ItiswellknownthatduringserviceZ-phasegrowsattheexpenseofMXin9-12%Crferritic-martensiticheat-resistantsteelscontainingNborVandahighcontentofnitrogen[17].ThisfactisclearlyevidentfromFigure7.Althoughthereisnosigni cantcoarseningseeninthecaseofMXprecipitates,Z-phase,LavesandM23C6seemtoundergocoarseningoncontin-uedthermalexposure(Figure8).

Reducedactivationferritic-martensitic(RAFM)steel:Figure9showsthevariationofthephasefractionofprecipitatesasafunctionoftimeforRAFMsteel.TheamountofM23C6,whichappearsduringearlystagesoftheheattreat-ment,remainsnearlythesameevenafter100,000hofther-malexposure.PhasefractionofZ-phaseexceedsthatofMXallthroughout.IncreaseinfractionofZ-phasebeyond10,000hattheexpenseofMXprecipitateisclearlyidenti ableinFig-ure9.CoarseningoftheM23C6precipitatetowardslaterstagesofthermalexposureisevidentfromFigure10.ThereisslightreductionintheaveragesizeofMXprecipitatesbeyond10,000h.Figure11showsvariationinthecompositionofM23C6as

afunctionoftime.ItisseenthatitsCrcontentincreasesandthereisacorrespondingdecreasetheFecontentasthethermalexposureadvances[22].

Conclusions

Comparisonofcalculatedphasetransformationtempera-tureswithexperimentalvaluesshowsthattheGibbsenergydatabaseusedhereisreliableinpredictingphasetransformationfeaturesofferritic-martensiticsteels.M23C6isamajorcarbideinallthesteelsconsideredhere,followedbyMXcarbide.InP91andRAFM,Z-phaseisalmostnon-existent.Whenitispresent,itsamountincreasesattheexpenseofMXcarbidesduringther-malexposure.E9016ismostseriouslyaffectedwithcoarseningoftheZ-phase.InP92mostsigni cantcoarseningisfortheLavesphase,althoughitsamountisquitelow.InmostvarietiesofsteelconsideredhereM23C6tendstoresistcoarseninguptoabout10,000hafterwhichittendstocoarsen.ItisalsoseenthatinM23C6theamountofCrincreasesandthereisacorrespondingdecreasetheFecontentasthethermalexposureadvances.Thekineticsimulationagreeswiththeevolutionofelementalabun-danceintheM23C6phaseinRAFMsteelobservedexperimen-tally.

Acknowledgement

Authorsthankfullyacknowledgemanyfruitfule-maildis-cussionswithProfessorErnstKozeschnik,MatCalcdeveloper,ViennaUniversityofTechnology,Austria.

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