造型铁素体合金元素对降水的影响

ModellingtheeffectsofalloyingelementsonprecipitationinferriticsteelsYouFaYin,RoyG.Faulkner
InstituteofPolymerTechnologyandMaterialsEngineering(IPTME),LoughboroughUniversity,Loughborough,Leicestershire,LE113TU,UKReceived9September2001;receivedinrevisedform16May2002AbstractThispaperreportsasystematicstudyontheeffectsofdifferentalloyingelementsontheprecipitationofM23C6particlesinpowerplantsteelsusingamodifiedmodellingtechnique.MTDATAisusedtocalculatesomeofthethermodynamicparametersforthealloyingsystem.Theelementsstudiedincludetungsten,aluminium,silicon,manganese,cobalt,copper,nickelandmolybdenumwithvaryingdegreesofconcentration.ResultsshowthatalltheelementsarebeneficialinreducingthecoarseningrateofM23C6precipitatesatlowconcentrations.CoarseningratedecreasesprogressivelywithincreasingconcentrationofSi,Co,AlandCu.Nicanbeveryharmfulbyincreasingcoarseningratewhenitsconcentrationisgreaterthanabout0.4at.%.BothMnandWarequiteeffectiveinreducingcoarseningratewhentheconcentrationsarelow.Extremelysharpincreasesincoarseningratearefoundwhentheconcentrationsofthesetwoelementsexceedabout0.5at.%.Moissignificantinreducingthecoarseningrateevenatverylowconcentrationsandtheeffectismorepronouncedwithincreasingconcentration.Alltheseeffectsresultfromthechangeofsolubilityofchromiuminthematrixcausedbythealloyingelements.#2002ElsevierScienceB.V.Allrightsreserved.Keywords:Precipitation;Modelling;Ferriticsteels;Alloyingelements;Grainboundary1.IntroductionInrecentyears,thereisincreasingdemandforincreasingpowerplantefficiencytoreduceboththecostofgeneratingelectricityandenvironmentaldamage.Thispromotesthedevelopmentofnewsteelsusedinsupercriticalpowerplant[1].Itiswellknownthatmicrostructuralevolutionofthesteelsdeterminesthelifeandlongtimebehaviourofthematerial.Oneofthemainfeaturesofthemicrostructureofsteelsisitsprecipitatepopulation.Thecoarseningrateofthesesecondphaseparticlesdeterminesthedegradationofthemechanicalpropertiesofthesteels.Thestabilityoftheseprecipitatesdependsonmainlythechemicalcomposi-tionandheattreatmentofthematerial.Itisofvitalimportanceinthedevelopmentofnewalloystounderstandtheeffectsofdifferentalloyingelementsonthemicrostructureandpropertiesofthematerial.Therefore,numerousexperimentalstudieshavebeencarriedoutontheeffectsofdifferentalloyingelementsonthemicrostructureandespeciallyonpropertiesofsteels[2,3].Thedifficultyofthiskindofexperimentalworkonsuchacomplexsystemisobvious.Inrecentyears,computermodellingand/orsimulationofmicrostructureevolutionandmechanicalpropertieshasbecomemoreandmoreimportant.Comparedwithexperimentalwork,computermodellingdoesnothavethedifficultiesofbeinglengthyorcostly.Computermodellingcangiveinsightsofstructureandpropertyevolutionandthereforeisabletopredictlongtimebehaviourofthematerial[4,5].ExamplesofsuchmodellingincludethemodeldevelopedbyBhadeshia’sgroup[6,7]andthemodelusedintheDICTRAsoftware[8,9].CarolanandFaulkner[10],andmorerecently,JiangandFaulkner[11]proposedamodelforinter-granularprecipitationthatisbasedontheconceptofcollectorplate.Itisunliketheabovemodelsthatareconcentratedonintra-granularprecipitation.Thismodelhasbeenreasonablysuccessfulwhenappliedtoaluminiumalloysandausteniticsteels[12,13].However,
Correspondingauthor.Tel.:/44-1509-223-153;fax:/44-1509-223-949E-mailaddress:[e-mail](R.G.Faulkner).MaterialsScienceandEngineeringA344(2003)92/102:S0921-5093(02)00418-5therearenosystematicstudiesoftheeffectsofdifferentalloyingelementsinsteels,duetothecomplexityofthesystemandthelackofthermodynamicdataavailable.MTDATA[14]developedbytheNationalPhysicalLaboratory(NPL)providesausefultooltocalculatethermodynamicparameters.Thispaperreportsasystematicstudyontheeffectofmainalloyingelementsontheprecipitationandcoar-seningofinter-andintra-granularM23C6precipitatesinpowerplantsteelsusingcomputermodellingbasedon[10,11]andMTDATA.2.Modellingdetails2.1.Inter-granularprecipitationmodelDetailsofthemodelforinter-granularprecipitationcanbefoundelsewhere[10,11].Themodelassumesthattheinter-granularprecipitatesarecap-shaped,asshowninFig.1.Basically,themodellingincludesfourstagesofcalculation.(1)Thesoluteconcentrationdistributionacrossthematrixisdetermined,i.e.segregationofsoluteatomstograinboundariesduringquenching.(2)Calcu-lationofnucleationsitedensityattheageingtempera-tureatgrainboundariesaccordingtothesoluteconcentrationcalculatedduringstageone.(3)Thenucleiformedinstagetwoareallowedtogrowattheageingtemperatureduetosolutediffusionfromgraincentretotheboundary.(4)Whenallsolutehasdiffusedfromcentretograinboundaries,coarseningoccurswherelargeprecipitatesgrowwhilesmalleronesdissolve,althoughonlytheaveragesizeofparticlesisconsideredintheactualmodelling.2.1.1.Non-equilibriumandequilibriumsegregationSegregationofsoluteatomstograinboundariescausessolutere-distributioninthematerial.Itisthoughtthatatageingtemperatureshigherthan0.55themeltingpointofthealloy,non-equilibriumsegregation(NES)mustbeconsidered,whilee