59-paper-The Nature and Consequences of Grain Refi

J.W.Morris,Jr.ProfessorofMetallurgyDepartmentofMaterialsScienceandEngineering210HearstMiningBuildingUniversityofCaliforniaBerkeley,California94720Tel:[phone]Fax:[phone]e-mail:[e-mail]enceandEngineering,Univ.ofCalifornia,Berkeley,CA,USAAbstract:Grainrefinementisusedintheprocessingofsteelfortwodifferentpurposes,whichareatleastsuperficiallycontradictory.High-strengthlathmartensiticsteelsaregrain-refinedtoimprovelow-temperaturetoughnessbyloweringtheductile-brittletransition.Forthismethodtobesuccessfulthegrainrefinementprocessmustincreaseresistancetocleavagefracturewithoutdramaticallyincreasingthestrength.Modernstructuralsteelsaregrain-refinedtoimprovestrength.Forthismethodtobesuccessfultherefinementprocessmustpreserveresistancetocleavagewhiledramaticallyincreasingstrength.Whiletheprocessesthataccomplishthesetwocontradictoryobjectivesaredescribedbythesameterm,“grainrefinement”,itseemsclearthattherearefundamentaldifferencesbetweenthem.Recentresearchsuggeststhatthekeyissueisthenatureofthephasetransformationthatisusedinrefinement.Inthecaseofmartensiticsteels,therefiningtransitionsarecrystallographicallycoherent,whileinmodernstructuralsteelstherefiningtransitions(e.g.,DIFTferrite)appeartobeincoherent.Thispaperaddressesthisissuefromtheperspectiveofhowthetwodifferentkindsoftransformationinfluencetheeffectivegrainsizesfor,respectively,transgranularcleavageandintergranularfracture.Keywords:Steel,grainrefinement,strength,ductile-brittletransition,transformations1.IntroductionGrainrefinementisusedintheprocessingofsteelforatleasttwodifferentpurposes.High-strengthlathmartensiticsteelsaregrain-refinedtoimprovelow-temperaturetoughnessbyloweringtheductile-brittletransition.Modernstructuralsteelsaregrain-refinedtoimprovestrengthwithoutsignificantlossoftoughness.Whiletheprocessesthataccomplishthesetwoobjectivesaredescribedbythesameterm,“grainrefinement”,therearefundamentaldifferencesbetweenthem.TheinfluenceofgrainsizeonthemechanicalpropertiesofsteelismostcommonlyexpressedinaseriesofconstitutiveequationsthathavetheHall-Petchform.Overtherangeofconventionalgrainsizes,thevaluesoftypicalmechanicalpropertiesincreasewiththereciprocalrootofthegrainsize.TheclassicHall-Petchequationrelatestheyieldstrengthtothegrainsize:y=0+Kyd-1/2(1)whereKyistheHall-Petchslopeanddisthemeangrainsize.Anidenticalrelationholdsfortheindentationhardness.Asimilarequationappliestothecleavagefracturestress(f)ofhigh-strengthsteels,f=Kfd-1/2(2)andanequationoftheHall-Petchformisalsooftenusefulforpredictingtheductile-brittletransitiontemperature:TB=T0-KBd-1/2(3)Whilethesameparameter,thegrainsize,d,appearsineachoftheseequations,itisimportanttorecognizethatgrainsizehasadifferentmeaningforeachofthepropertiesofinterest.2.TheEffectiveGrainSize2.1TheeffectivegrainsizeforstrengthSeveraldifferenttheorieshavebeenadvancedtoexplaintheHall-Petchrelationforstrength[1].However,theyhavethecommonfeaturethatgrainsizelimitsthedistanceoverwhichfreeslipcanoccur.InFe,theprimaryslipplanesarethe{110}planes,soslipislimitedbythedimensionofthe{110}planeswithinagrain.Hencetheappropriatemeasureofgrainsizewouldappeartobethecoherencelengthalong{110}.ThedependenceoftheHall-Petchcoefficient,Ky,onthepropertiesofthesteelarealsoatleastqualitativelycommontothevarioustheories.Forexample,theclassicpile-upmodeloftheHall-Petchrelationgivesy~32Gbbq1/2(4)whereqisageometricfactoroforderunity,Gistheshearmodulus,btheBurgersvector,andbisthecriticalshearstressforthetransmissionofslipacrossagrainboundary.Ifweexamineeq.(4)fromtheperspectiveofthecontrollabilityoftheHall-Petchcoefficient,thevariableparameterisb.Thisparameterissensitivetothemisorientationofthedominant110slipplanes.Increasingthetypicalmisorientationshouldincreaseb,and,hence,Ky,whichshouldbeaslargeaspossibletomaximizestrength.Thereis,however,aconstraintontheusefulstrengththatcanberealizedthroughgrainrefinement[2-4].Fortypicalsteels,theHall-Petchslopefortheultimatestrengthislessthanthatfortheyieldstrength,withtheconsequencethatthereisasignificantlossofductilityatfinegrainsize.Whilerecentresultssuggestthatitmaybepossibletomaintainusefulductilityinultrafine-graineddual-phasesteels[5],lowtensileductilityisaseriousconcernforsteelswithgrainsizebelowabout2µm.2.3TheeffectivegrainsizeforcleavageandthebrittletransitionSinceironcleaveson{100}planes,theappropriatemeasureofgrainsizeforcleavageshouldbethecoherencelengthon{100}.TheHall-Petchcoefficientforcleavagefracture,Kf,shouldincreasewiththemisorientationof{100}planesacrosstheboundaries.Inmanysteels,particularlymartensiticsteels,thiscoherencelengthissubtleanddifficulttomeasure[1,6].Themicrostructureofatypicallathmartensiticsteelismadeupofprioraustenitegrains,eachofwhichcontainsaverylargenumberofdiscretelathsofdislocatedmartensite.Themartensitelathsareorganizedintopackets,inwhichthelathssharethesamehabitplane,andthepacketsmaybesubdividedintoblocksinwhichthelathsarecrystallographic