DOC

Morphology

By Ann Lopez,2014-07-24 00:03
7 views 0
MorphologyMorpho

    Morphology

;!!:8,Volume2,No.12(SerialNo.13)

    ;Journalof

    ;

    ;MaterialsScienceandEngineering,ISSN1934.8959,USA ;Morphologyandformationmechanismofbainitecarbide ;LIUZong-chang,WANGHai-yan,LIWen-xue,RENHui-ping,DUANBao-yu ;(SchoolofMaterialandMetallurgy,InnerMongoliaUniversi~ofScienceandTechnology,

    Baotou014010,

    ;China)

    ;Abstract:TEMobservationandtheoreticanalysiswerecarried ;outonbainitecarbideofindustrialsteels.Theresultsindicated ;thatthemorphologyofbainitecarbideshowmultifariousshapes, ;likelath,fibrouswedgeandsoon.Therewasnoconditionof ;carbideformedinintracrystallineofbainiteferrite.bainite ;carbidenucleatedontheboundaryofbainiteferriteandaustenite ;(BF/7),andgrownalongtheboundary.The0-(Fe,M)3Cof ;upperbainitearrangedwithparalleltoferritebyandlarge.The ;lowerbainitegrownintogapofferritesubunitswith ;wadgeshapeorgrownintocarbiderichareasofaustenite,and

    ;presentcertainanglewithprincipalaxisdirectionofferritelath. ;Thecarbideofupperandlowerbainite,wouldbebesiegedby ;ferritesaftergrowthstopped;andthecarbidedistributedin ;bainiticferriteintheform.Intheformationprocessofbainite. ;carbonsdiffusewithlongrange,ironsmovealongboundaryand

    ;reconstructlatticebymeansofheatactivationtransition. ;Keywords:bainite;carbide;diffusion;integrateinasystematic ;way;heatactivationtransition

    ;1.IntrOductiOn

    ;Manyinvestigationshavebeencarriedoutonthe

    ;bainitetransformationsdynamicsfortheremarkable ;characterinapplicationofbainiticsteels.Bainite ;carbide(BC)isoneoftheimportantphasesinbainite, ;andthebainitewithoutcarbideiscarbide.freebainite. ;Therearemanyworkscarriedoutontheformation

    ;mechanismOfBC.AndtheBChastwotype

    ;s,9_Fe3Cands-Fe2.

    ;4C,buttheformationmechanism

    ;ofwhichhasn’tbeenstudiedclearly,andthe

    ;viewpointsaboutprecipitationplaceofBChasnot ;agreeonyet.TheproblemoftheBCseparatedoutfrom ;a,ortheinterfaceofa/yhasbeendisputedindifferent ;papersInthispaper,GCrl5steel,P20steel,9Cr2MO ;steel,718steel,23MnNiCrMosteel,T8steel,20CrMo

    chang(1940),male, ;C0rrespondingauthor:LIUZong

    ;professor;researchfields:solidstatephasetransformationand

    mail:lzchang75@163.com. ;heattreatment.E

    ;58

    ;steelwereisothermalquenchedintonitratebathafter ;austenize,andformedbainite.ThemorphologyofBC ;wasobservedbySEMofQUANTA400andTEMof

    ;JEM2010.TheintegratedmechanismofBC

    ;nucleationgrowthwasputforwardthroughtheoretical ;analysis.

    ;2.Morphologyofbainitecarbide

    ;2.1Carbidemorphologyofupperbainite

    ;Itiswellknownthatthecarbideofupperbainiteis ;mainlythe0(Fe,M)3C,upperbainiteofthe

    ;carbidefreebainiteorgranularbainitehavenocarbide, ;andthereis0.carbideonlyinsomefeatherybainite. ;Thespecimen0fGCr15steelswereaustenitized ;at1000?,isothermalquenchedat450?.then

    ;observedbymeansofSEMofOUANTA400after

    ;polishingandetching.Fig.1andFig.2show ;feathershapebainite.Lath.shapebainitenucleatedon ;interphaseofausteniteinFig.1,andgrownlikefeather, ;andcarbidedistributedparalleltoaxialdirectionofthe ;bainite1aths.

    ;Fig.1SEMupperbainiteofGCrl5steel

    ;(thesecondaryelectronimage)

    ;Morphologyandformationmechanismofbainitecarbide ;AscanbeseenfromFig.2,thecarbideand

    ;bainiteferritedistributedaltematelywithlamellar ;structure.Thebrightlamellasarecarbidelikefilament ;intwo.dimension.Andthefilamentshavedifierent ;lengthandfigure,someofthemarestraightwhile ;othersarebend.Mostofthecarbidearrangealong ;principalaxisoftheferrite.Aroundbainitelaththere ;iSlathmartensiteandretainedaustenite. ;Fig.2SEMupperbainiteofGCrI5steel

    ;(thesecondaryelectronimage)

    ;Fig.3showstheTEMimagesofupperbainiteof ;plasticmoldsteel35Cr2Mo(P20).Therodshapedor

    ;flakeshapedcarbidedistributedinbainiteferriteand ;paralleledwiththeferritelathsgenerally.Theanalysis ;ofdiffractionshowedthatthecarbideare0-(Fe,M)3C. ;Fig.3TEMofupperbainiteof35Cr2Mosteel

    ;Itisgenerallybelievedthattheupperbainite ;carbidedistributesamongsubsliporsubunitofferrite ;,

    ;andarrangesparalleledalongprincipalaxisofthe ;ferritewithrodshapedorgranuleshaped.Andsome

    ;scholarsconsiderthattheBCISirregular

    ;riband—shaped[“.

    ;2.2Carbidemorphologyoflowerbainite

    ;LowerbainiteiSanintricatestructureintegratedin ;asystematicwaywhichismadeupwithbainiteferrite, ;bainitecarbide.retainedausteniteandSOon.ThereiS ;nocarbideinmetalowerbainite,butretainedaustenite ;exists.Thelowerbainiticcarbidemaybe0carbideor

    ;8-carbide,andthecarbideiSlathshape,coshor

    ;discontinuousspottedstate,distributedinferriteshaped ;lath.pinorlentoidandmad55-60.angelwithlongaxes ;offerrite.Fig.4showsthelowerbainitestructurefor ;plasticmoldsteelP20isothermalquenchedat370?.

    ;Therodshapedorflakeshapedcarbidemade5560.

    ;angelwithlongaxesofferrite(Fig.4).Analysisof ;diffractionshowedthatthecarbideiS0cementite.The

    ;e-carbideseparatedveryfastfromlowerbainite,and ;translatedinto0-cementiteafterisotherlTlaquenchina

    ;longtime1.2].

    ;Fig.4TEMoflowerbainiteofP20steel

    ;(isothermalat370*(2)

    ;59

    ;Morphologyandformationmechanismofbainitecarbide ;Lotsof0-cementitewasseparatedoutinthe

    ;lowerbainiteofhighcarbonstee1.

    ;Lowerbainiteand

    ;martensitestructurewereobtainedafteraustenitiseof ;9Cr2Mosteelsat1100?,thenfollowedisotherInal

    ;quenchat350?.themicrostructurewasgivenin

    ;Fig.5.Itindicatesthatthelowerbainitelathcould ;nucleateonboundaryorinintracrystalline. ;Fig.5SEMoflowerbainiteandmartensiteof9Cr2Mo ;steel(thesecondaryelectronimage)

    ;Fig.6SEMoflowerbainiteofGCrl5steel

    ;(thesecondaryelectronimage)

    ;Fig.6showsthelowerbainitestructureofGCrl5 ;steelsspecimen,austenitizedat1000?,and

    ;isothermalquenchedat300~C.

    ;TheFig.6(a)shows

    ;oneoflowerbainitelathisothermalquenchedfor ;10min,whichwassurroundedbyretainedaustenite ;andmartensite.Thelathmadeupwithsomesub..1ath ;andthecarbideinthesublathhadanangelwith

    ;principalaxisofthelath.Fig.

    ;6(b)showslower

    ;bainitestructureisothermalquenchedfor10min,in ;whichthenumberofbainitelathhaveincrease.the ;mostofsheetshapedandfibreshapedcarbidehadan

    ;differentangelwiththeprincipalaxisofthebainitic ;ferritelath.

    ;Tosumup,themorphologylowerbainitic

    ;carbideiscomplex,likerod,flake,fibre,lathandso ;on.Andmostofthemdistributedinintracrystalline ;andhadananglewiththeprincipalaxisofthebainite ;ferrite1ath

    ;3.Nucleationofbainitecarbide

    ;3.1Sourcesofbainitecarbide

    ;Thecarboncontentofbainitecarbide.0Fe1C

    ;anda-Fe24C,isveryhigh,whichisdifferentfromthat ;ofaustenite.Sotheformationofbainitecarbideisa ;masstransferprocessanddiffusetapephase

    ;transitions.

    ;Thesourcesofbainitecarbidethatfrom0,1,or

    ;theboundaryofa/y,hasbeenacontroversialfocus.In ;fact.itisaproblemaboutthenucleationplaceof ;carbide.Thatmeansthecarbidenucleatedatbainite ;ferrite(BF),austeniteorheboundaryofBF/y. ;Thecognitionaboutthesourcesofupperbainite ;carbide,0.cementite,wasbasicallyconsistentthatthe ;cementiteseparatedfromtheausteniteofrich.carbon ;andexistedamongbainiteferriteorexistedwith ;remainedausteniteiointly.Butaboutthe1owerbainite ;carbide.itwasnotconsistent.Onepointwasthat ;thoseinitialbainiteresembledmartensiteswere ;providedwithcarboniccontentofsaturation.The ;bainiteferritecontainedwithsupersaturatingcarbon ;andthecarbideseparatedoutfromsupersaturation ;carbonofferriteButsomescholarswhoholdthe ;Mornholo~vandformationmechanismofbainitecarbide

    ;pointofdiffusiondidnotagreewiththat.They ;thoughtthatBCcamefromausteniteandthecarbon ;contentofbainiticferritewasatequilibriumstate.In ;thenucleationgrowthprocessofbainiticferrite,a ;massofcarbondiffusedintoausteniteatthebeginning ;offormationofferrite,Thecarbidewererootedin ;richcarbonofaustenitewhichwasinthefrontedgeof ;ferrite’sgrowth,andthecarbideweresurroundedby

    ;ferriteintheprocessofgrowth.Sothecarboncontent ;isclosetoequilibrium(<0.03wt%)[‘.

    ;3.2Precipitationofe-carbidefrombainite ;ferrite

    ;Thecarboncontentofferritewhichisconfined ;accordingtoironcarbonequilibriumdiagramcarbon ;is0.0218%Cfmassfraction%)at727?.whichisthe

    ;mostsolubilityofcarboninC-Fe.Thesolubilityof

    ;carboninferriteatroomtemperatureislessthan ;0.001%Cfmass%1[4..Thevalueshowsthatferritedo ;notcontaincarbonalmost.Thesolubilityofcarbonin ;austeniteisgreater,becausecarbonistheelementof ;enlargeausteniteregion.

    ;Itiswellknownthatthebainitetransformationis ;nonequilibriumtransformationofsupercooling ;austeniteatintermediatetemperature.Atinitialstages ;ofbainiteformation,thecarboncontentis ;supersaturating,butitisdifficulttomeasurethe ;degreeofsupersaturatingpractically.Itisassumed ;thatthereisoneFeCalloywhichcarboncontextisC.

    ;Fig.7showsthattheausteniteissupercooledto ;intermediatetemperature(T),whichenteredintothe ;metastableregionofA+Fandtranslatedintothe ;bainiteferritewithCBFandretainedaustenitewithCy. ;Soifbainitewerecoolingtotheroomtemperature, ;thatcarboncontentofthebainiteissupersaturatedand ;thedegreeofsuperstitionishigherthan0.0218%.Itis ;impossibletomeasurethecarboncontentofbainite ;ferrite.Infact,thereisnowaytomakethefleshBF. ;Accordingtoinvestigationthetimeofventingcarbon ;for500nmthicklathofupperbainitic,

    ;ferritewas

    ;estimatedonlyafewmilliseconds)J_Thecarbon

    ;contentofupperbainiticferritewasabout0.03% ;(mass%)whichwasestimatedbyAaronsonet.al.

    ;ButFig.4showsthatthegreatestdislocationdensity

;withinthebainiteferriteis2×1010cm.Whenthe

    ;dislocationdensityofmartensitereachedto ;2xl010cm.thecarboncontentwasabsorbed0.2% ;(mass%)[,squaredegreewasnotappearing.

    ;Thereforetheupperdislocationdensityofbainite ;ferriteabsorbedlotsofcarbonandthecontentof ;supersaturatedcarbonisgreaterthantheequilibrium ;ferrite’s.Infact,bainitewereformedinintermediate

    ;temperatureandthencooledtoroomtemperature ;rapidly,whichisdifficultforcarbontoprecipitate ;equilibriumfromferrite.Soferritewassupersaturated ;bycarbon.

    ;

    ;p

    ;\

    ;L,

    ;,

    ;

    ;//,s’

    ;P{}:I

    ;CFCBvC0

    ;Fig.7Metastablebalancesketchmapof

    ;supercoolingaustenite

    ;Tosumup,thecarbonofbainiteferriteis

    ;supersaturated.Thedegreeofsupersaturationisnot ;greatandlessthan0.2%(mass%).Atthedegreeof ;supersaturating,theBFcouldnotseparatedout ;E-carbide.

    ;3.3Nucleationspotofbainitecarbide

    ;Theformationofbainiticcarbideincludes ;nucleationandgrowth.Accordingtotheruleof ;transformation,nucleationofbainitecarbideis ;heterogeneousnucleationthatnucleatesonthecrysta1 ;defectfirst.1ikeboundary11.Thebainitecarbide

    ;nucleatedontheausteniticboundarybetweenbainite ;ferritelathsornucleatedbetweensubunitsobserved ;bySEM.TheFig.8(a)and(b)showthelowerbainite ;ofP20steeIandthe23MnNiCrMostee1.respectively. ;TheBCindicatedbyarrowheadwas0carbide.BC

    ;61

    ;Morphologyandformationmechanismofbainitecarbide ;nucleatedontheboundary,grownwiththeshapeof ;cuniformandthenwassurroundedbyferritewhenthe ;growthwasstopped.Fig.8(b)alsoshowsthatcarbide

    ;distributeintheinnerofferritelaths(shownby ;arrow).

    ;Fig.8TEMof(a)lowerbainiteofP20steeland ;(b)23MnNiCrMosteel

    ;Fig.9showsthatthelowerbainite0fGCrl5 ;steelwasaustenizedat950?andisothermaltreated

    bynitratesaltbath.thecarbidegrownwith ;at220?

    ;theshapeofcuniforn1.Thusitcanbeseenthatit ;nucleatedonboundary,sothesizeatboundarywas ;big,andbecametapergraduallytotheshapeof ;cuniforillwhenitgrownahead.

    ;Fig.9SEMoflowerbainiteofGCrl5steel

    ;(thebackscatteredelectronimage)

    ;Therehavebeenfewreportsaboutnucleation ;mechanismstudyofbainiticcarbide.Somescholars ;broughtforwardstep?nucleationmechanism,they ;consideredthatbainiticcarbidenucleatednearby ;stepfaceofa/yboundaryandcarbidecomesfrom ;richaustenite.

    ;62

    ;Thecarboncontentofs-Fe2.

    ;4Cande.Fe3Cis

    ;7.9%and6.7%(wt%)respectively,andwhen ;formationofcarbide,itneedsthecarbonconcentration ;fluctuationhigh.Butitisdifficulttoreachthehigh ;concentrationfluctuationinferrite,andthes-Fe24C ;andthe0-Fe3Cformedthestructurefluctuationhardly ;too.soitisverydifficulttoformthecarbidenucleate ;inthebainiteferrite.

    ;Carbonistheelementenlargedtheregion.

    ;Whenthesubunitsofbainiticferriteformed,carbon ;wouldbecontinuouslyventedtoaustenite,and ;austenitedissolvedlotsofcarbonsandbecome ;richcarbonaustenite.Especially,ontheboundaryof ;BF/y.Itiseasyforcarbontoadsorbandprecipitate. ;Thediffusingrateofcarbonalongboundaryisvary ;fast.andtheboundaryisadjacentwithrich.carbon ;austenite.soitisadvantagedforthenucleationof ;carbide.andcOncentrationfluctuationandstructure ;fluctuationmeettheneedofcarbidenucleuseasily. ;Itiseasytoappearoftherandomfluctuationon ;BF/yboundary.Threekindsofnonlinearitypositive ;feedbackactionsmakethecrystalcellofBCstructure ;formedandreducethefreeenthalpyofsystem.Thus

    ;wecansaythatBF/yboundaryistheadvantagedplace ;ofcarbidenucleation,andthatinagreewith ;experiment.

    ;4.Growthofbainitecarbide

    ;Bainitecarbidegrowsincessantlywith

    ;repetitiousformofferritesubunit.Thisprocessisan

    rangdiffusetransportprocessof ;incessantlong

    ;carbonsalongboundary,andisaheatactivation ;transitionprocessofironsontheboundary. ;Upperbainiteferritenucleatedataustenitic ;boundary,andthengrownalonghabitplaneinto ;austenite,arrangingparallelwiththeshapeoflaths, ;andthereisrichcarbonaustenitebetweenlaths(Fig. ;10).TheupperbainitecarbidenucleatesontheBFb/ ;boundaryandgrowsintorichcarbonaustenite,hence,

    ;rod--shapedorfibroid??shaped0-cementitearranges ;Morphologyandformationmechanism——

    ;ofbai

    ;..

    ;nite.

    ;c

    ;.....

    ;a

    ;.......

    ;r

    ;...

    ;b

    ;...

    ;i

    ;....

    ;d

    ;..

    ;e

    ;

    ;parallelalongprincipalaxisofferritedeservedly(Fig ;1andFig.21.

    ;Fig.10SEMthegrowthofbainiticferritelamellarwith ;parallelfromgrainboundarytointracrystalline ;Fig.11istheformedprocesssketchmapabout

    ;upperbainiticcarbide,andFig.12isthesketchmap ;offormationoflowerbainiteferritelathsandsubunit, ;andthesketchmapaboutnucleationgrowthof

    ;carbide.

    ;f-~-poor-carb.on

;YY

    ;.b

    ;(c)

    ;bounda~

    ;(b)

    ;0fBF

    ;carbon

    ;cboundaw

    ;Fig.11Formationprocesssketchmapof

    ;carbideinupperbainite

    ;Fig.12(a)showsthatpoorcarbonand

    ;rich.carbonareaswereformedduetofluctuationon ;boundaryordeficiency.Wjththenonlinearityaction ;oftheenergyandstructurefluctuation,subunitofBF ;nucleated(Fig.I2(b)),andthegrowthofferrite ;dependsontherepeatedformationofthesubunit.The ;repeatedformationofsubunitmakesthearound ;austenitecarbonrichcontinuously,andnucleusof ;0-cementiteformedontheBF/7boundarydueto ;fluctuation(Fig.12(c)),thentheBc/BFboundaryand ;,

    ;I

    ;,l

    ;..rr

    ;;ysubun-tcBy

    ;rich-

    ;I

    ;c

    ;

    ;a

    ;

    ;rbon

    ;.

    ;

    ;,

    ;,

    ;,,

    ;subunit

    ;.

    ;?

    ;

    ;/

    ;(

    ;Fig.12Sketchmapofnucleationandgrowthoflower

    ;bainiteferritesubcellandbainiticcarbide ;Theexperimentobservationslndicatethatthe ;lowerbainitecarbidenucleatedontheB|boundary. ;Becausethediffusingrateofboundaryisveryfast,itis ;easyforcarbidegrowingalongboundary.Andcarbon ;wasventedwhentheferritegrown,itaccelerated ;growthofcarbideadjacentferrite.Intheprocess,the ;growthofcarbidemadethecarboncontentlessin ;austeniteatthecircumambienceofcarbide,andit ;acceleratedthegrowthofferriteunceasinglytoo.The ;sustaininggrowthofcarbidewascircumscribedby ;longrangediffusionofcarbon.Whenthesupplyin ;short,thegrowthofcarbidewouldbeboggeddownand ;stopped,andbeoutflankedbyferritewhichisgrown. ;Simultaneity,carbidecouldgrowintotheaustenite, ;whichmadecircumjacentaustenitecarbonpoorand ;madetheaustenitcchangeintoferrite.Allthatmade ;carbideiSsurroundedbvferrite.

    ;63

    ;Morphologyandformationmechanismofbainitecarbide ;5.Displacementofatomsonnucleation

    ;growthofbainitecarbide

    ;Theformationprocessof0.carbideisacarbon ;atomsmasstransferprocesswithlong-rangediffusion, ;andisalsoanironstransferprocess.Thecontent

Report this document

For any questions or suggestions please email
cust-service@docsford.com