三维形态形成的铁素体与夹杂物在低碳钢

Three-dimensionalmorphologyofferriteformedinassociationwithinclusionsinlow-carbonsteelK.M.Wua,1,Y.Inagawab,2,M.Enomotoc,*aJapanSpaceUtilizationPromotionCenter,Nishi-Waseda,Tokyo169-8624,JapanbFacultyofEngineering,IbarakiUniversity,Hitachi316-8511,JapancDepartmentofMaterialsScience,IbarakiUniversity,Hitachi316-8511,JapanReceived4November2003;accepted10April2004AbstractThree-dimensional(3D)morphologyandgrowthbehaviorofferriteformedatintragranularinclusionsinlow-carbonsteelwerestudiedbyserialsectioningandcomputer-aidedvisualization.Thespecimenstakenfromthewelddepositwereaustenitizedandisothermallyreactedforvaryingtimes.Thelength,width,andthicknessofferriteplatesweremeasuredfrom3D-reconstructedimages.Fromthelength-to-widthratio,themorphologyofferriteislikelytobealathratherthanaplateinthisalloy.Multivariantferriteplates(orlaths)wereobservedtobenucleatedatinclusionsandgrowinthedirectionsclosetoh110igwithhabitplanesnear{111}g,probablykeepingafixedorientationrelationshipwithaustenite.D2004ElsevierInc.Allrightsreserved.Keywords:Low-carbonsteel;Acicularferrite;Widmansta¨ttenferrite;Three-dimensionalreconstruction;Inclusion1.IntroductionAgoodcombinationofstrengthandtoughnessoflow-carbonsteelweldsisachievedbyso-calledacic-ularferritemicrostructure,consistingofsmallinter-weavingferriteplatesformedwithinaustenitegrains[1–6].Thetermacicularisusedfrequentlyinweldswhereferriteplatesarerelativelysmallandcoveralargeproportionofthematrix.Theinterweavingmicrostructureisattributedtonucleationofmulti-variantferriteplatesatintragranularinclusions,growththereof,andnucleationofsecondaryplatesatthesurfaceofpreexistingferriteplates[2,6,7].Sinceferritetransformationoccursfastandthemi-crostructureisoftenobservedaftertheweldiscooledtoambienttemperature,studiesofmorphologyandgrowthbehaviorofferritecrystalsatearlystagesarerelativelyscarceinlow-carbonsteel.Inrecentyears,bothcomputerhardwareandsoft-wareforimageprocessingandthree-dimensional(3D)visualizationhavedevelopedrapidly.Thesedevelop-mentspromptedtheapplicationof3Dreconstructiontoopaquemetallicmicrostructuresforthepastdecade,asreviewedbyKraletal.[8].Thecharacterizationofmicrostructureinthree-dimensionsoftenplaysanessentialroleinidentificationoftransformationmech-1044-5803/$-seefrontmatterD2004ElsevierInc.Allrightsreserved.doi:10.1016/j.matchar.2004.04.004*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](M.Enomoto).1Presentaddress:DepartmentofAppliedPhysics,WuhanUniversityofScienceandTechnology,Wuhan430081,PRChina.2Presentaddress:DietecInc.,Mibu-cho,Shimotuga-yun,Tochigi321-0215,Japan.MaterialsCharacterization52(2004)121–127anism[8–14]andquantitativeanalysisofmicrostruc-ture[15].Inthisreport,low-carbonsteelspecimenswerereactedforveryshorttimestoobtainpartiallytransformedmicrostructureandthemicrostructurewas3D-reconstructedtoobservethemorphologyofferriteplatesformedwithintheaustenitematrix.Then,thelength,width,andthicknessofindividualferritecrys-talsweremeasuredfrom3D-reconstructedimagesandthegrowthbehaviorofferriteplateswasdiscussed.2.ExperimentalprocedureTable1showsthecompositionsofthebaseplate(SM490),low-carbonweldwireandspecimenstakenfromthewelddeposit.Theweldingwasconductedunderashieldinggasof100%CO2.Itisseenthattheconcentrationsofcarbonandsubstitutionalelementswereallreducedafterwelding,whiletheoxygenconcentrationislarge.Thespecimenswerefirstaus-tenitizedat1200jCfor20minandfurnace-cooledtoobtainagrainsizeaslargeas80Am.Theyweresubsequentlyaustenitizedat1350jCfor150sunderapurifiedargonatmosphere,rapidlycooled(atarateoff50jC/s)andisothermallyheldat570jCfor1and5s.Theseheattreatmentswereconductedinthehotdeformationsimulator.Thesecondaustenitizationandisothermalholdingwasalsoconductedinasaltbathtoachievefastquenchingtotheisothermalholdingtemperature.Forserialsectioning,specimenswerepolishedwithanautomaticgrinder–polisher.Polishedsurfaceswerelightlyetchedwith3%nital.Microhardnessindentswereappliedtomarktheareaofinterest(f12090Am2).TheywereusedasfiducialmarksforhorizontalimagealignmentandcalibrationoftheTable1Chemicalcompositionofthebaseplate,weldwire,andspecimen(mass%)CSiMnTiONSBaseplate0.160.391.42––––Weldwire0.111.252.200.26–––Welddeposit0.0780.901.570.030.03200.00500.0059Fig.1.Opticalmicrographsofspecimenisothermallyheld(a)at570jCfor1sand(b)at570jCfor5s.Fig.2.3D-reconstructedimageofferriteplatesinthespecimenisothermallyheldat570jCfor1s.K.M.Wuetal./MaterialsCharacterization52(2004)121–127122depthofremoval.Typically,0.32F0.04Amwasremovedpersection.Afterimagealignment,maskingofobjectiveferriteplate(s)wasmade.Then,astackof2Dimagesofthemaskedobjectwastransformedintoa3DimageusingAVSsoftware.Theseprocedureshavebeendescribedelsewhere[14].3.Resultsanddiscussion3.1.Three-dimensionalimageofferriteplateFig.1aandbshowsthelightopticalmicrographsofspecimensreactedat570jCfor1and5s,respective-ly.ItisseeninFig.1athatthinlongplatesarenucleatedinthematrix,presumablyatoxideinclu-sions.Atthisstage,theappearanceofferriteplatesisverysimilartointragranularfe