Deformation behavior of an ultrahigh carbon steel (UHCS-3.0Si) at elevated temperature
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MaterialsScienceandEngineeringA379(2004)
302–307
Deformationbehaviorofanultrahighcarbonsteel
(UHCS-3.0Si)atelevatedtemperature
GuangminLuoa,∗,JianshengWua,JunfeiFanb,HaishengShib,
YijianLinb,JingguoZhangb
aKeyLaboratoryforHighTemperatureMaterialsandTestsofMinistryofEducation,ShanghaiJiaotongUniversity,Shanghai200030,ChinabShanghaiBaosteelResearchInstitute,Shanghai201900,China
Received18September2003;receivedinrevisedform17February2004
Abstract
Isothermalcompressiontestswerecarriedoutonaspray-formedultrahighcarbonsteel(UHCS-3.0Si)byGleeble1500system.Thetruestressincreaseswiththedecreaseoftemperatureandincreaseofstrainrate.Thedeformationactivationenergies,Q,ofUHCS-3.0Siarenotonlytemperaturedependentbutalsostrainratedependent.ThevalueofQincreasesasthedeformationtemperaturedecreasesanditisinproportiontotheweightpercentofundissolvedcarbides.Athighdeformationstrainrates(5–30s−1),dislocationslipandcrosssliparethemaindeformationmechanismandatlowdeformationstrainrates(0.1–1s−1),climb-controlleddislocationcreepisthemaindeformationmechanism.Microstructuralevolutionwasanalyzedduringdeformationat800◦C,cementitelamellashavefairformabilityandsomearebrokenupwhichleadstogreatsoftening.©2004ElsevierB.V.Allrightsreserved.
Keywords:Ultrahighcarbonsteel;Physicalsimulation;Activationenergy;Dynamicrecrystallization
1.Introduction
Ultrahighcarbonsteels(UHCS)arehypereutectoidsteels
containing1.0–2.1%CanditwasfirstinvestigatedinStan-
fordUniversitybySherbyetal.inthe1970sandsubse-
quentlystudiedatLawrenceLivemoreNationalLaboratory,
California,becauseofitsexcellenthightemperaturesuper-
plasticity.Severalpatents[1–5]havebeenpublishedabout
theirwork.ResearchesaboutUHCSinothercountrieswere
alsopublished[6,7].Mostoftheinvestigations[8–13]are
aboutUHCSscontainingsiliconoraluminum,whichin-
creasesthecarbonactivityandsuppressestheformationof
grainboundarycarbides.Whenprocessedtoproducemi-
crostructureswithfinespheroidizedcarbides,thesesteels
canpossessuniquepropertieswithhighstrengthandhigh
ductilityatroomtemperature.
Toobtainhighsuperplasticity,UHCSsmustbehot–warm
processedtoafinegrainsizewhichisstablewhenitisheated
∗Correspondingauthor.Tel.:+86-21-62932440;fax:+86-21-52540011.E-mailaddress:gmluo@vip.sina.com(G.Luo).forsuperplasticforming[1,7,14].Thepowder–metallurgy
techniquewasusedtoobtainhigh-strain-ratesuperplastic-
ityinUHCS-10Al[15].Weproposedasprayformingtech-
niquetoproduceUHCSandachievecomparativelyhigh
elongationsathightemperaturewithoutanyadditionalpro-
cessing[16].Manyvoids,however,aredistributedinthe
spray-formedUHCSandhotdeformationisnecessaryto
closeupthevoids.Thepurposeofthepresentpaperisto
describeandassessthebehaviorandmechanismofdefor-
mationofthespray-formedUHCSatelevatedtemperature.
2.Experimentalprocedure
Theas-receivedingotofUHCSwasheatedina
induction-furnace,andsoakedfor20minatatemperature
ofabout150◦Cabovemeltingpoint.Themoltenmetalwas
atomizedintoasprayofliquiddropletsbyahighpressure
ofnitrogen.Thedropletswerecooledanddriventowardsa
revolvingsubstratetoformacondensedproduct.
Thespray-formedUHCS(UHCS-3.0Si)contained,by
weight:1.28%C,3.0%Si,1.49%Cr,0.50%Mn,anda
0921-5093/$–seefrontmatter©2004ElsevierB.V.Allrightsreserved.doi:10.1016/j.msea.2004.02.056G.Luoetal./MaterialsScienceandEngineeringA379(2004)302–307303
balanceofFe.Isothermalcompressionwascarriedoutby
aGleeble1500thermo-mechanicalsystem.Thespecimens
(12mminheightand8mmindiameter)wereheatedtothe
deformationtemperature(800,850,900,950,1000,1050,
and1100◦C)ataheatingrateof5◦Cs−1,heldfor5min,
isothermallycompressedwithfivestrainrates(0.1,1,5,
10,and30s−1)tothetruestrainof1.2(70%reduction)
andcooledat5◦C/sto600◦C,thenaircooledtotheroom
temperature.Truestrain–truestresscurveswereobtained.
Toinvestigatethedeformationbehavioratdifferentstrains
at800◦C,thespecimenswereisothermallycompressedto
thetruestrainof0.22(20%)and0.51(40%)at800◦Cat
strainrateof5s−1,respectively,followedbyrapidcooling
ofwaterquenching.TheAc1phasetransformationpointwas
measuredbydifferentialthermalanalysisataheatingrate
of20◦C/min.
Sectionswerecutfromthespecimensalongthelongitu-
dinalaxis.Thesampleswerepreparedbymechanicalpol-
ishingfollowedbyetchingin4%Nital.Scanningelectron
microscope(SEM)analyseswereperformedusingaS4200
fieldemissionSEM.Thesampledeformedat800◦Cwas
analyzedbyJEM200cxtransmissionelectronmicroscope
(TEM).
3.Results
TheSEMmicrographs,showninFig.1,illustratethe
microstructuresoftheas-castandspray-formedUHCS.As
canbeseen,thegrainsizeandthepearlitelamellaofthe
spray-formedUHCSarebothmuchfinerthanthoseofthe
as-castingot.Fewgrainboundarycarbidesareobserved
inthesprayformedUHCSbecauseofrapidsolidifica-
tion.Voids,however,aredistributedinthespray-formed
UHCS.
Truestrain–truestresscurvesofUHCS-3.0Sideformed
