IRONIRON-CARBONDIAGRAM

IRONIRON-CARBONDIAGRAMFerriteAusteniteSteelCastironPearlitePearliteandCementinePearliteandCarbideEutecticeutectoidOutlineIntroductionCoolingcurveforpureironDefinitionofstructuresIron-Carbonequilibriumphasediagram–SketchTheIron-IronCarbideDiagram-ExplanationTheAustenitetoferrite/cementitetransformationNucleation&growthofpearliteEffectofC%ageonthemicrostructureofsteelRelationshipb/wC%age&mechanicalpropertiesofsteelCoolingcurveforpureironDefinitionofstructuresVariousphasesthatappearontheIron-Carbonequilibriumphasediagramareasunder:AusteniteFerritePearliteCementiteMartensite*LedeburiteUnitCellsofVariousMetalsFIGURE-Theunitcellfor(a)austentite,(b)ferrite,and(c)martensite.Theeffectofthepercentageofcarbon(byweight)onthelatticedimensionsformartensiteisshownin(d).Notetheinterstitialpositionofthecarbonatomsandtheincreaseindimensioncwithincreasingcarboncontent.Thus,theunitcellofmartensiteisintheshapeofarectangularprism.MicrostructureofdifferentphasesofsteelDefinitionofstructuresFerrite

isknownasαsolidsolution.Itisaninterstitialsolidsolutionofasmallamountofcarbondissolvedinα

(BCC)iron.stableformofironbelow912deg.CThemaximumsolubilityis0.025%Cat723C

anditdissolvesonly0.008%Catroomtemperature.Itisthesofteststructurethatappearsonthediagram.DefinitionofstructuresFerrite

Averagepropertiesare:Tensilestrength=40,000psi;Elongation=40%in2in;Hardness >RockwellC0or >RockwellB90 DefinitionofstructuresPearlite

istheeutectoidmixturecontaining0.80%Candisformedat723°Converyslowcooling.Itisaveryfineplatelikeorlamellarmixtureofferriteandcementite.Thewhiteferriticbackgroundormatrixcontainsthinplatesofcementite(dark).DefinitionofstructuresPearlite

Averagepropertiesare:Tensilestrength=120,000psi;Elongation=20%in2in.;Hardness=RockwellC20,Rock-wellB 95-100,orBHN250-300.DefinitionofstructuresAustenite

isaninterstitialsolidsolutionofCarbondissolvedin(F.C.C.)iron.Maximumsolubilityis2.0%Cat1130°C.Highformability,mostofheattreatmentsbeginwiththissinglephase.Itisnormallynotstableatroomtemperature.But,undercertainconditionsitispossibletoobtainausteniteatroomtemperature.DefinitionofstructuresAustenite

Averagepropertiesare:Tensilestrength=150,000psi;Elongation =10percentin2in.;Hardness =RockwellC40, approx;andtoughness =highDefinitionofstructuresCementite

orironcarbide,isveryhard,brittleintermetalliccompoundofiron&carbon,asFe3C,contains6.67%C.Itisthehardeststructurethatappearsonthediagram,exactmeltingpointunknown.Itscrystalstructureisorthorhombic.Itishaslowtensilestrength(approx.5,000psi),buthighcompressivestrength.DefinitionofstructuresLedeburite

istheeutecticmixtureofausteniteandcementite.Itcontains4.3percentCandisformedat1130°C.DefinitionofstructuresMartensite-asuper-saturatedsolidsolutionofcarboninferrite.Itisformedwhensteeliscooledsorapidlythatthechangefromaustenitetopearliteissuppressed.TheinterstitialcarbonatomsdistorttheBCCferriteintoaBC-tetragonalstructure(BCT).;responsibleforthehardnessofquenchedsteelTheIron-IronCarbideDiagramAmapofthetemperatureatwhichdifferentphasechangesoccuronveryslowheatingandcoolinginrelationtoCarbon,iscalledIron-CarbonDiagram.Iron-Carbondiagramshowsthetypeofalloysformedunderveryslowcooling,properheat-treatmenttemperatureandhowthepropertiesofsteelsandcastironscanberadicallychangedbyheat-treatment.VariousFeaturesofFe-CdiagramPeritecticL+d=gEutecticL=g+Fe3CEutectoidg=a+Fe3CPhasespresent

LReactionsdBCCstructureParamagneticgausteniteFCCstructureNon-magneticductileaferriteBCCstructureFerromagneticFairlyductileFe3CcementiteOrthorhombicHardbrittleMax.solubilityofCinferrite=0.022%Max.solubilityofCinaustenite=2.11%ThreePhaseReactionsPeritectic,at1490deg.C,withlowwt%Calloys(almostnoengineeringimportance).Eutectic,at1130deg.C,with4.3wt%C,alloyscalledcastirons.Eutectoid,at723deg.Cwitheutectoidcompositionof0.8wt%C,two-phasemixture(ferrite&cementite).Theyaresteels.HowtoreadtheFe-CphasediagramFerriteAusteniteSteelCastironPearlitePearliteandCementinePearliteandCarbideEutecticeutectoidTheIron-IronCarbideDiagramThediagramshowsthreehorizontallineswhichindicateisothermalreactions(oncooling/heating):

Firsthorizontallineisat1490°C,whereperitecticreactiontakesplace:

Liquid+?austeniteSecondhorizontallineisat1130°C,whereeutecticreactiontakesplace:

liquid?austenite+cementiteThirdhorizontallineisat723°C,whereeutectoidreactiontakesplace:

austenite?pearlite(mixtureofferrite&cementite)DeltaregionofFe-FecarbidediagramLiquid+?austeniteFerriteregionofFe-FeCarbidediagramSimplifiedIron-Carbonphasediagramaustenite?pearlite(mixtureofferrite&cementite)TheAustenitetoferrite/cementitetransformationinrelationtoFe-CdiagramTheAustenitetoferrite/cementitetransformationinrelationtoFe-Cdiagram

Inordertounderstandthetransformationprocesses,considerasteeloftheeutectoidcomposition.0.8%carbon,beingslowcooledalonglinex-x‘.

Attheuppertemperatures,onlyausteniteispresent,withthe0.8%carbonbeingdissolvedinsolidsolutionwithintheFCC.Whenthesteelcoolsthrough723°C,severalchangesoccursimultaneously.TheAustenitetoferrite/cementitetransformationinrelationtoFe-CdiagramTheironwantstochangecrystalstructurefromtheFCCaustenitetotheBCCferrite,buttheferritecanonlycontain0.02%carboninsolidsolution.Theexcesscarbonisrejectedandformsthecarbon-richintermetallicknownascementite.PearliticstructureThenetreactionattheeutectoidistheformationofpearliticstructure.Sincethechemicalseparationoccursentirelywithincrystallinesolids,theresultantstructureisafinemixtureofferriteandcementite.Schematicpictureoftheformationandgrowthofpearlite

FerriteCementiteAusteniteboundaryNucleation&growthofpearliteTheAustenitetoferrite/cementitetransformationinrelationtoFe-CdiagramHypo-eutectoidsteels:

Steelshavinglessthan0.8%carbonarecalledhypo-eutectoidsteels(hypomeans"lessthan").Considerthecoolingofatypicalhypo-eutectoidalloyalongliney-y‘.Athightemperaturesthematerialisentirelyaustenite.Uponcoolingitentersaregionwherethestablephasesareferriteandaustenite.Thelow-carbonferritenucleatesandgrows,leavingtheremainingaustenitericherincarbon.TheAustenitetoferrite/cementitetransformationinrelationtoFe-CdiagramHypo-eutectoidsteels-

At723°C,theremainingaustenitewillhaveassumedtheeutectoidcomposition(0.8%carbon),andfurthercoolingtransformsittopearlite.Theresultingstructure,isamixtureofprimaryorpro-eutectoidferrite

(ferritethatformsbeforetheeutectoidreaction)andregionsofpearlite.TheAustenitetoferrite/cementitetransformationinrelationtoFe-CdiagramHyper-eutectoidsteels

(hypermeans"greaterthan")arethosethatcontainmorethantheeutectoidamountofCarbon.Whensuchasteelcools,asalonglinez-z',theprocessissimilartothehypo-eutectoidsteel,exceptthattheprimaryorpro-eutectoidphaseisnowcementiteinsteadofferrite.TheAustenitetoferrite/cementitetransformationinrelationtoFe-CdiagramAsthecarbon-richphasenucleatesandgrows,theremainingaustenitedecreasesincarboncontent,againreachingtheeutectoidcompositionat723°C.Thisaustenitetransformstopearliteuponslowcoolingthroughtheeutectoidtemperature.Theresultingstructureconsistsofprimarycementiteandpearlite.Thecontinuousnetworkofprimarycementitewillcausethematerialtobeextremelybrittle.TheAustenitetoferrite/cementitetransformationinrelationtoFe-CdiagramHypo-eutectoidsteelshowingprimarycementitealonggrainboundariespearliteTheAustenitetoferrite/cementitetransformationinrelationtoFe-CdiagramItshouldbenotedthatthetransitionsasdiscussed,areforequilibriumconditions,asaresultofslowcooling.Uponslowheatingthetransitionswilloccurinthereversemanner.TheAustenitetoferrite/cementitetransformationinrelationtoFe-CdiagramWhenthealloysarecooledrapidly,entirelydifferentresultsareobtained,sincesufficienttimemaynotbeprovidedforthenormalphase