1、IRON IRON-CARBON DIAGRAMIRON IRON-CARBON DIAGRAMFerriteAustenite SteelCast ironPearlitePearlite and CementinePearlite andCarbideEutecticeutectoidOutline?Introduction?Cooling curve for pure iron?Definition of structures?Iron-Carbon equilibrium phase diagram Sketch?The Iron-Iron Carbide Diagram-Explan
2、ation?TheAustenitetoferrite/cementitetransformation?Nucleation&growth of pearlite?Effect of C%age on the microstructure of steel?Relationshipb/wC%age&mechanicalproperties of steelCooling curve for pure ironDefinition of structuresVarious phases that appear on theIron-Carbonequilibriumphasediagram ar
3、e as under:?Austenite?Ferrite?Pearlite?Cementite?Martensite*?LedeburiteUnit Cells of Various Metals?FIGURE-The unit cell for(a)austentite,(b)ferrite,and(c)martensite.The effect of the percentage of carbon(by weight)on the lattice dimensionsfor martensite is shown in(d).Note the interstitial position
4、 of the carbonatoms and the increase in dimension c with increasing carbon content.Thus,the unit cell of martensite is in the shape of a rectangular prism.Microstructure of different phases of steelDefinition of structures?Ferrite is known as solid solution.?It is an interstitial solid solution of a
5、 smallamount of carbon dissolved in (BCC)iron.?stable form of iron below 912 deg.C?The maximumsolubility is 0.025%C at723?C and it dissolves only 0.008%C atroom temperature.?It is the softest structure that appears on thediagram.Definition of structuresFerrite?Average properties are:?Tensile strengt
6、h=40,000 psi;?Elongation=40%in 2 in;?Hardness Rockwell C 0 or Rockwell B 90Definition of structures?Pearlite is the eutectoid mixturecontaining0.80%Candisformed at 723C on very slowcooling.?Itisaveryfineplatelikeorlamellar mixture of ferrite andcementite.?The white ferritic background ormatrixcontai
7、nsthinplatesofcementite(dark).Definition of structuresPearlite?Average properties are:?Tensile strength=120,000 psi;?Elongation=20%in 2 in.;?Hardness=Rockwell C 20,Rockwell B95-100,or BHN 250-300.Definition of structures?Austeniteis an interstitial solid solution ofCarbon dissolved in?(F.C.C.)iron.?
8、Maximum solubility is 2.0%C at 1130C.?High formability,most of heat treatmentsbegin with this single phase.?It is normally not stable at room temperature.But,under certain conditions it is possible toobtain austenite at room temperature.Definition of structuresAustenite?Average properties are:?Tensi
9、le strength=150,000 psi;?Elongation=10 percent in 2 in.;?Hardness=Rockwell C 40,approx;and?toughness=highDefinition of structures?Cementiteor iron carbide,is very hard,brittleintermetalliccompoundofiron&carbon,as Fe3C,contains 6.67%C.?It is the hardest structure that appears on thediagram,exact melt
10、ing point unknown.?Its crystal structure is orthorhombic.?It is has?low tensile strength(approx.5,000 psi),but?high compressive strength.Definition of structures?Ledeburiteistheeutecticmixtureofausteniteandcementite.?It contains 4.3 percent C and isformed at 1130C.Definition of structures?Martensite
11、-a super-saturated solid solution of carbon in ferrite.?It is formed when steel is cooled so rapidly that the change from austenite to pearlite is suppressed.?The interstitial carbon atoms distort the BCC ferrite into a BC-tetragonal structure(BCT).;responsible for the hardness of quenched steelThe
12、Iron-Iron Carbide Diagram?A map of the temperature at which differentphase changes occur on very slow heatingand cooling in relation to Carbon,is calledIron-Carbon Diagram.?Iron-Carbon diagram shows?the type of alloys formed under very slowcooling,?proper heat-treatment temperature and?how the prope
13、rties of steels and cast ironscan be radically changed by heat-treatment.Various Features of Fe-C diagramPeritectic L+d=?Eutectic L=?+Fe3CEutectoid?=a+Fe3CPhases presentLReactionsdBCC structureParamagnetic?austeniteFCC structureNon-magneticductileaferriteBCC structureFerromagneticFairly ductileFe3Cc
14、ementiteOrthorhombicHardbrittleMax.solubility of C in ferrite=0.022%Max.solubility of C in austenite=2.11%Three Phase Reactions?Peritectic,at 1490 deg.C,with low wt%C alloys(almost no engineering importance).?Eutectic,at 1130 deg.C,with 4.3wt%C,alloys called cast irons.?Eutectoid,at 723 deg.C with e
15、utectoid composition of 0.8wt%C,two-phase mixture(ferrite&cementite).They are steels.How to read the Fe-C phase diagramFerriteAustenite SteelCast ironPearlitePearlite and CementinePearlite andCarbideEutecticeutectoidThe Iron-Iron Carbide DiagramThe diagram shows three horizontal lines whichindicatei
16、sothermalreactions(oncooling/heating):?Firsthorizontallineisat1490C,whereperitectic reaction takes place:Liquid+d?austenite?Second horizontal line is at 1130C,whereeutectic reaction takes place:liquid?austenite+cementite?Thirdhorizontallineisat723C,whereeutectoid reaction takes place:austenite?pearl
17、ite(mixture of ferrite&cementite)Delta region of Fe-Fe carbide diagramLiquid +d?austeniteFerrite region of Fe-Fe Carbide diagramSimplified Iron-Carbon phase diagramaustenite?pearlite(mixture of ferrite&cementite)The Austenite to ferrite/cementite transformation in relation to Fe-C diagramThe Austeni
18、te to ferrite/cementite transformation in relation to Fe-C diagramInordertounderstandthetransformationprocesses,consider a steel of the eutectoidcomposition.0.8%carbon,being slow cooledalong line x-x.?At the upper temperatures,only austenite ispresent,with the 0.8%carbon being dissolvedin solid solu
19、tion within the FCC.When the steelcools through 723C,several changes occursimultaneously.The Austenite to ferrite/cementite transformation in relation to Fe-C diagram?Theironwantstochangecrystalstructure from the FCC austenite to theBCC ferrite,but the ferrite can onlycontain 0.02%carbon in solid so
20、lution.?Theexcesscarbonisrejectedandformsthecarbon-richintermetallicknown as cementite.Pearlitic structure?Thenetreactionattheeutectoidis the formationof pearlitic structure.?Sincethechemicalseparation occurs entirelywithincrystallinesolids,the resultant structure is afine mixture of ferrite andceme
21、ntite.Schematic picture of the formation and growth of pearliteFerriteCementiteAustenite boundaryNucleation&growth of pearliteThe Austenite to ferrite/cementite transformation in relation to Fe-C diagram?Hypo-eutectoidsteels:Steels having less than0.8%carbon are called hypo-eutectoidsteels(hypo mean
22、s less than).?Consider the cooling of a typical hypo-eutectoidalloy along line y-y.?At high temperaturesthe materialis entirelyaustenite.?Upon cooling it enters a region where the stablephases are ferrite and austenite.?Thelow-carbonferritenucleatesandgrows,leaving the remaining austenite richer in
23、carbon.The Austenite to ferrite/cementite transformation in relation to Fe-C diagram?Hypo-eutectoid steels-At 723C,the remaining austenite will have assumed the eutectoid composition(0.8%carbon),and further cooling transforms it to pearlite.?The resulting structure,is a mixture of primary or pro-eut
24、ectoid ferrite(ferrite that forms before the eutectoid reaction)and regions of pearlite.The Austenite to ferrite/cementite transformation in relation to Fe-C diagram?Hyper-eutectoidsteels(hypermeansgreaterthan)are those that contain morethan the eutectoid amount of Carbon.?When such a steel cools,as
25、 along line z-z,the process is similar to the hypo-eutectoidsteel,except that the primary or pro-eutectoidphase is now cementite instead of ferrite.The Austenite to ferrite/cementite transformation in relation to Fe-C diagram?As the carbon-rich phase nucleates and grows,the remaining austenite decre
26、ases in carboncontent,againreachingtheeutectoidcomposition at 723C.?This austenite transforms to pearlite upon slowcooling through the eutectoid temperature.?Theresultingstructureconsistsof primarycementite and pearlite.?The continuous network of primary cementitewill cause the material to be extrem
27、ely brittle.The Austenite to ferrite/cementite transformation in relation to Fe-C diagramHypo-eutectoid steel showing primary cementite along grain boundaries pearliteThe Austenite to ferrite/cementite transformation in relation to Fe-C diagram?It should be noted that the transitionsasdiscussed,aref
28、orequilibriumconditions,as a result of slow cooling.?Upon slow heating the transitions willoccur in the reverse manner.The Austenite to ferrite/cementite transformation in relation to Fe-C diagram?When the alloys are cooled rapidly,entirelydifferent results are obtained,since sufficienttime may not
29、be provided for the normal phasereactions to occur.?In these cases,the equilibrium phase diagramisnolongeravalidtoolforengineeringanalysis.?Rapid-cool processes are important in the heattreatment of steels and other metals(to bediscussed later in H/T of steels).Principal phases of steel and their Ch
30、aracteristicsPhaseCrystal structureCharacteristicsFerriteBCCSoft,ductile,magneticAusteniteFCCSoft,moderate strength,non-magneticCementiteCompound of Iron&Carbon Fe3CHard&brittle24?Teutectoidchanges:?Ceutectoidchanges:Alloying Steel with more ElementsCast Irons-Iron-Carbon alloys of 2.11%C or more ar
31、e cast irons.-Typical composition:2.0-4.0%C,0.5-3.0%Si,less than 1.0%Mn and less than 0.2%S.-Si-substitutes partially for C and promotes formation of graphite as the carbon rich component instead Fe3C.Applications?It is used tailor properties of steel and to heat treat them.?It is also used for comparison of crystal structures for metallurgists in case of rupture or fatigue ConclusionThanks
