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1、Ferrous Casting AlloysFerrous Casting AlloysIntroductionIron-Carbon Phase DiagramBasic Cast Iron MetallurgyCast IronsMicrostructural FormationSolidificationSpecifications,properties and applicationsCast SteelsFerrous Casting AlloysLiterally iron-based casting alloysCast Iron iron alloy with signific

2、ant C and SiGrayDuctileCompacted GraphiteAustempered DuctileMalleableSpecialtyCast Steel iron alloy with low C and SiFerrous Casting AlloysAnderson,p.269Iron Carbon Phase DiagramASM Vol.3,p.2.110Iron Carbon Phase DiagramPoints of notePhase changes of pure iron（）Eutectic,eutectoid&#peritectic reactio

3、nsGraphite is stable,but metastable Fe3C（aka cementite or iron carbide）occurs frequentlyDesired for cast irons:#graphite eutectic product,cementite eutectoid productIron Carbon Phase DiagramWhat microconstituents are possible on cooling from austenite?#Depends on cooling ratePearliteBainite upper an

4、d lowerMartensiteIron Carbon Phase DiagramASM Vol.3,p.2.110PearliteLowest relative cooling ratesAt eutectoid composition（0.78%C）The eutectoid product ferrite+cementite grow in lamellar formNucleates at prior austenite grain boundariesPearliteSamuels,p.317PearliteSlowest relative cooling ratesAt eute

5、ctoid composition（0.78%C）The eutectoid product ferrite+cementite grow in lamellar formNucleates at prior austenite grain boundariesCooling rate determines interlamellar spacingLower rates coarse spacingsMore massive cementite platelets mean lower strength&#better machinabilitys20 23 HRCHigher rates

6、fine spacingsHigher strength&#poorer machinabilitys30 35 HRCPearliteRC&#W,p.361PearliteOff eutectoid composition HypoeutectoidFerrite nucleates at grain boundariesPearlite grows at eutectoid temperatureHypereutectoidCementite nucleates at grain boundariesPearlite grows at eutectoid temperatureCast i

7、rons are（extremely）hypereutectoidWell discuss this further with steelsBainite&#MartensiteBainiteIntermediate cooling ratesFerrite“lath”with Fe3CTwo typesUpper bainite-“lower intermediate”cooling rateLower bainite-“higher intermediate”cooling rate MartensiteHighest cooling rates（quenching required）Ca

8、rbon-austenite solid solutionUnique crystal structure BCTThese are unimportant in cast iron metallurgyIron CastingsBrief History3000 BC First cast objects（copper）600 BC-First cast iron object（a 600 lb.tripod）cast in China1500 AD Sand first used a molding media1646 First American iron foundry near Bo

9、ston1722 Malleable iron is developed1750 Parliament prohibits iron casting in American colonies1794 Cupola melting developed1863 Metallography developed,metallographs become available in 19111899 First electric arc furnace begins production1900 Brinell hardness test introducedIron CastingsBrief Hist

10、ory1906 First induction furnace installed1918 First automated foundry grenade hulks1930 First coreless induction furnace installed1943 Ductile iron discovered;#patented&#commercialized in 1949 （Ford switches 100%of crankshaft production to ductile iron in 1951）1957 Vertically-parted molding is inven

11、ted（DISA）1962 First automated green sand molding machine1976 Compacted graphite iron is developed1990 Key Core process employed for precision gray iron castingsSource:#Modern Casting,November 2002Cast IronsCharacterized by free graphite in matrix of ferrite and cementite（eutectoid pearlite）Types are

12、 differentiated by the form of the free graphiteICH,p.145Cast IronsCharacterized by free graphite in matrix of ferrite and cementite（ferrite or pearlite）Types are differentiated by the form of the free graphiteEssentially Fe-C-Si alloy（ternary）Basic Cast Iron MetallurgyFe-Fe3C-Si 2%SiICH,p.135Basic

13、Cast Iron MetallurgyEutectic composition varies with silicon（and phosphorus）contentCarbon EquivalentCE=%C+（%Si+%P）/3CE at eutectic=4.3Gray iron is hypoeutectic（CE=3.9 4.1）Ductile iron is hypereutectic（CE=4.3 4.6）Basic Cast Iron MetallurgyC becomes less soluble in austenite as temperature drops below

14、 eutectic temperatureC diffuses to graphite particlesGray IronFully pearlitic matrix around（normal）coarse flakesFerrite matrix around fine graphite flakesDuctile IronFerrite“bulls eye”around nodulesRapid cooling results in fully pearlitic matrixAt eutectoid temperature,remaining carbon in austenite

15、becomes pearliteBasic Cast Iron MetallurgyChill cast iron cooled too rapidly forms austenite-carbide eutectic（white iron）instead of austenite-graphite eutecticCheck for chill by pouring a chill wedgeMeasure of section thickness that can be cast free of carbideComposition effects depth of chillHigher

16、 CE=thinner sectionsBasic Cast Iron MetallurgyShrinkage three componentsLiquid contraction on coolingSolidification volumetric change on changing from liquid to solidSolid contraction on coolingControl liquid and solidification shrinkage by maintaining supply of hot metal-risersControl solid shrinkage by building patterns slightly oversizedGray IronGray IronGraphite in flake form（ASTM A247 Type VII）in a matrix of pearliteFerrite near fine graphite flakesGraphite and austenite grow simultaneously by eutect