*Cast ironsEF420 Lecture 9

Overview of cast ironIron with 1.7 to 4.5% carbon and 0.5 to 3% siliconLower melting point and more fluid than steel (better castability)Low cost material usually produced by sand castingA wide range of properties, depending on composition & cooling rateStrengthHardnessDuctilityThermal conductivityDamping capacity

*Iron carbon diagramLiquidAustenitea + Fe3Cdg+ La + gL + Fe3C723C910C0%0.8%~2%~3%g + Fe3CCast IronCarbon Steel

*Production of cast ironPig iron, scrap steel, limestone and carbon (coke)CupolaElectric arc furnaceElectric induction furnaceUsually sand cast, but can be gravity die cast in reusable graphite mouldsNot formed, finished by machining

*Types of cast ironGrey cast iron - carbon as graphiteWhite cast iron - carbides, often alloyedDuctile cast iron nodular, spheroidal graphiteMalleable cast ironCompacted graphite cast ironCG or Vermicular Iron

*Effect of cooling rateSlow cooling favours the formation of graphite & low hardnessRapid cooling promotes carbides with high hardnessThick sections cool slowly, while thin sections cool quicklySand moulds cool slowly, but metal chills can be used to increase cooling rate & promote white iron

*Effect of compositionA CE over 4.3 (hypereutectic) leads to carbide or graphite solidifying first & promotes grey cast ironA CE less than 4.3 (hypoeutectic) leads to austenite solidifying first & promotes white iron

*Grey cast ironFlake graphite in a matrix of pearlite, ferrite or martensiteWide range of applicationsLow ductility - elongation 0.6%Grey cast iron forms whenCooling is slow, as in heavy sectionsHigh silicon or carbon

*Typical propertiesDepend strongly on casting shape & thicknessAS1830 & ASTM A48 specifies propertiesLow strength, A48 Class 20, Rm 120 MPaHigh carbon, 3.6 to 3.8%Kish graphite (hypereutectic)High conductivity, high dampingHigh strength, A48 Class 60, Rm 410 MPaLow carbon, (eutectic composition)

*Graphite formUniformRosetteSuperimposed (Kish and normal)Interdendritic randomInterdendritic preferred orientationSee AS5094 designation of microstructure of graphite

*Matrix structurePearlite or ferriteTransformation is to ferrite whenCooling rate is slowHigh silicon contentHigh carbon equivalencePresence of fine undercooled graphite

*Properties of grey cast ironMachineability is excellentDuctility is low (0.6%), impact resistance lowDamping capacity highThermal conductivity highDry and normal wear properties excellent

*ApplicationsEnginesCylinder blocks, liners, Brake drums, clutch platesPressure pipe fittings (AS2544)Machinery bedsFurnace parts, ingot and glass moulds

*Ductile ironInoculation with Ce or Mg or both causes graphite to form as spherulites, rather than flakesAlso known as spheroidal graphite (SG), and nodular graphite ironFar better ductility than grey cast ironSee AS1831

*MicrostructureGraphite spheres surrounded by ferriteUsually some pearliteMay be some cementiteCan be hardened to martensite by heat treatment

*Production Composition similar to grey cast iron except for higher purity. Melt is added to inoculant in ladle. Magnesium as wire, ingots or pellets is added to ladle before adding hot iron. Mg vapour rises through melt, removing sulphur.

*VerificationTesting is required to ensure nodularisation is complete. Microstructural examinationMechanical testing on standard test bars (ductility)Ultrasonic testing

*PropertiesStrength higher than grey cast ironDuctility up to 6% as cast or 20% annealedLow costSimple manufacturing process makes complex shapesMachineability better than steel

*ApplicationsAutomotive industry 55% of ductile iron in USACrankshafts, front wheel spindle supports, steering knuckles, disc brake callipersPipe and pipe fittings (joined by welding) see AS2280

*Malleable ironGraphite in nodular formProduced by heat treatment of white cast ironGraphite nodules are irregular clustersSimilar properties to ductile ironSee AS1832

*MicrostructureUniformly dispersed graphiteFerrite, pearlite or tempered martensite matrixFerritic castings require 2 stage anneal. Pearlitic castings - 1st stage only

*Annealing treatmentsFerritic malleable ironDepends on C and Si1st stage 2 to 36 hours at 940C in a controlled atmosphereCool rapidly to 750C & hold for 1 to 6 hoursFor pearlitic malleable ironSimilar 1st stage above (2 - 36 h at 940C)Cool to 870C slowly, then air cool & temper to specificationHarden and temper pearlitic iron for martensitic castings

*PropertiesSimilar to ductile ironGood shock resistanceGood ductilityGood machineability

*ApplicationsSimilar applications to ductile ironMalleable iron is better for thinner castingsDuctile iron better for thicker castings >40mmVehicle componentsPower trains, frames, suspensions and wheelsSteering components, transmission and differential parts, connecting rodsRailway componentsPipe fittings AS3673

*Joining cast ironWeldingBraze-weldingBrazingSolderingMechanical connections

*WeldingWeldability of cast iron is low and depends on the material type, thickness, complexity of the casting, and on whether machinability is important

*Braze weldingRepair of cracked or broken cast ironOxy-fuel gas process using filler which melts between 450C and the melting temperature of the castingJoint is similar to that for weldingLow dilutionPreheat 320 to 400CCopper-zinc filler with suitable flux

*BrazingUsed for capillary jointsAny brazing processThose with automatic temperature control are bestLower melting silver brazing alloys are bestMust not contain phosphorus

*WeldabilityWhite cast iron - not weldableSmall attachments onlyGrey cast iron - low weldabilityWelding largely restricted to salvage and repairDuctile and malleable irons - good weldability (inferior to structural steel)Welding increasingly used during manufacture

*Welding problemsHigh carbon contentTendency to form martensite and cementite in HAZLoss of ductility, cracking and impairment of machinabilityDifficulty wettingPre-cleaning important, fluxingLow ductility of castingResidual stress causes cracking

Oxyfuel gas weldingLow power process with wide HAZ600C preheat of whole casting, 90 to 120 included angleFiller is cast iron of matching compositionInoculating fluxes available for ductile ironsWeld closely matches base material (machinability and corrosion resistance)Particularly suited to repair of casting defects at foundry in grey cast iron

MMAW cold methodSuits small repairs in grey cast ironDrill crack ends, 70 included angleUse nickel or Ni-55Fe alloy electrodesLow strength ductile weld metalKeep casting coldSmall diameter, 2.5mm, short weld runsBackstep weld runs, max interpass temperature 100CHAZ contains martensite and is unmachinablePeen each weld run

*Ductile and malleable ironsMMAW, SAW, GTAW, GMAW and FCAWPreheat may not be requiredPreheat is not recommended for ferritic ductile and malleable ironsPreheat of up to 320C for pearlitic irons Ni, 55Ni-45Fe, 53Ni-43Fe-4.5Mn fillers used

*White cast ironWhite fracture surfaceNo graphite, because carbon forms Fe3C or more complex carbidesAbrasion resistantOften alloyedAustralian Standard DR20394 Wear resistant white cast irons

*Effects of alloy elementsPromote graphite (Si, Ni)Promote carbides (Cr)Affect matrix microstructureFerrite, pearlite, martensite or austeniteCorrosion resistance (Cr)Specific effects

*Increasing carbonIncreases depth of chill in chilled ironIncreases hardnessIncreases brittlenessPromotes graphite during solidification

Increasing siliconLowers carbon content of eutecticPromotes graphite on solidificationReduces depth of chillNegative effect on hardenabilityPromotes pearlite over martensiteRaises Ms if martensite formsCan improve resistance to scaling at high temperature

*Manganese and sulphurEach alone increases depth of chillTogether reduces effect of other (MnS)Mn in excess scavenges S and stabilises austeniteSolid solution strengthener of ferrite / pearliteSulphur lowers abrasion resistance

*PhosphorusMild graphitiserReduces chill depthConsidered detrimental in alloy cast irons

*ChromiumMain uses: Forms carbidesGives corrosion resistanceHigh temperature stabilityUp to 3% - no effect on hardenabilityMore than 10% - M7C3 carbides stronger and tougher than M3C

*High chromium irons12 to 28% chromiumLess effect on hardenability than in steelsMo, Ni, Mn, and Cu also added for hardenability to give martensite

*NickelPromotes graphiteIncreases strength of pearliteIncreases hardenability 2.5 to 4.5% Ni-Hard ironsStabilises austeniteOver 6.5%

*Ni-hard ironsGrinding balls1-2.2 Si, 5-7 Ni, 7-11 CrM7C3 eutectic carbides in martensite

*CopperSuppresses pearlite formation in martensitic ironsSynergistic effect with Mo3 to 10% in some high Ni grey irons

*MolybdenumIncreases depth of chill mildlyHardens and toughens pearliteSuppresses pearliteIncreases hardenability

*VanadiumPotent carbide stabiliserIncreases depth of chill0.1 to 0.5%

*InoculantsFerrosilicon as graphitiserMg and Ce as spheroidisersTellurium, bismuth and vanadium promote carbides in white irons

*Abrasion resistant ironsPearlitic white ironsCheap but wear more quicklyMartensitic white ironsMore expensive but better wearingASTM A532-75A

*Can be heat treatedStress relief up to 700CTempering of martensiteSubzero treatment to remove retained austeniteAnnealing for machining followed by QT

*MicrostructuresPearlite and ferrite in Fe3C matrixAustenite / martensite in Fe3C matrixM7C3 in a martensite matrix

*Abrasion resistanceDepends on cast ironDepends also on abrasive and environmentEg Silicon carbide wears martensitic and pearlite equallySilica wears martensitic irons much less than pearlitic ones