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Heat Treatment of Metals

Heat Treatment of Metals. Heat - Treatment.  Heat treatment is a method used to alter the physical, and sometimes chemical properties of a material. The most common application is metallurgical

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Heat Treatment of Metals

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  1. HeatTreatmentofMetals

  2. Heat-Treatment • Heattreatmentisamethodusedtoalterthephysical,andsometimeschemicalpropertiesofamaterial.Themostcommonapplicationismetallurgical • Itinvolvestheuseofheatingorchilling,normallytoextremetemperatures,toachieveadesiredresultsuchashardeningorsofteningofamaterial • Itappliesonlytoprocesseswheretheheatingandcoolingaredoneforthespecificpurposeofalteringpropertiesintentionally • Generally, heat treatment uses phase transformation during heating and cooling to change a microstructure in a solid state.

  3. TypesofHeat-Treatment(Steel) Annealing Tempering,andQuenching Precipitationhardening Casehardening

  4. Annealing Aheattreatmentprocessinwhichametalisexposedtoanelevatedtemperatureforanextendedtimeperiodandthenslowlycooled. Purpose: Relievestressesofcoldworking Increasesoftness,ductilityandtoughness Producespecificmicrostructure

  5. Annealing ThreeStagesofAnnealing 1.Heatingtoadesiredtemperature 2.Holdingorsoakingatthattemperature 3.Coolingusuallytoroomtemperature Note:Timeinaboveproceduresisimportant -Duringheatingandcoolingtempgradientsexitb/winsideandoutsideportionsofpart.Ifrateoftempchangeistoolhigh,tempgradientswillinduceinternalstressinpartandhencecracking 2   1 3 T α+Fe3C T α+Fe3C Time Time

  6. TypesofAnnealing Stress-ReliefAnnealing(orStress-relieving) Normalizing FullAnnealing SpheroidizingAnnealing(orSpheroidizing) IsothermalAnnealing

  7. Iron-CPhaseDiagram A B

  8. TempRangesinFe-CPhaseDiagram - A1.Lower criticalTempA3.UppercriticalTempforHypo-eutectoidsteels Acm.Upper criticalTempforHyper-eutectoidsteels  Fe3C T  Eutectoid α+Fe3C

  9. TempRangesforAnnealingProcesses

  10. 1.Stress-ReliefAnnealing ItisanannealingprocessbelowthetransformationtemperatureA1,withsubsequentslowcooling,theaimofwhichistoreducetheinternalresidualstressesinaworkpiecewithoutintentionallychangingitsstructureandmechanicalproperties

  11. 1.Stress-ReliefAnnealing Forplaincarbonandlow-alloysteelsthetemperaturetowhichthespecimenisheatedisusuallybetween450and650˚C,whereasforhot-workingtoolsteelsandhigh-speedsteelsitisbetween600and750˚C Thistreatmentwillnotcauseanyphasechanges,butrecovery&recrystallizationmaytakeplace. Machiningallowancesufficienttocompensateforanywarping/distrotionresultingfromstressrelievingshouldbeprovided

  12. CausesofResidualStresses Mechanicalfactors(e.g.,cold-workingduringmetalforming/machining) Thermalfactors(e.g.,thermalstressescausedbytemperaturegradientswithinthework-pieceduringheatingorcooling) Metallurgicalfactors(e.g.,phasetransformationuponcoolingwhereinparentandproductphaseshavedifferentdensities -Intheheattreatmentofmetals,quenchingorrapidcoolingisthecauseofthegreatestresidualstresses

  13. StressReliefAnnealing– Temperature&TimeVsStresses Highertemperaturesandlongertimesofannealingbringresidualstressestolowerlevels Allkindsoftimes(heatingtime,soakingtime,coolingtime)

  14. StressReliefAnnealing– CoolingRateVsStresses Theresidualstresslevelafterstress-reliefannealingwillbemaintainedonlyifthecooldownfromtheannealingtemperatureiscontrolledandslowenoughthatnonewinternalstressesarise. Newstressesthatmaybeinducedduringcoolingdependon: Coolingrate Cross-sectionalsizeofthe work-piece,and (3)Compositionof thesteel

  15. 2.Normalizing Aheattreatmentprocessconsistingofaustenitizingattemperaturesof50–80˚C aboveuppercriticaltemperature(A1,Acm)followedbyslowcooling(usuallyinair) Theaimofwhichistoobtainafine-grained,uniformlydistributed,ferrite– pearlitestructure Normalizingisappliedmainlytounalloyedandlow-alloyhypo-eutectoidsteels Forhypereutectoidsteelstheaustenitizingtemperatureis50–80˚C abovetheACmtransformationtemperature

  16. Normalizing–Heatingand Cooling Purposeofsoaking: Toallowmetalto attainuniformtemp Alltheaustenite A3 transforminto pearlite,especially forhyper-eutectoid A1 compositions

  17. Normalizing–Austenitizing TemperatureRange Dependon composition IncreaseinC%reducestempforhypo-eutectoidsteels IncreaseinC%increasestempforhypo-eutectoidsteels

  18. EffectofNormalizingonGrainSize Normalizingrefines(reduces)thegrainsofasteelthathavebecomecoarse(longandirregular)asaresultofheavydeformationsasinforgingorinrolling Thefinegrainshavehighertoughnessthancoarsegrains, Steelwith0.5%C

  19. NormalizingafterRolling Afterhotrolling,thestructureofsteelisusuallyorientedintherollingdirection Toremovetheorientedstructureandobtaintheuniformmechanicalpropertiesinalldirections,anormalizingannealinghastobeperformed

  20. NormalizingafterForging • Afterforgingathightemperatures,especiallywithwork-piecesthatvarywidelyincrosssectionalsize,becauseofthedifferentratesofcoolingfromtheforgingtemperature,aheterogeneousstructureisobtainedthatcanbemadeuniformbynormalizing • Normalizingisalsodonetoimprove • machinabilityoflow-csteels

  21. Normalizing–HoldingTime Holdingtimeataustenitizingtemperaturemaybecalculatedusingtheempiricalformula: t=60+D wheretistheholdingtime(min)andDisthemaximumdiameteroftheworkpiece(mm).

  22. 3.FullAnnealing • Forcompositionslessthaneutectoid,themetalisheatedaboveA3linetoformaustenite • Forcompositionslargerthaneutectoid,themetalisheated • aboveA1linetoformausteniteandFe3C • CooledslowlyinafurnaceinsteadinairasinNormalizing. • Furnaceisswitchedoff,bothmetalandfurnacecoolatthesame • rate • Microstructureoutcome:Coarse • Pearlite.InNormalizing,structure? • StructureisrelativelysofterthanthatinNormalizing • Fullannealingisnormallyusedwhenmaterialneedstobedeformedfurther. Usuallyappliedforlow andmediumCsteel

  23. 4.SpheroidizingAnnealing ItisalsocalledasSoftAnnealing Anyprocessofheatingandcoolingsteelthatproducesaroundedorglobularformofcarbide(Fe3C) ItisanannealingprocessattemperaturesclosebeloworcloseabovetheA1temperature,withsubsequentslowcooling UsedforMedium&HighC-Steels -Spheroiditecanform atlowertemperaturesbutthe timeneededdrasticallyincreases,asthisisadiffusion-controlledprocess. Fe3C Fe3C

  24. Spheroidizing:HowtoPerform ByheatingalloyatatempjustbelowA1(700C).Ifpre-cursorstructureispearlite,processtimewillrangeb/w15&25Hrs HeatingalloyjustaboveA1lineandtheneithercoolingveryslowlyinthefurnaceorholdingataTempjustbelowA1 Heating&coolingalternativelywithin±50CoftheA1line.

  25. Spheroidizing-Purpose Theaimistoproduceasoftstructurebychangingallhardmicro-constituentslikepearlite,bainite,andmartensite(especiallyinsteelswithcarboncontentsabove0.5%andintoolsteels)intoastructureofspheroidizedcarbidesinaferriticmatrix amedium-carbonlow-alloysteelaftersoftannealingat720C; ahigh-speedsteelsoftannealedat820C.

  26. Spheroidizing-Uses Suchasoftstructureisrequiredforgoodmachinabilityofsteelshavingmorethan0.6%Candforallcold-workingprocessesthatincludeplasticdeformation. Spheroiditesteelisthesoftestandmost ductileformofsteel

  27. 5.IsothermalAnnealing SpheroidizingismoreusefulforimprovingmachinabilityofhighCsteelthanthatoflowandmediumCsteels. Infact,spherodizedlowandmediumCsteelsbecomeoversoftformachiningandgivelongshavingswhichaccumulateontoolcuttingedgeandproducepoorsurface. Hypoeutectoidlow-carbonsteelsaswellasmedium-carbonstructuralsteelsareoftenisothermallyannealed,forbestmachinability Anisothermallyannealedstructureshouldhavethefollowingcharacteristics: Highproportionofferrite Uniformlydistributedpearlitegrains Finelamellarpearlitegrains

  28. Process–IsothermalAnnealing Austenitizingfollowedbyafastcoolingtothetemperaturerangeofpearliteformation(usuallyabout650˚C.) Holdingatthistemperatureuntilthecompletetransformationofpearlite andcoolingtoroomtemperatureatanarbitrarycoolingrate ?  Fe3C

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