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Heat Treatment of Metals

Heat treatment alters properties of metals through controlled heating and cooling. Learn about annealing, tempering, quenching, and more techniques used on steel. Discover the phases and diagrams involved, such as the Iron-C Phase Diagram. Understand how stress-relief annealing works, influencing residual stresses and microstructure changes. Find out about normalizing processes for improving grain size and mechanical properties. Enhance your knowledge of heat treatment to optimize metal properties effectively.

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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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