材料的热处理.docx

材料的热处理.docx

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材料的热处理

外文资料

HEATTREATMENTOFMETALS

Theunderstandingofheattreatmentisembracebythebroaderstudyofmetallurgy.Metallurgyisthephysics,chemistry,andengineeringrelatedtometalsfromoreextractiontothefinalproduct.Heattreatmentistheoperationdoheatingandcoolingametalinitssolidstatetochangeitsphysicalproperties.Accordingtotheprocedureused,steelcanbehardenedtoresistcuttingactionandabrasion,oritcanbesoftenedtopermitmachining.Withtheproperheattreatmentinternalductileinterior.Theanalysisofthesteelmustbeknownbecausesmallpercentagesofcertainelements,notablycarbon,greatlyaffectthephysicalproperties.

Alloysteelsowetheirpropertiestothepresenceofoneormoreelementsotherthancarbon,namelynickel,chromium,manganese,molybdenum,tungsten,silicon,vanadium,andcopper.Becauseoftheirimprovedphysicalpropertiestheyareusedcommerciallyinmanywaysnotpossiblewithcarbonsteels.

Thefollowingdiscussionappliesprincipallytotheheattreatmentofordinarycommercialsteelknownasplain-carbonsteels.Withthisprovestherateofcoolingisthecontrollingfactor,producestheoppositeeffect.

ASIMPLIFIEDIRON-CARBONDAGRAM

Ifwefocusonlyonthematerialsnormallyknownassteels,asimplifieddiagramisoftenused.Thoseportionsoftheiron-carbondiagramnearthedeltaregionandthoseabove2%carboncontentareoflittleimportancetotheengineerandaredeleted.Asimplifieddiagram,suchastheoneinFig.2.1focusesontheeutectoidregionandisquiteusefulinunderstandingthepropertiesandprocessingofsteel.

Thekeytransitiondescribedinthisdiagramisthedecompositionofsingle-phaseaustenite（

）tothetwo-phaseferritepluscarbidestructureastemperaturedrop.Controlofthisreaction,whicharisesduetothedrasticallydifferentcarbonsolubilitiesofausteniteandferrite,enablesawiderangeofpropertiestobeachievedthroughheattreatment.

Tobegintounderstandtheseprocesses,considerssteeloftheeutectoidcomposition,0.77%carbon,beingslowcooledalongline

inFig.2.1Attheuppertemperatures,onlyausteniteispresent,the0.77%carbonbeingdissolvedinsolidsolutionwiththeiron.Whenthesteelcoolsto727

severalchangesoccursimultaneously.TheironwantstochangefromthebccaustenitestructuretothebccferriteStructure,buttheferritesanonlycontain0.02%carboninsolidsolution.Therejectedcarbonformsthecarbon-richcementiteintermetallicwithcomposition

.Inessence,thenetreactionattheeutectoidis:

Austenite

ferrite+cementite

Sincethischemicalseparationofthecarboncomponentoccursentirelyinthesolidstate,theresultingstructureisafinemechanicalmixtureofferriteandcementite.Specimentspreparedbyplolishingandetchinginaweaksolutionlfnitricacidandalcoholrevealthelamellarstructurelfalternatingplatesthatformsonslowcooling.Thisstructureiscomposedoftwodistinctphases,buthasitsownsetofcharacteristicpropertiesandgoesbythenamepearlite,becauseofitsresemblancetomother-of-pearlatlowmagnification.

Steelshavinglessthantheeutectoidamountofcarbon（lessthan0.77%）areknownashypoeutectoidsteels.Considernowthetransformationofsuchamaterialrepresentedbycoolingalongliney-y′inFig.2.1.Athightemperatures,thematerialisentrirelyaustenite,butuponcoolingentersaregionwherethestablephasesareferriteandaustenite.Tie-lineandlever-lawcalculationsshowthatlow-carbonferritenucleatesandgrows,leavingtheremainingaustenitericherincarbon.At727°C（1341°F）,theausteniteisofeutectoidcompositon（0.77%carbon）andfurthercoolingtransformstheremainingaustenitetopearlite.Theresultingstructureisamixturelfprimaryorproeutectoidferrite（ferritethatformedabovetheeutectoidreaction）andregionsofpearlite.

Hypereutectoidsteelsaresteelsthatcontaingreaterthantheeutectoidamountofcarbon.Whensuchasteelcools,asinz-z′ofFig.2.1theprocessissimilartothehypoeutectoidcase,exceptthattheprimaryorproeutectoidphaseisnowcementiteinsteadlfferrite.Asthecarbon-richphaseforms,theremainingaustenitedecreasesincarboncontent,reachingtheeutectoidcompositionat727°C（1341°F）.Asbefore,anyremainingaustenitetransformstopearliteuponslowcoolingthroughthistemperature.

Itshouldberememberedthatthetransitionsthathavebeendescribedbythephasediagramsareforequilibriumconditions,whichcanbeapproximatedbyslowcooling,Withslowheating,thesetransitionsoccurintherevertsemanner.However,whenalloysarecooledrapidly,entirelydifferentresultsmaybeobtained,becausesufficienttimeisnotprovidedforthenormalphasereactionstooccur,Insuchcases,thephasediagramisnolongerausefultoolforengineeringanalysis.

HARDENING

Hardeningistheprocessofheatingppieceofsteeltoatemperaturewithinoraboveitscriticalrangeandthencoolingitrapidly.Ifthecarboncontentofthesteelisknown,thepropertemperaturetowhichthesteelshouldbeheatedmaybeobtainedbyreferencetotheiron-ironcarbidephasediagram.However,ifthecompositionofthetsteelisunknown,alittlepreliminaryexperimentationmaybenecessarytodeterminetherange.Agoodproceduretofollowistoheat-quenchanumberlfsmallspecimenslfthesteelatvarioustemperatureslfthesteelatvarioustemperaturesandobservetheresults,eitherbyhardnesstestingorbymicroscopicexamination.Whenthencorrecttemperatureisobtained,therewillbemarkedchangeinhardnessandotherproperties.

Inanyheat-treatingoperationtherateofheatingisimportant.Heatflowsfromtheexteriortotheinteriorofsteelatadefiniterate.Ifthesteelisheatedtoofast,theoutsidebecomeshotterthantheinterioranduniformstructurecannotbeobtained.Ifapieceisirregularinshape,aslowrateisallthemoreessentialtoeliminatewarpingandcracking.Theheavierthesection,thelongermustbetheheatingtimetoachieveuniformresults.Evenafterthecorrectremperaturehasbeenreached,thepieceshouldbeheldatthattemperatureforasufficientperiodoftimetopermititsthickestsectiontoattainauniformtemperature.

Thehardnessobtainedfromagiventreatmentdependsonthequenchingrate,thecarboncontent,andtheworksize,Inalloysteelsthekindandamountlfalloyingelementinfluencesonlythehardenability（theabilitylftheworkpiecetobehardenedtodepths）lfthesteelanddoesnotaffectthehardnessexceptinunhardenedorpartiallyhardenedsteels.

Steelwithlowcarboncontentwillnotrespondappreciablytohardeningtreatments.Asthecarboncontentinsteelincreasesuptoaround0.60%,thepossiblehardnesscanbeincreasedonlyslightly,becausesteelsabovetheeutectoidpointaremadeupentirelyofpearliteandcementiteintheannealedstate.Pearliterespondsbesttoheat-treatingoperations;anysteelcomposedmostlyofpearlitecanbetransformedintoahardsteel.

Asthesizeofpartstobehardenedincreases,thesurfacehardnessdecreasessomewhateventhoughallotherconditionshaveremainedthesame.Thereisalimittotherateofheatflowthroughsteel.Nomatterhowcoolthesame.Thereisalimittotheratelfheatflowthroughsteel.Nomatterhowcoolthequenchingmediummanybe,iftheheatinsidealargepiececannotescapefasterthanacertaincriticalrate,thereisadefinitelimittotheinsidehardness.However,brineorwaterquenchingiscapablelfrapidlybringingthesurfacelfthequenchedparttoitowntemperatureandmaintainingitatorclosetothistemperature.Underthesecircumstancestherewouldalwaysbesomefinitedepthofsurfacehardeningregardlesslfsize.Thisisnottrueinoilquenching,whenthesurfacetemperaturemaybehighduringthecriticalstagesofquenching.

TEMPERING

Steelthathasbeenhardenedbyrapidquenchingisbrittleandnotsuitableformostuses.Bytemperingordrawing,thehardnessandbrittlenessmaybereducedtothedesiredpointforserviceconditions.Asthesepropertiesarereducedthereisalsoadecreaseintensilestrengthandanincreaseintheductilityandtoughnessofthesteel.Theoperationconsistslfreheatingquench-hardenedsteeltosometemperaturebelowthecriticalrangefollowedbyanyratelfcooling.Althoughthisprocesssoftenssteel,itdiffersconsiderablyfromannealinginthattheprocesslendsitselftoclosecontrollfthephysicalpropertiesandinmostcasesdoesnotsoftenthesteeltotheextentthatannealingwould.Thefinalstructureobtainedfromtemperingafullyhardenedsteeliscalledtemperedmartensite.

Temperingispossiblebecauseoftheinstabilityofthemartensite,theprincipalconstituentofhardenedsteel.Low-temperaturedraws,from300°to400°F（150°-205°C）,donotcausemuchdecreaseinhardnessandareusedprincipallytorelieveinternalstrains.Asthetemperingtemperaturesareincreased,thebreakdownofthemartensitetakesplaceatafasterrate,andatabout600°F（315°C）thechangetoastructurecalledtemperedmartensiteisveryrapid.Thetemperingoperationmaybedescribedasonelfprecipitationandagglomerationorcoalescenceofcementite.Asubstantialprecipitationlfcementitebeginsat600°F（315°C）,whichproducesadecreaseinhardness.Increasingthetemperaturecausescoalescencelfthecarbideswithcontinueddecreaseinhardness.

Intheprocessoftempering,someconsiderationshouldbegiventotimeaswellastotemperature.Althoughmostofthesofteningactionoccursinthefirstfewminutesafterthetemperatureisreached,thereissomeadditionalreductioninhardnessifthe