外文翻译材料的热处理.docx
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外文翻译材料的热处理
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,thereissomeadditionalreductioninhardnessif