英文翻译外文文献翻译39材料的热处理.docx
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英文翻译外文文献翻译39材料的热处理.docx
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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,thephasediagramisnolongerausefultoolforenginee
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