国外热处理教程WORD版.docx
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国外热处理教程WORD版.docx
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国外热处理教程WORD版
HeatTreatment
OBJECT:
ToperformaJominyend-quenchtestinordertoobserveheattreatmenthardeningandpreparethehardenabilitycurveforasteelbar.
THEORY:
Steelisthemostimportantengineeringandconstructionmaterial;itaccountsforapproximately80%ofallmetalsproduced.Steelhasattainedthisdegreeofprominencebecauseitcombinesstrength,easeoffabricabilityintomanyshapes,andawiderangeofpropertiesalongwithlowcost.Alsoitispossibletogiveawiderangeofmechanicalpropertiestosteelsbychangingthesizeadshapeofthegrainsorchangingitsmicroconstituents.Thispropertyowestoseveraldifferentwaysthataustenitecandecompose.
Fundamentally,allsteelsarealloysofironandcarbon.So-calledplaincarbonsteelsalsogenerallyhavesmallbutspecifiedamountsofphosphorusandsulfur.Alloysteelsarethosewhichcontainspecifiedpercentagesofotherelementsintheirchemicalcompositions.
Figure1.Iron-CarbonEquilibriumDiagram[1]
HARDENABILITY:
Ingeneralstrengthofagivensteelisproportionaltoitshardness;thehigherthehardness,thestrongerthesteel.Thecarboncontentofasteeldeterminesthemaximumhardnessattainable.Themostimportantfactorinfluencingthemaximumhardnessismassofthemetalbeingquenched.Inasmallsection,theheatisextractedquickly,thusexceedingthecriticalcoolingrateofthespecificsteel.Thecriticalcoolingrateisthatrateofcoolingwhichmustbeexceededtopreventformationofnon-martensiteproducts.
Hardenabilityistheeasewithwhichhardnessmaybeattained.Asteelthattransformsrapidlyfromaustenitetoferritepluscarbidehaslowhardenabilitybecause(+carbide)isformedattheexpenseofmartensite.Converselyasteelthattransformsslowlyfromaustenitetoferritehasgreaterhardenability.
Foranygivensteel,thereisadirectandconsistentrelationshipbetweenhardnessandcoolingrate.Howevertherelationshipishighlynon-linear.Thereisastandardizedtestthatletsusmakenecessarypredictionsofhardness.ThisistheJominyend-quenchedtest.Aroundbarwithastandardsizeisheatedtoformausteniteandisthanend-quenchedwithawaterstreamofspecifiedflowrateandpressure.HardnessvaluesalongthebararedeterminedonaRockwellharnesstesterandaHardenabilitycurveisplotted.
Thequenchedendiscooledveryfastandthereforehasthemaximumpossiblehardnessfortheparticularcarboncontentofthesteelthatisbeingtested.Thecoolingratesatpointsbehindthequenchedendareslowerandconsequentlythehardnessvaluesarelower.
Figure2.TransformationProcesses[3].
HEATTREATMENT:
Theamountofcarbonpresentinplaincarbonsteelhasapronouncedeffectonthepropertiesofasteelandontheselectionofsuitableheattreatmentstoattaincertaindesiredproperties.Belowaresomemajortypesofheattreatmentprocesses:
Annealing:
Steelisannealedtoreducethehardness,improvemachinability,facilitatecold-working,produceadesiredmicrostructure.Fullannealingistheprocessofsofteningsteelbyaheatingandcoolingcycle,sothatitmaybebentorcuteasily.Inannealing,steelisheatedabovethetransformationtemperaturetoformaustenite,andcooledveryslowly,usuallyinthefurnace.
Thereareseveraltypesofannealinglikeblackannealing,blueannealing,boxannealing,brightannealing,flameannealing,intermediateannealing,isothermalannealing,processannealing,recrystallisationannealing,softannealing,finishannealingandspheroidizing.Thesearepracticedaccordingtotheirdifferentfinalproductpropertiesintheindustry.
Thetwo-stageheattreatingprocessofquenchingandtemperingisdesignedtoproducehighstrengthsteelcapableofresistingshockanddeformationwithoutbreaking.Ontheotherhand,theannealingprocessisintendedtomakesteeleasiertodeformormachine.Inmanufacturingsteelproducts,machiningandseverebendingoperationsareoftenemployed.Eventemperedsteelmaynotcutorbendveryeasilyandannealingisoftennecessary.ProcessannealingconsistsofheatingsteeltoatemperaturejustbelowtheA1forashorttime.Thismakesthesteeleasiertoform.Thisheattreatmentiscommonlyappliedinthesheetandwireindustries,andthetemperaturesgenerallyusedarefrom1020to12000F(550to6500C).Fullannealing,wheresteelisheated50to1000F(90to1800C)abovetheA3forhypoeutectoidsteels,andabovetheA1forhypereutectoidsteels,andslowcooled,makesthesteelmucheasiertocut,aswellasbend.Infullannealing,coolingmusttakeplaceveryslowlysothatacoarsepearliteisformed.Slowcoolingisnotessentialforprocessannealing,sinceanycoolingratefromtemperaturesbelowA1willresultinthesamemicrostructureandhardness.
Normalizing:
Innormalizingsteelisalsoheatedaboveaustenitizingtemperature,butcoolingisaccomplishedbystillaircoolinginafurnace.Steelisnormalizedtorefinegrainsize,makeitsstructuremoreuniform,ortoimprovemachinability.Whensteelisheatedtoahightemperature,thecarboncanreadilydiffusethroughout,andtheresultisareasonablyuniformcompositionfromoneareatothenext.Thesteelisthenmorehomogeneousandwillrespondtotheheattreatmentinamoreuniformway.
Theprocessmightbemoreaccuratelydescribedasahomogenizingorgrain-refiningtreatment.Withinanypieceofsteel,thecompositionisusuallynotuniformthroughout.Thatis,oneareamayhavemorecarbonthantheareaadjacenttoit.Thesecornpositionaldifferencesaffectthewayinwhichthesteelwillrespondtoheattreatment.Becauseofcharacteristicsinherentincaststeel,thenormalizingtreatmentismorefrequentlyappliedtoingotspriortoworking,andtosteelcastingsandforgingspriortohardening.
Hardening:
Hardeningiscarriedoutbyquenchingasteel,thatiscoolingitrapidlyfromatemperatureabovethetransformationtemperature.Steelisquenchedinwaterorbrineforthemostrapidcooling,inoilforsomealloysteels,andinairforcertainhigheralloysteels.Withthisfastcoolingrate,thetransformationfromaustenitetopearlitecannotoccurandthenewphaseobtainedbyquenchingiscalledmartensite.Martensiteisasupersaturatedmetastablephaseandhavebodycenteredtetragonallettice(bct)insteadofbcc.Aftersteelisquenched,itisusuallyveryhardandstrongbutbrittle.Martensitelooksneedle-likeundermicroscopeduetoitsfinelamellarstructure.
Tempering:
Tempering(formerlycalleddrawing),consistsofreheatingaquenchedsteeltoasuitabletemperaturebelowthetransformationtemperatureforanappropriatetimeandcoolingbacktoroomtemperature.Freshlyquenchedmartensiteishardbutnotductile.Temperingisneededtoimpartductilitytomartensiteusuallyatasmallsacrificeinstrength.
Theeffectoftemperingmaybeillustratedasfollows.Iftheheadofahammerwerequenchedtoafullymartensiticstructure,itprobablywouldcrackafterthefirstfewblows.Temperingduringmanufactureofthehammerimpartsshockresistancewithonlyaslightdecreaseinhardness.Temperingisaccomplishedbyheatingaquenchedparttosomepointbelowthetransformationtemperature,andholdingitatthistemperatureforanhourormore,dependingonitssize.
Themicrostructuralchangesaccompanyingtemperingincludelossofacicularmartensitepatternandtheprecipitationoftinycarbideparticles.Thismicrostructuralisreferredtoastemperedmartensite.
StressRelieving:
Whenametalisheated,expansionoccurswhichismoreorlessproportionaltothetemperaturerise.Uponcoolingametal,thereversereactiontakesplace.Thatis,acontractionisobserved.Whenasteelbarorplateisheatedatonepointmorethanatanother,asinweldingorduringforging,internalstressesaresetup.Duringheating,expansionoftheheatedareacannottakeplaceunhindered,andittendstodeform.Oncooling,contractionispreventedfromtakingplacebytheunyieldingcoldmetalsurroundingtheheatedarea.Theforcesattemptingtocontractthemetalarenotrelieved,andwhenthemetaliscoldagain,theforcesremainasinternalstresses.Stressesalsoresultfromvolumechangeswhichaccompanymetaltransformationsandprecipitation.
Thetermstresshaswideusageinthemetallurgicalfield.Itisdefınedsimplyasbadorforcedividedbythecross-sectionalareaoftheparttowhichthebadorforceisapplied.Internal,orresidualstresses,arebadbecausetheymaycausewarpingofsteelpartswhentheyaremachined.Torelievethesestresses,steelisheatedtoaround11000F(5950C)assuringthattheentirepartisheateduniformly,thencooledslowlybacktoroomtemperature.Thisprocedureiscalledstressreliefannealing,ormerelystressrelieving.
ALLOYINGELEMENTSINQUENCHING:
Becausethesectionstreatedareoftenrelativelylargeandbecausethealloyingelementshavethegeneraleffectofloweringthetemperaturerangeatwhichmartensiteisformed,thethermalandtransformationalstressessetupduringquenchingtendtobegreaterinthesealloysteelpartsthanthoseencounteredinquenchingthenecessarilysmallersectionsofplaincarbonsteels.Ingeneral,thegreaterstressesresultindistortionandriskofcracking.
Alloyingelements,however,havetwofunctionsthattendtooffsetthesedisadvantages.Firstandprobablymostimportantisthecapacitytopermituseofalowercarboncontentforagivenapplication.Thedecreaseinhardenabilityaccompanyingthedecreaseincarboncontentmaybereadilyoffsetbythehardenabilityeffectoftheaddedalloyingelementsandthelowercarbonsteelwillexhibitamuchlowersusceptibilitytoquenchcracking.Thislow
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