金属热处理外文文献及翻译Word下载.docx
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金属热处理外文文献及翻译Word下载.docx
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Likewise,thetypesof heat treatment thataresometimesusedforproductssuchas glassorplasticsarealsoexcludedfrom coverageby this definition.
TransformationCurves
The basisforheattreatmentisthetime—temperature—transformation curves orTTTcurveswhere,in asinglediagram allthethreeparametersareplotted.Becauseoftheshapeofthecurves,they arealsosometimescalledC—curves orS-curves.
To plotTTTcurves,theparticularsteelisheldatagiventemperatureandthestructureisexamined at predeterminedintervalstorecordtheamountoftransformation taken place.Itisknown that the eutectoidsteel(T80)under equilibriumconditions contains,all austeniteabove723℃,whereas below,itisthepearlite。
Toformpearlite,thecarbonatoms should diffusetoformcementite。
Thediffusion being arate process,wouldrequiresufficient timefor completetransformationofaustenitetopearlite.Fromdifferentsamples,itispossible to note the amountofthetransformation takingplace atanytemperature.Thesepointsarethenplottedonagraphwithtimeandtemperature as theaxes。
Throughthesepoints,transformationcurvescanbeplottedasshownin Fig.1 foreutectoidsteel.Thecurveatextreme leftrepresentsthetimerequiredforthetransformationofaustenitetopearlitetostartatany giventemperature.Similarly,thecurve at extremerightrepresents thetimerequiredforcompleting thetransformation.Betweenthetwocurvesarethepointsrepresentingpartialtransformation.ThehorizontallinesMsandMfrepresentthestart andfinishof martensitictransformation.
Classification of Heat Treating Processes
In someinstances,heattreatmentprocedures areclear-cutintermsof techniqueandapplication.whereasin otherinstances,descriptionsorsimpleexplanations areinsufficientbecausethesametechniquefrequently maybeused to obtaindifferentobjectives。
Forexample,stress relievingand temperingareoftenaccomplished with the sameequipmentand byuseofidentical timeandtemperaturecycles。
Theobjectives,however,aredifferentforthetwoprocesses.
Thefollowingdescriptionsofthe principalheattreatingprocessesaregenerallyarranged according totheirinterrelationships.
Normalizingconsistsofheatingaferrousalloyto a suitabletemperature(usually 50°
Fto 100°
For 28℃ to56℃)aboveitsspecificuppertransformationtemperature.Thisisfollowedbycoolingin stillairtoatleastsometemperaturewellbelowitstransformation temperature range.Forlow—carbonsteels,the resultingstructure andpropertiesarethesameasthoseachievedbyfullannealing;
formostferrousalloys,normalizingand annealingarenotsynonymous.
Normalizingusuallyisusedasa conditioningtreatment,notablyfor refining the grainsofsteels thathavebeensubjectedto hightemperaturesforforgingorotherhot workingoperations.The normalizingprocessusually issucceeded by anotherheattreating operationsuchasaustenitizingforhardening,annealing, ortempering.
Annealing isa generictermdenotingaheattreatmentthatconsistsofheating toandholding atasuitable temperature followedbycoolingat asuitablerate。
Itis usedprimarilytosoftenmetallicmaterials,butalsotosimultaneouslyproducedesiredchangesin other propertiesorin microstructure.Thepurposeofsuchchangesmaybe,butis notconfinedto,improvement ofmachinability,facilitation ofcoldwork(knownasin-process annealing), improvement ofmechanical orelectricalproperties,orto increase dimensional stability.When appliedsolelytorelivestresses,it commonlyis calledstress-reliefannealing,synonymouswithstressrelieving.
Whentheterm“annealing”isapplied to ferrousalloys withoutqualification,full annealingisapplied。
Thisisachievedby heatingabovethealloy’stransformationtemperature,thenapplying acoolingcyclewhich provides maximumsoftness. Thiscyclemayvarywidely,dependingon compositionandcharacteristicsofthe specificalloy。
Quenchingisarapidcoolingof asteeloralloy from the austenitizingtemperature byimmersingtheworkpiece inaliquidorgaseousmedium.Quenchingmedium commonly usedinclude water,5%brine,5%causticin anaqueoussolution, oil, polymer solutions,or gas(usuallyairornitrogen)。
Selectionofaquenching mediumdependslargelyon the hardenability ofmaterialandthe massofthe materialbeingtreating(principallysectionthickness).
The coolingcapabilities ofthe above-listedquenching mediavary greatly。
In selectingaquenchingmedium,itisbesttoavoidasolution thathasmore cooling power thanisneeded toachievetheresults,thusminimizing the possibilityofcrackingand warpof theparts being treated。
Modificationsofthetermquenchingincludedirectquenching,fogquenching,hotquenching,interruptedquenching, selectivequenching,spray quenching,and timequenching。
Tempering.In heattreatingofferrousalloys,temperingconsistsofreheatingthe austenitized andquench-hardenedsteelorirontosomepreselectedtemperaturethatis belowthelowertransformation temperature(generallybelow1300℃or705℃).Temperingoffers a meansofobtainingvariouscombinations of mechanical properties.Temperingtemperaturesusedforhardenedsteelsare often no higherthan 300℃(150℃).Theterm“tempering”shouldnot beconfusedwitheitherprocessannealingorstressrelieving。
Eventhoughtimeandtemperaturecycles forthe three processesmaybethesame,the conditionsof thematerialsbeingprocessed and the objectives maybedifferent。
Stress relieving。
Liketempering,stressrelievingis alwaysdonebyheatingtosometemperaturebelow the lowertransformationtemperatureforsteels andirons. Fornonferrous metals,thetemperature mayvaryfrom slightly aboveroomtemperaturetoseveral hundred degrees,dependingon thealloyandtheamountofstress reliefthatisdesired.
Theprimary purposeofstressrelievingistorelievestressesthathavebeenimpartedtotheworkpiecefrom suchprocessesas forming, rolling,machining or welding.The usual procedureisto heat workpiecetothe pre—establishedtemperature longenoughtoreducetheresidualstresses (thisisatime-andtemperature-dependentoperation) toanacceptablelevel;
thisis followed by coolingat arelativelyslowrate toavoidcreationofnewstresses.
Thegenerallyaccepteddefinition forheat treating metals andmetalalloysis “heatingandcoolinga solidmetalor alloy ina waysoastoobtainspecificconditionsorproperties.”Heating for thesolepurposeof hot working(as in forging operations)isexcludedfromthis definition.Likewise,thetypes ofheattreatmentthataresometimes used forproductssuchasglass orplasticsarealso excluded fromcoveragebythisdefinition.
Transformation Curves
Thebasisforheattreatment isthe time—temperature-transformationcurvesorTTT curves where,inasingle diagramall thethreeparametersareplotted.Becauseofthe shapeofthe curves,theyarealsosometimescalledC—curves or S—curves.
Toplot TTTcurves,theparticularsteelisheldata giventemperatureandthestructureis examinedatpredeterminedintervals torecordtheamountoftransformation takenplace.Itisknownthattheeutectoidsteel(T80)underequilibriumconditionscontains,all austeniteabove723℃,whereasbelow,it ispearlite.Toform pearlite,the carbon atoms shoulddiffuse toformcementite。
Thediffusionbeing arateprocess,wouldrequiresufficienttimefor completetransformationofausteniteto pearlite.Fromdifferentsamples,itispossibleto notetheamountofthe transformation takingplaceatanytemperature.Thesepoints are then plottedonagraph with timeandtemperatureas theaxes。
Through thesepoints,transformationcurvescan be plottedas showninFig.1foreutectoid steel.The curveatextremeleftrepresentsthetimerequiredforthetransformation ofaustenitetopearlite to startatanygiventemperature.Similarly,thecurveatextremerightrepresentsthetimerequiredfor completingthetransformation。
Betweenthetwo curvesarethe pointsrepresentingpartial transformation。
Thehorizontal linesMs andMf representthe startand finishofmartensitic transformation.
ClassificationofHeatTreatingProcesses
In someinstances,heattreatmentproceduresareclear-cutintermsoftechniqueand application.whereasinother instances,descriptions orsimple explanationsareinsufficientbecause the sametechniquefrequentlymaybeusedtoobtaindifferent objectives.Forexample, stressrelieving andtemperingareoftenaccomplished withthe same equipmentandbyuse ofidenticaltime andtemperaturecycles.Theobjectives,however,aredifferentfor thetwoprocesses.
Thefollowingdescriptions oftheprincipal heattreatingprocesses aregenerally arrangedaccordingtotheir interrelationships。
Normalizingconsistsofheatingaferrousalloytoasuitabletemperature (usually50°
Fto100°
F or28℃to56℃)aboveitsspecificuppertransformationtemperature.Thisisfollowedbycoolinginstill air toatleastsometemperaturewellbelowits transformationtemperature range.For low-carbonsteels,theresulting structureandpropertiesare thesameasthoseachieved byfull annealing;
for mostferrous alloys,normalizingandannealing arenotsynonymous.
Normalizing usuallyisusedasa conditioning treatment,notably forrefiningthegrainsofsteelsthat havebeensubjected tohightemperaturesforforgingorotherhotworkingoperations。
Thenormalizingprocessusually is
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