材料成型专业英语 Heat Treatment

EnglishinMaterialsScienceandEngineering材料科学与工程专业英语2020/2/2合肥工大材料学院何顺荣制作2EnglishinMaterialsScienceandEngineeringInitialheatingtoatemperatureupto900℃giveslamellarpearlite,withmoredisperse*structurebeingobtainedatlowertempera-ture.Thestructureofthesamesteelatthesametemperatureoftransformationbutuponalowtemperatureheating(780℃)isgranularpearlite.Cementitegrainsthatareformedinthestructurearefineratalowertemperaturetransformation.2020/2/2合肥工大材料学院何顺荣制作3EnglishinMaterialsScienceandEngineeringConsequently,thesizeofcementiteparticlesdependsonthetemperatureofaustenitetransformationandtheirformdependsonthetemperatureofheating(thetemperatureofaustenitization).ThetransformationattemperatureaboveandbelowthebendoftheCcurvedifferinthekineticsandtheshapeofdecompositionproducts.2020/2/2合肥工大材料学院何顺荣制作4EnglishinMaterialsScienceandEngineeringAbovethebendofCcurve,i.e.atsmalldegreesofundercooling,thetransformationbeginsfromafewcentersandpearliticcrystalsfreelygrowtointerference.BelowthebendofCcurvethereformsanacicularmicrostructure,sinceacicularplateletscannotgrowfreelyandthetransformationmainlyoccursthroughtheformationofnewcrystals.[1]2020/2/2合肥工大材料学院何顺荣制作5EnglishinMaterialsScienceandEngineeringAsindicatedabove,therateofisothermaltransformationofausteniteisdeterminedbytherateofnucleationandcrystalgrowth,NandG.Therateofcrystalgrowthisquitesensitivetovariouschangesinthesteelstructureandcanbestronglyaffectedbythemetallurgicalnatureofthesteel,thedegreeofdeoxidization,thepresenceofundissolvedparticles,thehomogeneityofaustenite,andthesizeofausteniticgrains.2020/2/2合肥工大材料学院何顺荣制作6EnglishinMaterialsScienceandEngineeringThepresenceofundissolvedparticlesincreasesthevalueofG,sincetheseparticlesserveasadditionalnuclei.ThevalueofGdiminisheswithincreasingsizeofausteniticgrains.Crystallizationnucleiappearmainlyongrainboundaries,therefore,theconditionsfornucleationareworseincoarsegrainedsteelwheretheextentofgrainboundariesissmaller.2020/2/2合肥工大材料学院何顺荣制作7EnglishinMaterialsScienceandEngineeringThefactorsmentionedhave,however,noeffectonthenucleationrate,N.ThevalueofNdependsonlyonsteelcompositionandforasteelofagivencomposition,isanaturalcharacteristicdependingonlyonthedegreeofundercooling.2020/2/2合肥工大材料学院何顺荣制作8EnglishinMaterialsScienceandEngineeringWehavediscussedtheaustenitetopearlitetransformationinsteelswhosecompositionisclosetoeutectoid*.Ifthecontentofcarboninasteeldiffersfromtheeutectoidvalue,thepearlitetransformationwillbeprecededwiththeprecipitation*offerriteorcementite(asfollowsfromtheironcarbonconstitutionaldiagram).2020/2/2合肥工大材料学院何顺荣制作9EnglishinMaterialsScienceandEngineeringInhypoeutectoid*steels,thetransformationofaustenitebeginswiththeformationofferriteandthesaturationoftheremainingsolutionwithcarbon,andinhypereutectoidsteels,withtheprecipitationofcementiteanddepletion*oftheausteniteofcarbon.[2]2020/2/2合肥工大材料学院何顺荣制作10EnglishinMaterialsScienceandEngineeringUnderequilibriumconditions,thedecompo-sitionofausteniteintoferriteandcementite(pearlitetransformation)beginswhenthecontentofcarboninaustenite,remaineduponprecipitationofexcessferriteorcementite,correspondsto0.8%carbon.2020/2/2合肥工大材料学院何顺荣制作11EnglishinMaterialsScienceandEngineeringTheeutectoidwhichformsfromunder-cooledausteniteandhasaconcentrationdifferingfromtheeutectoidvalueiscalledquasi*-eutectoid.Thequasi-eutectoidinhypereutectoidsteelscontainsmorethan0.8percentcarbonandthatinhypoeutectoidsteels,lessthan0.8percent,thedeviationfromthisvaluebeinggreateratalowertemperatureoftransform-ation.2020/2/2合肥工大材料学院何顺荣制作12EnglishinMaterialsScienceandEngineeringTherefore,thelowerthetemperatureoftransformation,thelesstheexcessferrite(orcementite)precipitatesbeforethepearlitetransformationbegins.AttemperaturesnearthebendoftheCcurveandatlowertemperatures,decom-positionofaustenitebeginswithoutprecipi-tationofexcessphases.2020/2/2合肥工大材料学院何顺荣制作13EnglishinMaterialsScienceandEngineeringIfwetakeahypereutectoidsteelinsteadofhypoeutectoid,thedecompositionofausteniteatsmalldegreesofundercoolingwillbeprecededwithprecipitationofcementite.Ifthecoolingrateishigher,thetransform-ationhasnotimetoproceedintheuppertemperaturerange,theaustenitewillbeundercooledtoalowtemperatureand2020/2/2合肥工大材料学院何顺荣制作14EnglishinMaterialsScienceandEngineeringandwilltransformintomartensite,suchacoolingwillresultinhardening.(淬火)Therefore,tohardensteel,itshouldbecooledatahighratesothataustenitehasnotimetodecomposeintheuppertemperaturerange.Thelowestcoolingrateneededtounder-coolausteniteuptomartensitetransformationiscalledthecriticalrateofhardening.Ifasteelistobehardened,itshouldbecooledataratenotlessthanthecritical.2020/2/2合肥工大材料学院何顺荣制作15EnglishinMaterialsScienceandEngineeringThecriticalrateislowerforsteelswhosecurveofthebeginningoftransformationpassesfarthertotheright.Inotherwords,withalowerrateofaustenitetopearlitetransformation,itiseasiertoundercooltheaustenitetothetemperatureofmartensitetransformationandthecriticalrateofhardeningwillbelower.2020/2/2合肥工大材料学院何顺荣制作16EnglishinMaterialsScienceandEngineeringIfcooli