机械工程第七课原文加翻译。

Heattreatmentisamethodusedtoalterthephysical,andsometimeschemical,propertiesofamaterial.热处理是一种用于改变材料物理、化学和力学性能的方法Themostcommonapplicationismetallurgical.最常见的应用是冶金Heattreatmentsarealsousedinthemanufactureofmanyothermaterials,suchasglass.热处理也被用于许多其他材料的制造，诸如玻璃Heattreatmentinvolvestheuseofheatingorchilling,normallytoextremetemperatures,toachieveadesiredresultsuchashardeningorsofteningofamaterial.热处理包括加热或冷却（通常是极端温度）的使用来获得期望的结果，诸如材料的硬化或软化Heattreatmenttechniquesincludeannealing,casehardening,precipitationstrengthening,temperingandquenching.热处理技术包括退火，表面硬化，沉淀硬化，回火和淬火。Itisnoteworthythatwhilethetermheattreatmentappliesonlytoprocesseswheretheheatingandcoolingaredoneforthespecificpurposeofalteringpropertiesintentionally,heatingandcoolingoftenoccurincidentallyduringothermanufacturingprocessessuchashotformingorwelding.值得注意的是，当术语热处理仅仅应用于为改变性能的专用目的采取加热和冷却的过程时，其它的制造工艺中，像热成型和焊接偶尔采用加热和冷却。Metallicmaterialsconsistofamicrostructureofsmallcrystalscalledgrainsorcrystallites.金属材料由称为晶粒的小晶体显微组织组成Thenatureofthegrains(i.e.grainsizeandcomposition)isoneofthemosteffectivefactorsthatcandeterminetheoverallmechanicalbehaviorofthemetal.晶粒的本质（晶粒大小和成分）是决定金属整个力学行为的最有效的因素Heattreatmentprovidesanefficientwaytomanipulatethepropertiesofthemetalbycontrollingrateofdiffusion,andtherateofcoolingwithinthemicrostructure.热处理通过控制扩散的速率以及显微组织内冷却的速率，为控制金属性能提供了有效的方法Complexheattreatingschedulesareoftendevisedbymetallurgiststooptimizeanalloy'smechanicalproperties.复杂的热处理安排经常由冶金学家设计以优化合金的力学性能Intheaerospaceindustry,asuperalloymayundergofiveormoredifferentheattreatingoperationstodevelopthedesiredproperties.在航空航天工业，超合金可能经历了5个或更多个不同的热处理操作以开发期望的性质Thiscanleadtoqualityproblemsdependingontheaccuracyofthefurnace‘stemperaturecontrolsandtimer.这能导致质量问题依赖于加热炉温度控制精度和定时器精度Annealingisatechniqueusedtorecovercoldworkandrelaxstresseswithinametal.退火是用于恢复冷加工和释放金属内的应力的一种技术Annealingtypicallyresultsinasoft,ductilemetal.退火使金属变软，具有延展性Whenanheatedpartisallowedtocoolinthefurnace,itiscalledafullannealheattreatment.当加热的零件被允许在炉内冷却时，被称为完全退火热处理Whenanheatedpartisremovedfromthefurnaceandallowedtocoolinair,itiscalledanormalizingheattreatment.当加热的零件从炉内取走在空气中冷却时，被称为正火热处理Duringannealing,smallgrainsrecrystallizetoformlargergrains.在退火过程中，小晶粒再结晶从而形成更大的晶粒Inprecipitationhardeningalloys,precipitatesdissolveintothematrix,solutionizingthealloy.在沉淀硬化合金中，沉淀物融入母体，固溶化合金Tohardenbyquenching,ametal(usuallysteelorcastiron)mustbeheatedintotheausteniticcrystalphaseandthenquicklycooled.为了通过淬火提高硬度，金属（通常为钢或铸铁）必须被加热为奥氏体结晶相，然后快速冷却Dependingonthealloyandotherconsiderations(suchasconcernformaximumhardnessvs.crackinganddistortion),coolingmaybedonewithforcedairorothergas(suchasnitrogen),oil,polymerdissolvedinwater,orbrine.依赖于合金和其他的考虑（诸如最大硬度、裂纹和变形），冷却必须通过强迫空气或其他气体、油、融入水中的聚合物或卤水Uponbeingrapidlycooled,aportionofaustentite(dependentonalloycomposition)willtransformtomartensite,ahardbrittlecrystallinestructure.基于快速冷却，部分奥氏体将转变为马氏体，一个硬且脆的晶体结构Thequenchedhardnessofametaldependsuponitschemicalcompositionandquenchingmethod.金属的淬火硬度依赖于它的化学成分和淬火方法Coolingspeeds,fromfastesttoslowest,gofrompolymer(i.e.silicon),brine,freshwater,oil,andforcedair.冷却速度，从最快到最慢依次为聚合物、卤水、清水、油和强迫空气However,quenchingacertainsteeltoofastcanresultincracking,whichiswhyHigh-tensilesteelslikeAISI4140shouldbequenchedinoil,toolsteelssuchas2767orH13hotworktoolsteelshouldbequenchedinforcedair,andlowalloyormedium-tensilesteelssuchasXK1320orAISI1040shouldbequenchedinbrineorwater.然而，某种钢淬火太快能够导致断裂，这就是为什么高强度钢（例如AISI4140）应该在油中冷却，诸如2767或H13热锻模具钢等工具钢应该在强迫空气中淬火，低合金或中强度钢应该在水或卤水中淬火的原因However,metalssuchasausteniticstainlesssteel(304,316),andcopper,produceanoppositeeffectwhenthesearequenched;theyanneal.然而，诸如奥氏体不锈钢等金属和铜，当它们淬火时产生相反的效果，它们退火Austeniticstainlesssteelsmustbequench-annealedtobecomefullycorrosionresistant,astheywork-hardensignificantly.奥氏体不锈钢必须被淬火软化成为耐腐蚀，因为它们加工硬化Untemperedmartensite,whileveryhardandstrong,istoobrittletobeusefulformostapplications.没有回火的马氏体，硬度高、强度大，太脆以至于不能用于很多的场合Amethodforalleviatingthisproblemiscalledtempering.消除这一问题的方法被称为回火Mostapplicationsrequirethatquenchedpartsbetempered(heattreatedatalowtemperature)toimpartsometoughness.许多应用需要淬火的零件回火处理（低温加热）提高韧性Highertemperingtemperaturesaresometimesusedtoimpartfurtherductility,althoughsomeyieldstrengthislost.较高的回火温度用于提高延展性，屈服强度降低Somemetalsareclassifiedasprecipitationhardeningmetals.一些金属被划分为沉淀硬化金属Whenaprecipitationhardeningalloyisquenched,itsalloyingelementswillbetrappedinsolution,resultinginasoftmetal.当沉淀硬化合金淬火时，它的合金元素将被熔化，成为较软的金属Agingasolutionizedmetalwillallowthealloyingelementstodiffusethroughthemicrostructureandformintermetallicparticles.时效以固溶金属将允许合金元素通过微结构慢慢混合形成金属间颗粒Theseintermetallicparticleswillnucleateandfalloutofsolutionandactasareinforcingphase,therebyincreasingthestrengthofthealloy.这些金属化合物将成核并从固溶体中沉淀脱落出来，起到强化相的作用，从而增加合金的强度。Alloysmayagenaturallymeaningthattheprecipitatesformatroomtemperature,ortheymayageartificiallywhenprecipitatesonlyformatelevatedtemperatures.合金可以自然时效意味着在室温时沉淀物形成，或者它们可以人工时效，沉淀物仅仅在不断提高的温度形成Casehardeningisspecifiedbyhardnessandcasedepth.表面硬化通过硬度和硬化层深度表述Thecasedepthcanbespecifiedintwoways:totalcasedepthoreffect