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《章跃机械制造工程专业英语---lesson 2 hardening and tempering》由会员上传分享，免费在线阅读，更多相关内容在行业资料-天天文库。
1、Lesson2HardeningandTemperingPlaincarbonsteelhasbeenvaluedfromearlytimebecauseofcertainproperties.Thissoftsilver-graymetalcouldbeconvertedintoasuperhardsubstancethatwouldcutglassandmanyothersubstances,includingitselfwhensoft.Furthermore,itshardnesscouldbecontrolled.Thisco
2、nvertingofcarbonsteelintoasteelofusefulhardnessisdonewithdifferentheattreatments,twoofthemostimportantofwhicharehardeningandtempering(drawing),whichyouwillinvestigateinthislesson.HardeningbyquenchingAssteelisheatedabovethelowercriticaltemperatureof1330F(721C),thecarbonth
3、atwasintheformoflayersofironcarbideinpearlitebeginstodissolveintheironandformasolidsolutioncalledaustenite.Whenthissolutionofironandcarbonissuddenlycooledorquenched,anewmicrostructureisformed.Thisiscalledmartensite.Martensiteisveryhardandbrittle,havingamuchhighertensiles
4、trengththanthesteelwithapearlitemicrostructure.Itisquiteunstable,however,andmustbetempered(drawn)torelieveinternalstressesinordertohavetheductilityandtoughnessneededtobeuseful.AISI-C1095,commonlyknownaswater-hardeningtool(W1)steel,willbegintoshowhardnesswhenquenchedfroma
5、temperaturejustover1330F(721C)butwillnothardenatallifquenchedfromatemperaturelowerthan1330F(721C).Thissteelwillbecomeashardasitcangetwhenheatedto1450F(788C)andquenchedinwater.Thisquenchingtemperaturechangesasthecarboncontentchanges.Itshouldbe50F(28C)abovetheuppercritical
6、temperatureforcarbonsteelscontaininglessthan0.83percentcarbon.Thereasoncarbonsteel,lessthaneutectoid,shouldbeheatedabovetheuppercriticaltemperatureisthattheferriteisnotalltransformedintoaustenitebelowthispoint,andwhenquenched,isretainedinthemartensiticstructure.Theretain
7、edferritecausesbrittlenessevenaftertempering.LowcarbonsteelssuchasAISI1020willnot,forallpracticalpurpose,hardenwhentheyareheatedandquenched.Oil-andair-hardeningsteelshaveahigherhardenabilityanddonothavetobequenchedasrapidlyasplaincarbonsteels.Consequently,theyaredeeperha
8、rdeningthanwaterhardeningtypes,whichmustbecooledto200Fwithin1or2seconds.Plaincarbonsteelscontaining0.83
