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1、CHAPTER2BASICHEATTREATMENTAsSteelworkers,weareinterestedintheheattreatmentofmetals,becausewehavetoknowwhateffectstheheatproducedbyweldingorcuttinghasonmetal.Wealsoneedtoknowthemethodsusedtorestoremetaltoitsoriginalcondition.Theprocessofheattreatingisthemethodbywhich
2、metalsareheatedandcooledinaseriesofspecificoperationsthatneverallowthemetaltoreachthemoltenstate.Thepurposeofheattreatingistomakeametalmoreusefulbychangingorrestoringitsmechanicalproperties.Throughheattreating,wecanmakeametalharder,stronger,andmoreresistanttoimpact.
3、Also,heattreatingcanmakeametalsofterandmoreductile.Theonedisadvantageisthatnoheat-treatingprocedurecanproduceallofthesecharacteristicsinoneoperation.Somepropertiesareimprovedattheexpenseofothers;forexample,hardeningametalmaymakeitbrittle.HEAT-TREATINGTHEORYThevariou
4、stypesofheat-treatingprocessesaresimilarbecausetheyallinvolvetheheatingandcoolingofmetals;theydifferintheheatingtemperaturesandthecoolingratesusedandthefinalresults.Theusualmethodsofheat-treatingferrousmetals(metalswithiron)areannealing,normalizing,hardening,andtemp
5、ering.Mostnonferrousmetalscanbeannealed,butnevertempered,normalized,orcase-hardened.Successfulheattreatmentrequiresclosecontroloverallfactorsaffectingtheheatingandcoolingofametal.Thiscontrolispossibleonlywhentheproperequipmentisavailable.Thefurnacemustbeofthepropers
6、izeandtypeandcontrolled,sothetemperaturesarekeptwithintheprescribedlimitsforeachoperation.Eventhefurnaceatmosphereaffectstheconditionofthemetalbeingheat-treated.Thefurnaceatmosphereconsistsofthegasesthatcirculatethroughouttheheatingchamberandsurroundthemetal,asitisb
7、eingheated.Inanelectricfurnace,theatmosphereiseitherairoracontrolledmixtureofgases.Inafuel-firedfurnace,theatmosphereisthemixtureofgasesthatcomesfromthecombinationoftheairandthegasesreleasedbythefuelduringcombustion.Thesegasescontainvariousproportionsofcarbonmonoxid
8、e,carbondioxide,hydrogen,nitrogen,oxygen,watervapor,andothervarioushydrocarbons.Fuel-firedfurnacescanprovidethreedistinctatmosphereswhenyo
