机械工程专业英语246

PART1MaterialsUnit1EngineeringMaterialsUnit2MechanicalPropertiesofMetalsUnit3SteelsUnit4CastIronUNIT5StainlessSteelsUnit1EngineeringMaterialsNewwordsandexpressionsUnit1EngineeringMaterialsAllproductsthatcomeoutofindustryconsistofatleastoneandoftenmanytypesofmaterials.Themostobviousexampleistheautomobile.Acarcontainsawidevarietyofmaterials,rangingfromglasstosteeltorubber,plusnumerousothermetalsandplastics.Thenumberofmaterialswhichareavailabletotheengineerinindustryisalmostinfinite.Thevariouscompositionsofsteelalonerunintothethousands.Ithasbeensaidthattherearemorethan10，000varietiesofglass,andthenumbersofplasticsareequallygreat.Itaddition,severalhundrednewvarietiesofmaterialsappearonthemarketeachmonth.Thismeansthatindividualengineersandtechnicianscannothopetobefamiliarwithallthepropertiesofalltypesofmaterialsintheirnumerousforms.Allhecandoistrytolearnsomeprinciplestoguidehimintheselectionandprocessingofmaterials.Thepropertiesofamaterialoriginatefromtheinternalstructureofthatmaterial.ThisisanalogoustosayingthattheoperationofaTVsetdependsonthecomponentsandcircuitswithinthatset.Theinternalstructuresofmaterialsinvolveatoms,andthewayatomsareassociatedwiththeirneighborsintocrystals,molecules,andmicrostructures.Itisconvenienttodividematerialsintothreemaintypes:(1)metals(2)plasticsorpolymersand(3)ceramics.Characteristically,metalsareopaque,ductile,andgoodconductorsofheatandelectricity.Plastics(orpolymers),whichusuallycontainlightelementsandhaverelativelylowdensity,aregenerallyinsulators,andareflexibleandformableatrelativelylowtemperatures.Ceramics,whichcontaincompoundsofbothmetallicandnonmetallicelements,areusuallyrelativelyresistanttoseveremechanical,thermal,andchemicalconditions.Metalsaredividedintoferrousandnon-ferrousmetals.Theformercontainironandthelatterdonotcontainiron.Certainelementscanimprovethepropertiesofsteelandarethereforeaddedtoit.Forexample,chromiummaybeincludedtoresistcorrosionandtungstentoincreasehardness.Aluminum,copper,andthealloys,bronzeandbrass,arecommonnon-ferrousmetals.Plasticsandceramicsarenon-metals;however,plasticsmaybemachinedlikemetals.Plasticsareclassifiedintotwotypes:thermoplasticsandthermosets.Thermoplasticscanbeshapedandreshapedbyheatandpressurebutthermosetscannotbereshapedbecausetheyundergochemicalchangesastheyharden.Ceramicsareoftenemployedbyengineerswhenmaterialswhichcanwithstandhightemperaturesareneeded.Unit2MechanicalPropertiesofMetalsNewwordsandexpressionsMechanicalpropertiesarethecharacteristicresponsesofamaterialtoappliedforces.Thesepropertiesfallintofivebroadcategories:strength,hardness,elasticity,ductility,andtoughness.Strengthistheabilityofametaltoresistappliedforces.Strengthpropertiesarecommonlyreferredtoastensilestrength,bendingstrength,compressivestrength,torsionalstrength,shearstrengthandfatiguestrength.Tensilestrengthisthatpropertywhichresistsforcesactingtopullthemetalapart.Itisoneofthemostimportantfactorsintheevaluationofametal.Compressivestrengthistheabilityofamaterialtoresistbeingcrushed.Compressionistheoppositeoftensionwithrespecttothedirectionoftheappliedload.Mostmetalshavehightensilestrengthandhighcompressivestrength.However,brittlematerialssuchascastironhavehighcompressivestrengthbutonlyamoderatetensilestrength.Unit2MechanicalPropertiesofMetalsBendingstrengthisthatqualitywhichresistsforces

from

causingamembertobendordeflectinthedirectioninwhichtheloadisapplied.Actuallyabendingstressisacombinationoftensileandcompressivestresses.Torsionalstrengthistheabilityofametaltowithstandforcesthatcauseamembertotwist.Shearstrengthreferstohow

well

amembercanwithstandtwoequalforcesactinginoppositedirections.Fatiguestrengthisthepropertyofamaterialtoresistvariouskindsofrapidlyalternatingstresses.Forexample,apistonrodoranaxleundergoescompletereversalofstresses

from

tensiontocompression.Bendingapieceofwirebackandforthuntilitbreaksisanotherexampleoffatiguestrength.Hardnessisthatpropertyin

steel

whichresistsindentationorpenetration.Hardnessisusuallyexpressedintermsoftheareaofanindentationmadebyaspecial

ball

undera

standard

load,orthedepthofaspecialindenterunderaspecificload.Elasticityistheabilitytospringbacktooriginalshape.Autobumpersandallspringsshouldhavethisquality.Ductilityistheabilitytoundergopermanentchangesofshapewithoutrupturing.Modern,deep-formedautobodiesandfenders,andotherstampedandformedproductsmusthavethisproperty.Toughnessistheabilitytoabsorbmechanicallyappliedenergy.Strengthandductilitydetermineamaterial’stoughness.Toughnessisneededinrailroadcars,automobileaxles,hammers,rails,andsimilarproducts.Unit3SteelsNewwordsandexpressionsWithoutalloyironsandsteels,thestateoftechnologywouldbesetbackconsiderably.Manyvarietiesofalloyshavebeendevelopedtomeetspecificneedsofanadvancingcivilization.However,theavailabilityofmanyvarietieshasoftenresultedinpoorselectionandexcesscostforanunnecessaryandexpensivealloymaterial.Itistheresponsibilityofthedesignandmanufacturingengineertobeknowledgeableinthisareaandtomakethebestselectionfromtheavailablealternatives.Unit3SteelsSteeltheoreticallyisanalloyofironandcarbon.Whenproducedcommercially,however,certainotherelements-notablymanganese,phosphorus,sulfurandsiliconarepresentinsmallquantities.Whenthesefourforeignelementsarepresentintheirnormalpercentages,theproductisreferredtoasplain-carbonsteel.Itsstrengthisprimarilyafunctionofitscarboncontent.Unfortunely,theductilitydecreasesasthecarboncontentisincreased,anditshardenabillityisquitelow.Inaddition,thepropertiesofordinarycarbonsteelsareimpairedbybothhighandlowtemperatures,andtheyaresubjecttocorrosioninmostenvironments.Plain-CarbonSteelPlain-carbonsteelsaregenerallyclassedintothreesubgroups,basedoncarboncontent.Low-carbonsteelshavelessthan0.30percentcarbon,possessgoodformabilityandweldability,butnotenoughhardenabilitytobehardentoanysignificantdepth.Theirstructuresusuallyareferriteandpearlite,andthematerialgenerallyisusedasitcomesfromthehot-formingorcold-formingprocess.Medium-carbonsteelshavebetween0.30and0.80percentcarbon,andtheycanbequenchedtoformmartensiteorbainiteifsectionsizeissmallandaseverewaterorbrinequenchisused.Thebestbalanceofpropertiesisattainedatthesecarbonlevels,thehighfatigueandtoughnessofthelow-carbonmaterialbeingingoodcompromisewiththestrengthandhardnessthatcomeswithhighercarboncontent[4].Thesesteelsfindnumerousapplications.High-carbonsteelshavemorethan0.80percentcarbon;toughnessandformabilityarequitelow,buthardnessandwearresistancearehigh.Severequenchescanformmartensite,buthardenabilityisstillpoor.Quenchcrackingisoftenaproblemwhenthematerialispushedtoitslimit.Plaincarbonsteelsarethelowest-coststeelmaterialandshouldbeconsideredformanyapplications.Often,however,theirlimitationsbecomerestrictive.Whenimprovedmaterialisrequired,steelscanbeupgradedbytheadditionofonemorealloyingelements.Thedifferentiationbetween“plaincarbon”and“alloy”steelisoftensomewhatarbitrary.Bothcontaincarbon,manganese,andusuallysilicon.Copperandboronalsoarepossibleadditionstobothclasses.Steelscontainingmorethan1.65percentmanganese,0.60percentsilicon,or0.60percentcopperaredesignatedasalloysteels.Also,steelisconsideredtobeanalloysteelifadefiniteamountorminimumofotheralloyingelementisspecifiedorrequired.Themostcommonalloyelementsarechromium,nickel,molybdenum,vanadium,tungsten,cobalt,boron,andcopper,aswellasmanganese,silicon,phosphorus,andsulfurinamountsgreaterthannormallyarepresent.AlloySteelsUnit4CastIronNewwordsandexpressionsCastiron,essentiallyanalloyofiron,carbon,andsilicon,iscomposedofironandfrom2to6.67percentcarbon,plusmanganese,sulfur,andphosphorus.Commerecialcastironcontainsnomorethan4percentcarbon.Castironisoftenalloyedwithelementssuchasnickel,chromium,molybdenum,vanadium,copper,andtitanium.Alloyingelementstoughenandstrengthencastirons.Unit4CastIronGraycastironisarelativelybrittlematerial,mainlybecauseofitslongthingraphiteflakesthatareveryweak.Graycastironisametalthatwillwithstandlargecompressiveloadsbutsmalltensileloads.GrayCastIronWhitecastironisveryhard,brittle,andvirtuallynonmachinable.Insomecasesitisusedwherethereisaneedforresistancetoabrasion.Whitecastironisoftenfoundincombinationwithothercastiron,suchasgraycastiron,toimprovethehardnessandwearresistantproperties.Therearebasicallytwowaysofobtainingwhitecastiron.Onewayisbyloweringtheiron`ssiliconcontent;thesecondisbyrapidcooling,whichinthiscaseyieldswhatiscalledchilledcastiron.Whencooledatarapidrate,theexcesscarbonformsironcarbideandnotgraphite,thusmakingwhitecastirons.WhiteCastIronMalleablecastironisnotedforitsstrength,toughness,ductility,andmachinability.Intheprocessofmakingmalleablecastiron,itisfirstnecessarytobeginwithwhitecastiro.Thewhitecastironisthenheattreatedasfollows.1.Heattoabout1700℉(927℃)；2.Holdatthistemperatureforabout15hours.Thisbreaksdowntheironcarbidetoausteniteandgraphite；3.Slowcooltoabout1300℉(704℃)；4.Holdatthistemperatureforapproximately15hours；5.Aircooltoroomtemperature.Theaboveprocessbreaksdowntheironcarbideintoadditionalausteniteandgraphite.Uponcoolingthegraphitewillformintoclustersorballs.Theaustenitewilltakeonanyoneofthetransformationproducts,dependingonthecoolingrate.MalleableCastIronNodularcastironisknownbyseveralnames:nodulariron,ductileiron,andspheroidalgraphiteiron.Itgetsthenamesfromtheball-likeformofthegraphiteinthemetalandtheveryductilepropertyitexhibits.Nodularcastironcombinesmanyoftheadvantageofcastironandsteel.Itsadvantagesincludegoodcastability,toughness,machinability,goodwearresistance,weldability,lowmeltingpoint,andhardenability.Theformationofthegraphiteintoaballformisaccomplishedbyaddingcertainelementssuchasmagnesiumandceriumtothemeltjustpriortocasting.Thevigorousmixingreactioncausedbyaddingtheseelementsresultsinahomogeneousspheroidalorball-likestructureofthegraphiteinthecastiron.Theironmatrixorbackgroundmaterialcanbeheattreatedtoformanyoneofthemicrostructuresassociatedwithsteelssuchasferrite,pearlite,ormartensite.NodularCastIronUNIT5StainlessSteelsNewwordsandexpressionsStainlesssteelsdonotrustintheatmosphereasmostothersteelsdo．Theterm“stainless”impliesaresistancetostaining，rusting，andbitingintheair,moistandpollutedasitis,andgenerallydefinesachromiumcontentinexcessof11％butlessthan30％.Andthefactthatthestuffis“steel”meansthatthebaseisiron.Stainlesssteelshaveroom-temperatureyieldstrengthsthatrangefrom205MPa(30ksi)tomorethan1725MPa(250ksi).Operatingtemperaturesaround750℃(1400ºF)arecommon,andinsomeapplicationstemperaturesashighas1090℃(2000ºF)arereached．Attheotherextremeoftemperaturesomestainlesssteelsmaintaintheirtoughnessdowntotemperaturesapproachingabsolutezero.UNIT5StainlessSteelsWithspecificrestrictionsincertaintypes，thestainlesssteelscanbeshapedandfabricatedinconventionalways．Theycanbeproducedandusedintheas-castcondition;shapescanbeproducedbypowder-metallurgytechniques;castingotscanberolledorforged(andthisaccountsforthegreatesttonnagebyfar).Therooledproductcanbedrawn，bent，extruded，orspun．Stainlesssteelcanbefurthershapedbymachining，anditcanbejoinedbysoldering,brazing,andwelding.Itcanbeusedasanintegralcladdingonplaincarbonorlowalloysteels.Thegenericterm“stainlesssteel”coversscoresofstandardcompositionsaswellasvariationsbearingcompanytradenamesandspecialalloysmadeforparticularapplications．Stainlesssteelsvaryintheircompositionfromafairlysimplealloyof,essentially,ironwith11％chromium，tocomplexalloysthatinclude30％chromium，substantialquantitiesofnickelandhalfadozenothereffectiveelements．Atthehigh-chromium，high-nickelendoftherangetheymergeintoothergroupsofheat-resistingalloys，andonehastobearbitraryaboutacutoffpoint．Ifthealloycontentissohighthattheironcontentisabouthalf，however，thealloyfallsoutsidethestainlessfamily．Evenwiththeseimposedrestrictionsoncomposition，therangeisgreat，andnaturally，thepropertiesthataffectfabricationandusevaryenormously.Itisobviouslynotenoughtospecifysimplya“stainlesssteel”.Thevariousspecifyingbodiescategorizestainlesssteelsaccordingtochemicalcompositionandotherproperties.Forexample,theAmericanIronandSteelInstitute(AISI)listsmorethan40approvedwroughtstainlesssteelcompositions;theAmericanSocietyforTestingandMaterials(ASTM)callsforspecificationsthatmayconformtoAISIcompositionsbutadditionallyrequirecertainmechanicalpropertiesanddimensionaltolerances;theAlloyCastingInstitute(ACI)specifiescompositionsforcaststainlesssteelswithinthecategoriesofcorrosion-andheat-resistingalloys;theSocietyofAutomotiveEngineers(SAE)hasadoptedAISIandACIcompositionalspecifications.MilitaryspecificationMIL-HDBK-5listsdesignvalues.Inaddition,manufacturers’specificationsareusedforspecialpurposesorforproprietaryalloys.Federalandmilitaryspecificationsandmanufacturers’specificationsarelaiddownforspecialpurposesandsometimesacquireageneralacceptance．However,allthestainlesssteels,whateverspecificationstheyconformto,canbeconvenientlyclassifiedintosixmajorclassesthatrepresentthreedistincttypesofalloyconstitution,orstructure．Theseclassesareferritic，martensitic,austenitic,manganese-substitutedaustenitic,duplexaustenitic-ferritic，andprecipitation-hardening.FerriticStainlesssteelissonamedbecausethecrystalstructureofthesteelisthesameasthatofironatroomtemperature.ThealloysintheclassaremagneticatroomtemperatureanduptotheirCurietemperature[about750℃1400ºF)]．Commonalloysintheferriticclasscontainbetween11％and29％chromium，nonickel,andverylittlecarboninthewroughtcondition．The11％ferriticchromiumsteels,whichprovidefaircorrosionresistanceandgoodfabricationatlowcost，havegainedwideacceptanceinautomotiveexhaustsystems，containers，andotherfunctionalapplications．Theintermediatechromiumalloys,with16％～17％chromium，areusedprimarilyasautomotivetrimandcookingutensils,alwaysinlightgages，theirusesomewhatrestrictedbyweldingproblems.Thehigh-chromiumsteels,with18％to29％chromiumcontent，havebeenusedincreasinglyinapplicationsrequiringahighresistancetooxidationand，especially，tocorrosion．Thesealloyscontaineitheraluminumormolybdenumandhavelowcarboncontent.Thehigh-temperatureformofiron(between910℃and1400℃，or1670ºFand2550ºF)isknownasaustenite(Strictlyspeakingthetermaustenitealsoimpliescarboninsolidsolution)．Thestructureisnonmagneticandcanberetainedatroomtemperaturebeappropriatealloying．Themostcommonausteniteretainerisnickel．Hence，thetraditionalandfamiliarausteniticstainlesssteelshaveacompositionthatcontainssufficientchromiumtooffercorrosionresistance,togetherwithnickeltoensureausteniteatroomtemperatureandbelow．Thebasicausteniticcompositionisthefamiliar18％chromium，8％nickelalloy．Bothchromiumandnickelcontentscanbeincreasedtoimprovecorrosionresistance，andadditionalelements(mostcommonlymolybdenum)canbeaddedtofurtherenhancecorrosionresistance.PART

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FormingTechnologyUnit6CastingUnit7ForgingUnit8HeatTreatmentofMetalsUnit9WeldingUnit10DieCastingUnit6CastingNewwordsandexpressionsUnit6CastingMetalcastingisoneoftheoldestofallindustries,bothancientandmedievalhistoryofferingexamplesofthemanufactureanduseofcasting.Fromsimpleaxeheadspouredfromcopperinopenmouldssome5000yearsago,castinginthepre-Christianworlddevelopedtoapointatwhichelaboratebronzestatuarycouldbeproducedintwo-pieceandcoredmoulds.Bytheendofthemedievalperiod,decoratedbronzeandpewtercastingshadbeguntobeusedinEuropeanchurchanddomesticlife.ThewidespreadadoptionofcastironasengineeringmaterialawaitedthesuccessofAbrahamDarbyin1709insmeltinginthecokeblastfurnace;thispavedthewayforthemassiveuseofcastironinconstructionduringtheyearsfollowingtheindustrialrevolution.Manyfoundriessprangupaftertheindustrialrevolution,thevastmajoritybeingforthemanufactureofthecastironthenbeingusedasastructuralmaterial.Thequantityproductionofironcastingsinthenineteenthcenturywasnotmatchedbyauniversaladvanceinqualityandtheengineeringuseoftheproductsencounteredmoreseriousrisksinanon-ductilematerial.Despitetheskillofthemolderinproducingcomplexforms,therewaslittlechangeinthemetallurgicalandengineeringsituationuntilthemodernerabroughtabetterunderstandingofthefactorsdeterminingquality.Withmoderntechniquesofprocesscontroltherudimentaryjudgmentoftheoperatorcouldgivewaytoobjectivemeasurementsofmetaltemperature,moldingmaterialpropertiesandotherproductionvariables.Theseimprovementshavebeenappliednotonlytocastironbuttoawiderangeofcastalloys.Therearefourbasiccastingmethods:sand-casting,die-casting,investment-casting,andcentrifugalcasting.Sand-castingisthemostwidelyusedmethodemployedinfoundry.Inthisprocess,sandmouldsarecontainedinmetalmoldingboxesthathavefoursidesbuttoporbottom.Duringthemoldingoperationtheboxesarelocatedtogetherbypinssothattheycanbeseparatedtoremovethepattern,andreplacedinthecorrectpositionbeforethemetalispouredin.Theboxesareclampedtogether,orthecope(topsection)weighteddownwhenpouringtopreventthecopefrom“floatingaway”fromthedrag(lowersection)whenthemouldisfullofmoltenmetal.Thesequencewhenmoldingthesimpletwo-partmouldtocastabracketisillustratedasfollows.Atthefirststagethepatternisseatedonthemouldingboard.Thepatterniscoveredwithfacingsand,whichisaspeciallypreparedsandofgoodquality,whichcantakeacleanandsmoothimpression,andcanresisttheheatfromthemetalthatwillbeincontactwithit.Thefacingsandisbackedupwithmoldingsand,whichisoldfacingsandfrompreviousmoulds.Themoldingsandiscarefullyrammedupsothatitisfairlytightaroundthepatterntoproduceagoodsolidmould,yetpermeableenoughtoallowthegasesproducedduringcastingtoescape.Thesandisfinallyleveledoff.Atthesecondstagethemouldwiththepatternstillinpositionisinverted;theexposedsandlightlycoveredwithpartingsand,andtheexposedpatternwithfacingsand.(Thepartingsandhasnocohesion,andisintroducedtopermitacleanseparationwhenthemouldisopeneduptoremovethepattern.)Thesecondmoldingboxislocatedinpositiononthefirstboxandfilledwithmoldingsand.Twoormoreplugsareintroducedwhenthesecondboxisbeingfilled(theseareremovedlater,leavingchannelsinthesand).Oneoftheseplugsispositionedtoonesideofthepattern.Thesandisrammedupandleveledoff.Now,atstage3,allowthepatterntoberemoved.Thisisdonebyscrewingabarwithathreadedendintoasuitableinsertinthepattern,dampingthesandaroundthepattern,andgentlyrappingthebarinalldirectionssothatthepatterncanbecarefullywithdrawn.Tofacilitatetheremovalofthepatternwithoutscuffingthesidesoftheimpression,allsurfacesthatlieinthedirectionofpatternremovalareinclinedslightlybyasmallamount(thedrawangle).Agroovecalledagateiscutinthesandfacetoallowthechannelproducedbytheplugthatisoutsidethepatterntoconnectwiththeimpression.Themetalispouredthroughthischannel(calledtherunner),andthegatepreventsitfromdroppingstraightintotheimpressionanddamagingit.Thecross-sectionofthegateisslightlysmallerthanthatofchannelsothatafullrunnerwillalwayssupplymetaltothegateataslightpressure.Finally,themouldisreassembled,carefullylocatingandsecuringthetwosections.Thetopsectionisknownasthecope,andthelowersectionisknownasthedrag.Thesandinthecopeisvented.Theseventsallowthesandtoberammedupmoretightlyattheearlierstageswithouttheriskofgasesbeingtrappedinthemoltenmetalandformingblowholesinthesolidmetal.Asand-feedinggate(alsocalledapouringorbowl)isaddedtomakeiteasiertopourthemetalintotherunner.Themoltenmetalispouredthroughtherunnerandtheairwillescapethroughtheriser.Theimpressionwillbefilledwithmoltenmetalwhenitiscompletelyfilled.Gasescanescapethroughtherunnerandtheriser,whichalsoactasheaderstosupplytheimpressionwithmoremetaltocompensateforthecontractionofthemetalwhencoolinginthemoltenstate..Unit7ForgingNewwordsandexpressionsPressforgingemploysaslowsqueezingactionindeformingtheplasticmetal,ascontrastedwiththerapid-impactblowsofahammer.Thesqueezingactioniscarriedcompletelytothecenterofthepartbeingpressed,thoroughlyworkingtheentiresection.Thesepressesaretheverticaltypeandmaybeeithermechanicallyorhydraulicallyoperated.Themechanicalpresses,whicharefasteroperatingandmostcommonlyused,rangeincapacityfrom500to10,000tons.Forsmallpressforgingsclosedimpressiondiesareused,andonlyonestrokeoftheramisnormallyrequiredtoperformtheforgingoperation.Themaximumpressureisbuiltupattheendofthestrokewhichforcesthemetalintoshape.Diesmaybemountedasseparateunits,orallthecavitiesmaybeputintoasingleblock.Forsmallforgingsindividualdieunitsaremoreconvenient.Thereissomedifferenceinthedesignofdiesfordifferentmetals;copper-alloyforgingscanbemadewithlessdraftthansteel,consequentlymorecomplicatedshapescanbeproduced.Thesealloysflowwellinthedieandarereaidlyextruded.Intheforgingpressagreaterproportionofthetotalworkputintothemachineistransmittedtothemetalthaninadrophammerpress[2].Muchoftheimpactofthedrophammerisabsorbedbythemachineandfoundation.Pressreductionofthemetalisfaster,andthecostofoperationisconsequentlylower.Mostpressforgingsaresymmetricalinshape,havingsurfaceswhicharequitesmooth,andprovideaclosertolerancethanisobtainedbyadrophammer.However,manypartsofirregularandcomplicatedshapescanbeforgedmoreeconomicallybydropforging.Forgingpressesareoftenusedforsizingoperationsonpartsmadebyotherforgingprocesses.Indropforging,apieceofmetal,roughlyorapproximatelyofthedesiredshape,isplacedbetweendiefaceshavingtheexactformofthefinishedpiece,andforcedtotakethisformbydrawingthediestogether.Thismethodiswidelyusedforthemanufactureofpartsbothofsteelandbrass.Largeingotsarenowalmostalwaysforgedwithhydraulicpressesinsteadofwithsteamhammers,sincetheworkdonebyapressgoesdeeper[3].Further,thepresscantakeacooleringotandcanworktocloserdimensions.Theforgingshouldbedoneataboutthesametemperatureastolling;theprocessimprovesthephysicalpropertiesofthesteeljustasrollingdoes[4].Inthefinalforgingitisimportantnottohavethesteeltoohot,foranoverheatedsteelwillhavepoormechanicalpropertieswhencooled[5].Inheatingforforgingthetemperatureisusuallyjudgedbytheeye,butwherelargenumbersofthesamepatternaremade,thepiecetobeforgedareheatedinfurnacesinwhichthetemperatureisindicatedbypyrometers,andoftenisautomaticallycontrolled.Unit8HeatTreatmentofMetalsNewwordsandexpressionsTheunderstandingofheattreatmentisembracedbythebroaderstudyofmetallurgy.Metallurgyisthephysics,chemistry,andengineeringrelatedtometalsfromoreextractiontothefinalproduct.Heattreatmentistheoperationofheatingandcoolingametalinitssolidstatetochangeitsphysicalproperties.Accordingtotheprocedureused,steelcanbehardenedtoresistcuttingactionandabrasion,oritcanbesoftenedtopermitmachining.Withtheproperheattreatmentinternalstressesmayberemoved,grainsizereduced,toughnessincreased,orahardsurfaceproducedonaductileinterior.Theanalysisofthesteelmustbeknownbecausesmallpercentagesofcertainelements,notablycarbon,greatlyaffectthephysicalproperties.Alloysteelowestheirpropertiestothepresenceofoneormoreelementsotherthancarbon,namelynickel,chromium,manganese,molybdenum,tungsten,silicon,vanadium,andcopper[1].Becauseoftheirimprovedphysicalpropertiestheyareusedcommerciallyinmanywaysnotpossiblewithcarbonsteels.Thefollowingdiscussionappliesprincipallytotheheattreatmentofordinarycommercialsteelsknownasplaincarbonsteels.Withthisprocess,therateofcoolingisthecontrollingfactor,rapidcoolingfromabovethecriticalrangeresultsinhardstructure,whereasveryslowcoolingproducestheoppositeeffect.Ifwefocusonlyonthematerialsnormallyknownas