《机械工程专业英语》课件-机械工程专业英语课件-PART2

PART

2

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.Ifwefocusonlyonthematerialsnormallyknownassteels,asimplifieddiagramisoftenused.Thoseportionsoftheiron-carbondiagramnearthedeltaregionandthoseabove2%carboncontentareoflittleimportancetotheengineerandaredeleted.Asimplifieddiagram,suchastheoneinFig.8-1,focusesontheeutectoidregionandisquiteusefulinunderstandingthepropertiesandprocessingofsteel.Thekeytransitiondescribedinthisdiagramisthedecompositionofsingle-phaseaustenite(γ)tothetwo-phaseferritepluscarbidestructureastemperaturedrops.Controlofthisreaction,whicharisesduetothedrasticallydifferentcarbonsolubilityofausteniteandferrite,enablesawiderangeofpropertiestobeachievedthroughheattreatment.ASimplifiedIron-carbonDiagramTobegintounderstandtheseprocesses,considerasteeloftheeutectoidcomposition,0.77%carbon,beingslowcooledalonglinex-x’inFig.8-1.Attheuppertemperatures,onlyausteniteispresent,the0.77%carbonbeingdissolvedinsolidsolutionwiththeiron.Whenthesteelcoolsto727℃(1341℉),severalchangesoccursimultaneously.TheironwantstochangefromtheFCCaustenitestructuretotheBCCferritestructure,buttheferritecanonlycontain0.02%carboninsolidsolution[3].Therejectedcarbonformsthecarbon-richcementiteintermetallicwithcompositionFe3C.Inessence,thenetreactionattheeutectoidisAustenite(0.77%C)→ferrite(0.02%C)+cementite(6.67%C).Sincethischemicalseparationofthecarboncomponentoccursentirelyinthesolidstate,theresultingstructureisafinemechanicalmixtureofferriteandcementite.Specimenspreparedbypolishingandetchinginaweaksolutionofnitricacidandalcoholrevealthelamellarstructureofalternatingplatesthatformsonslowcooling.Thisstructureiscomposedoftwodistinctphases,buthasitsownsetofcharacteristicpropertiesandgoesbythenamepearlite,becauseofitsresemblancetomother-of-pearlatlowmagnification.Steelshavinglessthantheeutectoidamountofcarbon(lessthan0.77%)areknownashypo-eutectoidsteels.Considernowthetransformationofsuchamaterialrepresentedbycoolingalongliney-yinFig.8-1.Athightemperatures,thematerialisentirelyaustenite,butuponcoolingentersaregionwherethestablephasesareferriteandaustenite.Tie-lineandlevel-lawcalculationsshowthatlow-carbonferritenucleatesandgrows,leavingtheremainingaustenitericherincarbon[5].At727℃(1341℉),theausteniteisofeutectoidcomposition(0.77%carbon)andfurthercoolingtransformstheremainingaustenitetopearlite.Theresultingstructureisamixtureofprimaryorpro-eutectoidferrite(ferritethatformedabovetheeutectoidreaction)andregionsofpearlite.Hypereutectoidsteelsaresteelsthatcontaingreaterthantheeutectoidamountofcarbon.Whensuchsteelcools,asshowninz-z’ofFig.8-1theprocessissimilartothehypo-eutectoidcase,exceptthattheprimaryorpro-eutectoidphaseisnowcementiteinsteadofferrite.Asthecarbon-richphaseforms,theremainingaustenitedecreasesincarboncontent,reachingtheeutectoidcompositionat727℃(1341℉).Asbefore,anyremainingaustenitetransformstopearliteuponslowcoolingthroughthistemperature.Itshouldberememberedthatthetransitionsthathavebeendescribedbythephasediagramsareforequilibriumconditions,whichcanbeapproximatedbyslowcooling.Withslowheating,thesetransitionsoccurinthereversemanner.However,whenalloysarecooledrapidly,entirelydifferentresultsmaybeobtained,becausesufficienttimeisnotprovidedforthenormalphasereactionstooccur,insuchcases,thephasediagramisnolongerausefultoolforengineeringanalysis.Hardeningistheprocessofheatingapieceofsteeltoatemperaturewithinoraboveitscriticalrangeandthencoolingitrapidly.Ifthecarboncontentofthesteelisknown,thepropertemperaturetowhichthesteelshouldbeheatedmaybeobtainedbyreferencetotheiron-ironcarbidephasediagram.However,ifthecompositionofthesteelisunknown,alittlepreliminaryexperimentationmaybenecessarytodeterminetherange.Agoodproceduretofollowistoheat-quenchanumberofsmallspecimensofthesteelatvarioustemperaturesandtoobservetheresult,eitherbyhardnesstestingorbymicroscopicexamination.Whenthecorrecttemperatureisobtained,therewillbeamarkedchangeinhardnessandotherproperties.HardeningInanyheat-treatingoperationtherateofheatingisimportant.Heatflowsfromtheexteriortotheinteriorofsteelatadefiniterate.Ifthesteelisheatedtoofast,theoutsidebecomeshotterthantheinterioranduniformstructurecannotbeobtained.Ifapieceisirregularinshape,aslowrateisallthemoreessentialtoeliminatewarpingandcracking.Theheavierthesection,thelongermustbetheheatingtimetoachieveuniformresults.Evenafterthecorrecttemperaturehasbeenreached,thepieceshouldbeheldatthattemperatureforasufficientperiodoftimetopermititsthickestsectiontoattainauniformtemperature.Thehardnessobtainedfromagiventreatmentdependsonthequenchingrate,thecarboncontent,andtheworksize.Inalloysteelsthekindandamountofalloyingelementinfluenceonlythehardenability(theabilityoftheworkpiecetobehardenedtodepths)ofthesteelanddoesnotaffectthehardnessexceptinunhardenedorpartiallyhardenedsteels.Steelwithlowcarboncontentwillnotrespondappreciablytohardeningtreatment.Asthecarboncontentinsteelincreasesuptoaround0.60%,thepossiblehardnessobtainablealsoincreases.Abovethispointthehardnesscanbeincreasedonlyslightly,becausesteelsabovetheeutectoidpointaremadeupentirelyofpearliteandcementiteintheannealedstate.Pearliterespondsbesttoheat-treatingoperations;anysteelcomposedmostlyofpearlitecanbetransformedintohardsteel.Asthesizeofpartstobehardenedincreases,thesurfacehardnessdecreasessomewhateventhoughallotherconditionshaveremainedthesame.Thereisalimittotherateofheatflowthroughsteel.Nomatterhowcoolthequenchingmediummaybe,iftheheatinsidealargepiececannotescapefasterthanacertaincriticalrate,thereisadefinitelimittotheinsidehardness.However,brineorwaterquenchingiscapableofrapidlybringingthesurfaceofthequenchedparttoitsowntemperatureandmaintainingitatorclosetothistemperature.Underthesecircumstancestherewouldalwaysbesomefinitedepthofsurfacehardeningregardlessofsize.Thisisnottrueinoilquenching,whenthesurfacetemperaturemaybehighduringthecriticalstagesofquenching.Steelthathasbeenhardenedbyrapidquenchingisbrittleandnotsuitableformostuses.Bytemperingordrawing,thehardnessandbrittlenessmaybereducedtothedesiredpointforserviceconditions.Asthesepropertiesarereducedthereisalsoadecreaseintensilestrengthandanincreaseintheductilityandtoughnessofthesteel.Theoperationconsistsofreheatingquench-hardenedsteeltosometemperaturebelowthecriticalrangefollowedbyanyrateofcooling.Althoughthisprocesssoftenssteel,itdiffersconsiderablyfromannealinginthattheprocesslendsitselftoclosecontrolofthephysicalpropertiesandinmostcasesdoesnotsoftenthesteeltotheextentthatannealingwould.Thefinalstructureobtainedfromtemperingafullyhardenedsteeliscalledtemperedmartensite.TemperingTemperingispossiblebecauseoftheinstabilityofthemartensite,theprincipalconstituentofhardenedsteel.Low-temperaturedraws,from300℉to400℉(150℃~205℃),donotcausemuchdecreaseinhardnessandareusedprincipallytorelieveinternalstrains.Asthetemperingtemperaturesareincreased,thebreakdownofthemartensitetakesplaceatafasterrate,andatabout600℉(315℃)thechangetoastructurecalledtemperedmartensiteisveryrapid.Thetemperingoperationmaybedescribedasoneofprecipitationandagglomerationorcoalescenceofcementite.Asubstantialprecipitationofcementitebeginsat600℉(315℃),whichproducesadecreaseinhardness.Increasingthetemperaturecausescoalescenceofthecarbideswithcontinueddecreaseinhardness.Intheprocessoftempering,someconsiderationshouldbegiventotimeaswellastotemperature.Althoughmostofthesofteningactionoccursinthefirstfewminutesafterthetemperatureisreached,thereissomeadditionalreductioninhardnessifthetemperatureismaintainedforaprolongedtime.Usualpracticeistoheatthesteeltothedesiredtemperatureandholditthereonlylongenoughtohaveituniformlyheated.Twospecialprocessesusinginterruptedquenchingareaformoftempering.Inboth,thehardenedsteelisquenchedinasaltbathheldataselectedlowertemperaturebeforebeingallowedtocool.Theseprocesses,knownasaustemperingandmartempering,resultinproductshavingcertaindesirablephysicalproperties.TheprimarypurposeofannealingistosoftenhardsteelsothatitmaybemachinedorcoldworkedThisisusuallyaccomplishedbyheatingthesteeltooslightlyabovethecriticaltemperature,holdingitthereuntilthetemperatureofthepieceisuniformthroughout,andthencoolingataslowlycontrolledratesothatthetemperatureofthesurfaceandthatofthecenterofthepieceareapproximatelythesame.Thisprocessisknownasfullannealingbecauseitwipesoutalltraceofpreviousstructure,refinesthecrystallinestructure,andsoftensthemetal.Annealingalsorelievesinternalstressespreviouslysetupinthemetal.AnnealingThetemperaturetowhichagivensteelshouldbeheatedinannealingdependsonitscomposition;forcarbonsteelsitcanbeobtainedreadilyfromthepartialiron-ironcarbideequilibriumdiagram.Whentheannealingtemperaturehasbeenreached,thesteelshouldbeheldthereuntilitisuniformthroughout.Thisusuallytakesabout45minforeachinch(25mm)ofthicknessofthelargestsection.Formaximumsoftnessandductilitythecoolingrateshouldbeveryslow,suchasallowingthepartstocooldownwiththefurnace.Thehigherthecarboncontent,theslowerthisratemustbe.Theprocessofnormalizingconsistsofheatingthesteelabout50℉to100℉(10℃~40℃)abovetheuppercriticalrangeandcoolinginstillairtoroomtemperature.Thisprocessisprincipallyusedwithlow-andmedium-carbonsteelsaswellasalloysteelstomakethegrainstructuremoreuniform,torelieveinternalstresses,ortoachievedesiredresultsinphysicalproperties.Mostcommercialsteelsarenormalizedafterbeingrolledorcast.Spheroidizingistheprocessofproducingastructureinwhichthecementiteisinaspheroidaldistribution.Ifsteelisheatedslowlytoatemperaturejustbelowthecriticalrangeandheldthereforaprolongedperiodoftime,thisstructurewillbeobtained.Theglobularstructureobtainedgivesimprovedmachinabilitytothesteel.Thistreatmentisparticularlyusefulforhypereutectoidsteelsthatmustbemachined.NormalizingandSpheroidizingTheprocessofnormalizingconsistsofheatingthesteelabout50℉to100℉(10℃~40℃)abovetheuppercriticalrangeandcoolinginstillairtoroomtemperature.Thisprocessisprincipallyusedwithlow-andmedium-carbonsteelsaswellasalloysteelstomakethegrainstructuremoreuniform,torelieveinternalstresses,ortoachievedesiredresultsinphysicalproperties.Mostcommercialsteelsarenormalizedafterbeingrolledorcast.Spheroidizingistheprocessofproducingastructureinwhichthecementiteisinaspheroidaldistribution.Ifsteelisheatedslowlytoatemperaturejustbelowthecriticalrangeandheldthereforaprolongedperiodoftime,thisstructurewillbeobtained.Theglobularstructureobtainedgivesimprovedmachinabilitytothesteel.Thistreatmentisparticularlyusefulforhypereutectoidsteelsthatmustbemachined.SurfaceHardeningCarburizingSteelforcarburizingisusuallyalow-carbonsteelofabout0.15%carbonthatwouldnotinitselfrespondsappreciablytoheattreatment.Inthecourseoftheprocesstheouterlayerisconvertedintohigh-carbonsteelwithacontentrangingfrom0.9%to1.2%carbon.Asteelwithvaryingcarboncontentand,consequently,differentcriticaltemperaturesrequiresaspecialheattreatment.Becausethereissomegraingrowthinthesteelduringtheprolongedcarburizingtreatment,theworkshouldbeheatedtothecriticaltemperatureofthecoreandthencooled,thusrefiningthecorestructure.Thesteelshouldthenbereheatedtoapointabovethetransformationrangeofthecaseandquenchedtoproduceahard,finestructure.Thelowerheat-treatingtemperatureofthecaseresultsfromthefactthathypereutectoidsteelsarenormallyaustenitizedforhardeningjustabovethelowercriticalpoint.Athirdtemperingtreatmentmaybeusedtoreducestrains.Carbonitriding,sometimesknownasdrycyanidingornicarbing,isacase-hardeningprocessinwhichthesteelisheldatatemperatureabovethecriticalrangeinagaseousatmospherefromwhichitabsorbscarbonandnitrogen.Anycarbon-richgaswithammoniacanbeused.Thewear-resistantcaseproducedrangesfrom0.003to0.030inch,(0.08~0.76mm)inthickness.Anadvantageofcarbonitridingisthatthehardenabilityofthecaseissignificantlyincreasedwhennitrogenisadded,permittingtheuseoflow-coststeels.CarbonitridingCyaniding,orliquidcarbonitridingasitissometimescalled,isalsoaprocessthatcombinestheabsorptionofcarbonandnitrogentoobtainsurfacehardnessinlow-carbonsteelsthatdonotrespondtoordinaryheattreatment.TheparttobecasehardenedisimmersedinabathoffusedsodiumcyanidesaltsatatemperatureslightlyabovetheAC1range[6],thedurationofsoakingdependingonthedepthofthecase.Thepartisthenquenchedinwateroroiltoobtainahardsurface.Casedepthsrangesfrom0.005to0.015inch.(0.13~0.38mm)maybereadilyobtainedbythisprocess.Cyanidingisusedprincipallyforthetreatmentofsmallparts.CyanidingNitridingissomewhatsimilartoordinarycasehardening,butitusesadifferentmaterialandtreatmenttocreatethehardsurfaceconstituents.Inthisprocessthemetalisheatedtoatemperatureofaround950℉(510℃)andheldthereforaperiodoftimeincontactwithammoniagas.Nitrogenfromthegasisintroducedintothesteel,formingveryhardnitridesthatarefinelydispersedthroughthesurfacemetal.Nitrogenhasgreaterhardeningabilitywithcertainelementsthanwithothers;hence,specialnitridingalloysteelshavebeendeveloped.Aluminumintherangeof1%to1.5%hasprovedtobeespeciallysuitableinsteel,inthatitcombineswiththegastoformaverystableandhardconstituent.Thetemperatureofheatingrangesfrom925℉to1,050℉(495℃~565℃).NitridingLiquidnitridingutilizesmoltencyanidesaltsand,asingasnitriding,thetemperatureisheldbelowthetransformationrange.Liquidnitridingaddsmorenitrogenandlesscarbonthaneithercyanidingorcarburizingincyanidebaths.Casethicknessof0.001to0.012in.(0.03~0.30mm)isobtained,whereasforgasnitridingthecasemaybeasthickas0.025inch.(0.64mm).Ingeneraltheusesofthetwo-nitridingprocessesaresimilar.Nitridingdevelopsextremehardnessinthesurfaceofsteel.Thishardnessrangesfrom900to1,100Brinell,whichisconsiderablyhigherthanthatobtainedbyordinarycasehardening.Nitridingsteels,byvirtueoftheiralloyingcontent,arestrongerthanordinarysteelsandrespondreadilytoheattreatment.Itisrecommendedthatthesesteelsbemachinedandheat-treatedbeforenitriding,becausethereisnoscaleorfurtherworknecessaryafterthisprocess.Fortunately,theinteriorstructureandpropertiesarenotaffectedappreciablybythenitridingtreatmentand,becausenoquenchingisnecessary,thereislittletendencytowarp,developcracks,orchangeconditioninanyway.Thesurfaceeffectivelyresistscorrosiveactionofwater,saltwaterspray,alkalies,crudeoil,andnaturalgas.Unit9WeldingNewwordsandexpressionsUnit9WeldingTheAmericanWeldingSocietydefinesweldingas“alocalizedcoalescenceofmetalswhereincoalescenceisproducedbyheatingtosuitabletemperatureswithorwithouttheapplicationofpressureandwithorwithouttheuseoffillermetal.Thefillermetaleitherhasameltingpointapproximatelythesameasthebasemetalsorhasameltingpointbelowthatofthebasemetalsbutabove800.”Thereare34differentweldingprocesses.Fig.9-1isamasterchartofthese