Casehardening c1914英文原版机械工程MACHINERY教程教材教案

Casehardening c1914英文原版机械工程MACHINERY教程教材教案 TJ7M3NRLFB3Dlfl25CENTSCASEHARDENINGMETHODS EMPLOYEDIN THEAUTOMOBILE,BICYCLE,BALL ANDROLLER BEARINGANDALLIED TRADES-CASEHARDENING SHAFT-INGNEW CASEHARDENINGMETHODS-CASEHARDENING BYGASMACHINERYS REFERENCEBOOK NO.141PUBLISHED BYMACHINERY,NEW YORKMACHINERYS REFERENCESERIESEACH NUMBERISONEUNITIN APLETELIBRARYOFMACHINE DESIGNAND SHOPPRACTICE REVISEDANDREPUBLISHED FROMMACHINERYNUMBER141CASEHARDENINGCONTENTSIntroduction-3Casehardening Practicein theBicycle,Automobile andAlliedTrades,byROBERT H.GRANT-5Carbonization ofShafting-19Casehardening RollerBearing Parts,by E.F.LAKE-26New CasehardeningMethods,by E.F.LAKE-36Copyright,1914,The IndustrialPress,Publishers ofMACHINEKT,140-148Lafayette Street,New YorkCityOther booksin thisseries dealingwithHeat-treatmentofSteel andkindredsubjectsare asfollows:No.46HARDENING ANDTEMPERINGNo.62HARDNESS TESTINGOF METALSNo.63HEAT-TREATMENT OFSTEELNo.117HIGH-SPEED ANDCARBON STEELNo.118PROPERTIES OFALLOY STEELSMACHINERYThe LeadingMechanicalJournalMACHINE DESIGNCONSTRUCTIONSHOPPRACTICETHE INDUSTRIALPRESS140-148LafayetteSt.New YorkCity51-52Chancery Lane,LondonINTRODUCTIONCasehardening is the processofincreasing the carbon contentof thesurface of steelparativelylow incarbon,so that it can be hardenedby the usualmethodof being heatedtothe hardeningtemperatureand quenchedin acooling medium.The termcasehardening,byit-self,implies the hardening of the surfaceor skinof anarticle,andin order to fullyunderstand theprocess andits object,it isnecessaryto brieflyconsider thefacts andlaws uponwhichit isfounded.Car-bon has a verygreataffinityfor ironand bineswithitatalltemperatures abovea faintredheat.Advantage wastaken of thisfact in the productionof steelby cementation,an oldprocess whichconsistedofrollingwrought ironintothinstripsand thenplacingthesein boxeswith somematerialcontaining afairproportionofcarbon.These boxeswere thenheatedto avery hightemperatureand thecarbon wasgradually absorbedby theiron.The processof casehardeningis,infact,only animprovement onthisoldcementationprocessusedintimespastfor makingsteelfromwroughtiron.Thesteelisheatedin packingboxesinthepresence of a carbonaceousmaterial and when the surfaceof the steelhas absorbed enoughcarbon so thatit will hardenthe sameas high-carbonsteel,it can be quenchedinoilorwater,aordingtotherequirements.For manypurposes,inmachinework,articlesarerequired which must haveaperfectlyhardsurface andyet beofsuchinternalstructurethatthereisnochanceof breaking themwheninuse.In manyinstances,this resultcan beobtained betterby using casehardenedmildsteel thanby usinghigh-classcruciblesteel.For example,in making axles,cups,cones,and manysimilarpartsfor bicycles,itis extremelydifficultto obtainperfect hardnessbinedwithgreatresistancetotorsional,shearingorbendingstresses.For suchpurposes,nothing meetstheserequirementssowellas doarticles whichhave beencasehardened.Agreat improvementhas been madeincasehardeningprocessesduring thelast fewyears.The advancewas begunwith thedevelop-mentofthebicycleindustry;andthenecessityfor casehardenedpartsof thehighest qualityin automobile manufacturehascauseda stillfurther improvementin thisfield.As anexampleofwhat has beenaomplished bypropercase-hardening methods,considerthetransmissiongearingof anauto-mobile.Who wouldthinkof throwingin theback-gears of a latheoranyother machioolwithoutfirst stopping the machine?Inan automobile,however,this verythingis actuallydone dozensoftimesa day,by aperson whogives littlethought towhat heisreallydoing.Yet thegears standup underthis treatmentbecause of beingmanufacturedofspecialsteelsdevelopedduringrecentyearsandbecause ofbeingheat-treated andcasehardened byimproved methods.3476054-/t:-No.141-CASEHARDENINGThere area number of differentquestions thatmust beconsideredin orderto obtaingood resultsin casehardening.In thefirstplace,the properkind ofsteel to be used for variouspurposes must be care-fully selected.Another mostessential thingisthat the casehardeningfurnace mustgive auniformheat.Asoil andgas haveto a greatextentsupersededcoalasfuelsforcasehardeningfurnaces,thechanges infurnace constructionhave,of late,been considerable.An-other itemwhichmust be givencareful considerationisthe boxinwhichthe materialispacked,as wellas thecarbonaceous materialitselfusedin packingthepartsto be casehardened.Stillanotherquestion to be dealtwithis themethod used for hardeningthe partsafterthey have been carbonized.Steel forCasehardened PartsAsthecasehardeningprocessconsistsinaddingcarbontothesteel,amaterial mustbeusedwhichwillabsorbcarbonwithoutnecessitating overheatingor burning.Theeffectof carbononsteelis,in general,it may besaid,to makeitdense,and thedenser thesteelthe higherthe heatnecessary toopen thepores throughwhich itmustabsorbthecarbon.A low-carbonsteelcontaining,say,from0.15to0.20per centof carbonis,therefore,most suitableforcase-hardening.It shouldalso bebornein mindwhen selectingthe ma-terial that thecasehardeningprocess doesnot eliminateanyof theimpuritiesordinarilyfoundiniron,suchassulphur,phosphorus,etc.,and hence,a materialas freeas possiblefrom theseimpuritiesshould!beselected;besides,the material should beperfectlysoundand freefrom mechanicalfaults orweaknesses caused by overheatingorimproper workingduring themanufacturing processes.Both ironand mildsteel have been employedas materialsfor case-hardening in the past;butthisis thesteelage,and iron has longpasseditsday.The steelemployed should be prepared,selected andcontrolledfrom thebeginning with the objectof makingitsuitablefor thefinalrequirements.There aremany pointswithrelation totheselection of the propersteel,its positionand treatment,whichcan onlybe gainedby longexperience anda studyof therequire-ments,but,as ageneralrule,the low-carbonsteelspecifiedinthepreceding paragraphwill be found suitablefor mostpurposes.In thefollowing chapterswill begiven directionsfor casehardening,as publishedinMACHINERY bya numberof authorities.It will beseen thatopinionsdiffer oncertain points,and,therefore,thestate-ments of each authorhavebeengiven infull.The informationgivenwill necessarilyoverlap somewhaton this aount,but,on theotherhand,a pleteand authoritativepresentation of the wholesubjecthas beenmade possible.CHAPTERICASEHAKDENING PRACTICEIN THEBICYCLE,AUTOMOBILE ANDALLIED TRADESInbuilding orconstructinga.furnacefor casehardening,thesizeof the workto be hardened should be thefirstconsideration.Itisfar bettertouse a smallfurnace with a smallbox wheneverpos-sible.If the work variesinsize,different sizesoffurnaces may beused.Smallfurnacesrequirelessfuel,andsmall workmust beplacedinsmall boxesas otherwise the piecespacked nearthe sideswill be overheatedwhile thosein the centerwill notreach there-quiredtemperature.Thefurnacesshould be maderight-andleft-hand so that they can be placed closetogether.Thick wallsshouldbe usedtoretaintheheat.These wallsshould besupported byasubstantial concretefoundation,so that they willretain theirpositionand shape,even whensubjected toa highheat.Large fluesshould beprovidedto carryaway thesmoke andgases.Thefurnace shouldalso besoconstructedthat as muchaspos-sible of the heatof thebustion gasesmay beextracted before theyaredischarged.Theflues andallpartsof thefurnaceshould beeasilyaessible,and adoor,thefull widthof theoven,should beprovidedsothat thetiles can be takenout andthefluescleaned.A pressureblower with a lightoilshould be used withall the pipesaessibleand placed,preferably,above the furnace.If,however,theyare placedbelow ground,they should be arrangedinpartmentswhich can be easilyreachedif repairsare required.The blowerpipes should be runthrough the furnace so asto pre-heat the air used;if coldair isused directlyit will reducethe heatin the furnace.The furnacefronts should be madein severalpartsto preventcracking,with the door properlybalanced andlined.Ashelf should be provided,projecting at the front,for holdingthe boxeswhen they aretaken outor putinto the furnace.The smokestackshould bemadeof sufficientheight toproduce agood draft.Burners should be placedbothat the front andrear of the ovenandshouldbe arrangedinseparate partments,so that the heatwill be uniformin theoven.The hot gases willthen passover thetopof the partmentwall andstrikethe boxeson thetop,afterwhichthey passoutthroughsmallopeningsinthecornerofthefurnace.They thentake azigzag courseunderthetiles andpassfrom therethrough aflue to the rearof the furnace.A largeconduitshould be providedjust belowtheground whichwillcatchallthesoot.This conduit should beprovided withironcovers whichcaneasily be taken offto removethe aumulationof soot.Thefurnace shouldnotbe heatedtooquickly,asthisis apttocrackthebrickwork.Thecoolingshouldalsobedonegradually.6No.141CASEHARDENINGAfter the work has been taken out and the heatshut offfor theday,allthedampers should be closedto holdthe heat.In thisway thefurnacewill coolslowly andcracking orbulging out of shapewill beprevented.In addition,itwill be easierto heatthe furnacethe nextmorning,asit willhave retainedsome of the heat.When workisto be annealed,it should be placedin the furnaceafterthework to be hardened hasbeen removed,and thenthe furnacebroughttothe proper heat.The materialto be annealed canthenCASEHARDENINGgrapple iron,as shownin theupper partof Fig.2;can be used forhandlingtheboxes.Thesidesofthe boxesshouldtaperslightlytowardsthe bottomso that the contents can beeasily dumpedoutof them;they arealso easierto castwhen madein thatway.When verylargeboxes arerequired,theyshould,ifpossible,beprovided witha holethrough thecenter sothat the heat canreachthe contentsfromtheinside,as wellasfrom theoutside.A boxofthis kindis shownin Fig.3.For longwork,such asshafts,tubing,etc.,a wrought-iron pipewithacap oneach endprovides anidealbox.Local Hardening-In many casesitisessential thatthe pieceofwork be hardenedat acertain placeand thatother partsbe leftsoft.There arethreewaysin whichthiscanbe aomplished:First,bycopper-platingFig.2.Grapple Ironor Forkused for Handling CasehardeningBoxesand TruckforHandlingHeavy Boxesandenameling;second,bycoveringthepart whichisnotto behardened byfireclay;and third,byusinga bushingor collarto coverthepartto be left soft.In thefirstcase thearticle shouldbe paintedwith enamelwhereit is to be hardened,the enamelbeing bakedafter havingbeen applied.The remainderof thepiece thatis to be left softis copper-plated.Inthe secondcase,if thearticle to be hardenedhas arecess,such asa hole,slot,etc.,this maybefilled withclay.The thirdmethodisused whena shaft,for example,is onlyto beleftsoft fora shortdistance.A collaris thenplaced on the shaft,and thisprovides theeasiestand leastexpensive meansfor aomplishingthe purpose.No.141CASEHARDENINGIn thecase whereenamel andcopper-platingisused,the enamelwillburn awayand allow the surfacecovered byitto absorbcarbonand,hence,to be hardened,whereasthe copperwillstandaveryhigh heat and preventhardening ofthose portionsthat arecoveredbyit.If the copperis burnedoff,itis anindication thatthe workhasbeen overheated.The clayprevents the hardening ofa portionof theworkin the same wayas doesthecopper.Itis alsoofad-vantage whendipping thework,asit preventsthe formationof steampocketswhich areaptto warpordistortthepiece.When asleeveMachineryFig.3.Large CircularBox withHole inCenter for the Circulationof the GasesofCombustionorcollarisused,thisshouldbe madeaboutone-halfinchlongerthan thepart whichistobeleft soft,soas to preventcarbonizationnear the ends of the collar.Packing-for Hardening-The packingroom should,ifpossible,beseparate from the roomcontainingthe furnaces,sothatthe packingcanbe done withoutthedisfort of the heat and dust.Tables onwheels,or trucks,providedwith shelvesof thesame heightas theshelf infront ofthe furnaceandlarge enoughto holdthe requirednumberofboxes foronefur-CASEHARDENING9nace,should beprovided,sothatthe packedboxes canbeeasilymovedtothe furnace andquickly placed init.The work tobehard-ened shouldbe classifiedaording toitssize and the percentageofcarbonrequired,asit willtake a higher heatfor largerwork,as wellasforpieces which are requiredtoabsorb ahigher percentageofcarbon.Thereareagreat manydifferentkindsof hardeningmaterials,buttheold-fashioned methodofusing groundbone canalways bereliedupontogivesatisfactoryresults.During thelastfewyears,however,the useof bonein variousmanufacturers hasincreasedsothat thepriceof groundbonefor casehardeningpurposesis almostprohibitive.Leather hasbee veryextensively usedforthis pur-pose,it beingfirst burnedand thenground andgraded.A mixingbin is a greatadvantage inconnection with the handlingofthecasehardeningmaterial.Somepartlyusedboneand somenewis thenused to make amixture suitablefor the size ofthe piecestobe hardened.Large piecesrequire aricher materialthan smallerones,as duringthe higherheat required for thelarger piecesand thelongerapplication ofthe heat,more ofthe carbonizing material willburnaway.When packinga box,first puta layerofthecasehardening materialon the bottom,the thicknessofthis layerdepending uponthesizeofthe piecestobehardened.If the articles areheavy,they donotrequire suchgreat carein packing,butif they are thinor long,orhave apeculiar shape,greater careis required.It hasfrequently beenstatedthat onepiece should never bepermitted totouch anotherwhenpacking,butit hasbeen foundthat thisprecautionis notnecessary.If aboxis properlysealed,the partscan toucheach otherwithoutinjury.Thin longpieces should,if possible,beplacedin anuprightpositionto preventtheir saggingout ofshape.Between eachlayerofpieces,casehardening materialispacked aordingtothesizeofthe piecestobehardened.It hasbeen foundfrom experiencethatif thereis notenough ofthe carbonizingmaterial inthe box,the workisliable tohave softspots.About twoinches from the topofthe box,sheet steelstrips about1/16inch thickshould belaidand theseshouldbecovered withalayerof aboutone inchormoreof powderedcharcoal.Thenthecoveris placedon thebox and the edgesare sealedwith fireclay.Ifthereis anydoubt aboutthe length of timerequiredforheating thepiecestoobtain acertain depthof case,wire acouple ofpiecesto-gether,allowing thewire toproject outofthebox.These piecescanthen be takenoutquickly andhardened,and,in thisway,it canbeascertained whetherthe partshave beensufficientlycarbonized.Incasehardening verysmall work,itisadvisableto wirethepiecestogethersothattheycanbe takenoutoftheboxat once;otherwise,they would have tobe pickedout withsmall tongs,asit isimpossibletosift verysmallworkin ascreen becausethe meshwouldhavetobe sofine thatit wouldtake along timeto dothe siftingand theworkwould beetoo coldfor hardening.Ifitis desirableto colorthe10No.141CASEHARDENINGwork,from one-third toone-half ofthe carbonizingmaterialshouldbeburnt leather.The boxesshouldneverbe put into the furnace underahighheat,but shouldbe placedinitwhenits temperatureisfrom800to900degrees F.Then theheat shouldbe slowlybrought uptofrom1500to1800degreesF.In placingthe boxesinthefurnace,great careshouldbe takenthatthehotgaseshave anopportunity tocirculateallaroundthem.A pyrometershouldbeputin someconvenientplaceand properlywired sothattheheat inthefurnacecanbe readilyascertained atany time.If thereisagreat dealof nightworktobedone,a recordingpyrometer shouldbeusedasit gives the maninchargea recordoftheheats duringthe night.By theaid ofthe pyrometerithasbeenfound thatitisnecessaryto have an expansiontank in orderto geta constantair pressure,otherwisethepulsation from the blowerwillaffect theheatin thefurnace.This expansiontank shouldbe situatedsothatthebloweris connecteddirectlywith oneend while the dischargepipeis con-nected atthe oppositeend.This willthen actasareservoir,produc-inga constant pressure.Whenoilis usedfor theheating,itis pre-ferableto pumpitfrom thestorage tankinthegroundto astandpipe,which willinsure aconstant flowof theoil.The intermittentactionofthepump,should theoilbeuseddirectly asites fromit,is objectionable.There is also anotheradvantage,in casethepumpshould havetobeshut downon aountofbreak-down.In thatcase,the furnacescouldstill continueto operate,as thestand pipeshouldhold asupply ofoil sufficient for severalhours.At nightand onholi-days theoilshouldbedrained backinto thestorage tankinordertominimize thedanger incidentto itsuse.The supplypipe forthe airshould efrom the outside andshouldbe soarranged thattheairpasses througha finewire ting,so astoprevent foreignsubstances fromentering theblower.On the outside ofeach furnacea cardshouldbeplaced tellingtheMnd ofwork thatisinthefurnace,when thework wasputin,theheat requiredforit,andwhenit istobe removed.These cardscan bekeptasarecord whichwill beof valuewhen parisonis madewiththe depthof caseobtained underany specifionditions.Carbonizing,Reheating-and Hardening1The heatrequiredfor casehardeningisa greatdealhigher thanthatrequiredfor ordinaryhardening.If,for example,the materialtobe casehardenedwas heatedonlyto1375degreesF.,which wouldbe sufficientforthehardeningof ordinarytool steel,the resultwouldbe veryunsatisfactory.In fact,there wouldbe noresult atall.Smallparts mustbe heatedtoat least1575degreesF.,in whichcase suf-ficient depthof carbonized surfacewillbe obtainedinfromsixtoeight hours.The timerecorded as the correctone forcasehardeningshould betaken fromthe time the boxesare heatedclear through.The correctwayin whichtocasehardenisfirstto carbonizethematerial and then toallowtheboxes tocool downwith thework inCASEHARDENING11them,after whichthey arereheated andhardened in water.There-heating refinesthe grainof thesteeland preventsthe formationofa distinct line betweenthe outerhardened caseand thesoft core.Ifthereis adistinctlinebetween thesetwo sections,the caseisliableto flakeoffwhen thehardened partissubjected tosevere stresses.A stillmore refinedmethod ofcasehardeningis torepack thework,afterit hasbeen carbonized,in oldbone,and afterheating fortwoor threehours takeitoutanddip thepieces inthehardeningtankdirectly asthey efromtheboxes.This willproduce avery finegrainand inmanycasesprevent warping.If thewo