MACHINABILITYThemachinabilityofamaterialusuallydefinedintermsoffourfactors:1、Surfacefinishandintegrityofthemachinedpart；2、Toollifeobtained；3、Forceandpowerrequirements；4、Chipcontrol.Thus,goodmachinabilitygoodsurfacefinishandintegrity,longtoollife,andlowforceAndpowerrequirements.Asforchipcontrol,longandthin(stringy)curedchips,ifnotbrokenup,canseverelyinterferewiththecuttingoperationbybecomingentangledinthecuttingzone.Becauseofthecomplexnatureofcuttingoperations,itisdifficulttoestablishrelationshipsthatquantitativelydefinethemachinabilityofamaterial.Inmanufacturingplants,toollifeandsurfaceroughnessaregenerallyconsideredtobethemostimportantfactorsinmachinability.Althoughnotusedmuchanymore,approximatemachinabilityratingsareavailableintheexamplebelow.Becausesteelsareamongthemostimportantengineeringmaterials(asnotedinChapter5),theirmachinabilityhasbeenstudiedextensively.Themachinabilityofsteelshasbeenmainlyimprovedbyaddingleadandsulfurtoobtainso-calledfree-machiningsteels.ResulfurizedandRephosphorizedsteels.Sulfurinsteelsformsmanganesesulfideinclusions(second-phaseparticles),whichactasstressraisersintheprimaryshearzone.Asaresult,thechipsproducedbreakupeasilyandaresmall；thisimprovesmachinability.Thesize,shape,distribution,andconcentrationoftheseinclusionssignificantlyinfluencemachinability.Elementssuchastelluriumandselenium,whicharebothchemicallysimilartosulfur,actasinclusionmodifiersinresulfurizedsteels.Phosphorusinsteelshastwomajoreffects.Itstrengthenstheferrite,causingincreasedhardness.Hardersteelsresultinbetterchipformationandsurfacefinish.Notethatsoftsteelscanbedifficulttomachine,withbuilt-upedgeformationandpoorsurfacefinish.Thesecondeffectisthatincreasedhardnesscausestheformationofshortchipsinsteadofcontinuousstringyones,therebyimprovingmachinability.LeadedSteels.Ahighpercentageofleadinsteelssolidifiesatthetipofmanganesesulfideinclusions.Innon-resulfurizedgradesofsteel,leadtakestheformofdispersedfineparticles.Leadisinsolubleiniron,copper,andaluminumandtheiralloys.Becauseofitslowshearstrength,therefore,leadactsasasolidlubricant(Section32.11)andissmearedoverthetool-chipinterfaceduringcutting.Thisbehaviorhasbeenverifiedbythepresenceofhighconcentrationsofleadonthetool-sidefaceofchipswhenmachiningleadedsteels.Whenthetemperatureissufficientlyhigh-forinstance,athighcuttingspeedsandfeeds(Section20.6)theleadmeltsdirectlyinfrontofthetool,actingasaliquidlubricant.Inadditiontothiseffect,leadlowerstheshearstressintheprimaryshearzone,reducingcuttingforcesandpowerconsumption.Leadcanbeusedineverygradeofsteel,suchas10xx,11xx,12xx,41xx,etc.LeadedsteelsareidentifiedbytheletterLbetweenthesecondandthirdnumerals(forexample,10L45).(Notethatinstainlesssteels,similaruseoftheletterLmeans“l(fā)owcarbon,”aconditionthatimprovestheircorrosionresistance.)However,becauseleadisawell-knowntoxinandapollutant,thereareseriousenvironmentalconcernsaboutitsuseinsteels(estimatedat4500tonsofleadconsumptioneveryyearintheproductionofsteels).Consequently,thereisacontinuingtrendtowardeliminatingtheuseofleadinsteels(lead-freesteels).Bismuthandtinarenowbeinginvestigatedaspossiblesubstitutesforleadinsteels.Calcium-DeoxidizedSteels.Animportantdevelopmentiscalcium-deoxidizedsteels,inwhichoxideflakesofcalciumsilicates(CaSo)areformed.Theseflakes,inturn,reducethestrengthofthesecondaryshearzone,decreasingtool-chipinterfaceandwear.Temperatureiscorrespondinglyreduced.Consequently,thesesteelsproducelesscraterwear,especiallyathighcuttingspeeds.StainlessSteels.Austenitic(300series)steelsaregenerallydifficulttomachine.Chattercanbesproblem,necessitatingmachinetoolswithhighstiffness.However,ferriticstainlesssteels(also300series)havegoodmachinability.Martensitic(400series)steelsareabrasive,tendtoformabuilt-upedge,andrequiretoolmaterialswithhighhothardnessandcrater-wearresistance.Precipitation-hardeningstainlesssteelsarestrongandabrasive,requiringhardandabrasion-resistanttoolmaterials.TheEffectsofOtherElementsinSteelsonMachinability.Thepresenceofaluminumandsiliconinsteelsisalwaysharmfulbecausetheseelementscombinewithoxygentoformaluminumoxideandsilicates,whicharehardandabrasive.Thesecompoundsincreasetoolwearandreducemachinability.Itisessentialtoproduceandusecleansteels.Carbonandmanganesehavevariouseffectsonthemachinabilityofsteels,dependingontheircomposition.Plainlow-carbonsteels(lessthan0.15%C)canproducepoorsurfacefinishbyformingabuilt-upedge.Caststeelsaremoreabrasive,althoughtheirmachinabilityissimilartothatofwroughtsteels.Toolanddiesteelsareverydifficulttomachineandusuallyrequireannealingpriortomachining.Machinabilityofmoststeelsisimprovedbycoldworking,whichhardensthematerialandreducesthetendencyforbuilt-upedgeformation.Otheralloyingelements,suchasnickel,chromium,molybdenum,andvanadium,whichimprovethepropertiesofsteels,generallyreducemachinability.Theeffectofboronisnegligible.Gaseouselementssuchashydrogenandnitrogencanhaveparticularlydetrimentaleffectsonthepropertiesofsteel.Oxygenhasbeenshowntohaveastrongeffectontheaspectratioofthemanganesesulfideinclusions；thehighertheoxygencontent,thelowertheaspectratioandthehigherthemachinability.Inselectingvariouselementstoimprovemachinability,weshouldconsiderthepossibledetrimentaleffectsoftheseelementsonthepropertiesandstrengthofthemachinedpartinservice.Atelevatedtemperatures,forexample,leadcausesembrittlementofsteels(liquid-metalembrittlement,hotshortness；seeSection1.4.3),althoughatroomtemperatureithasnoeffectonmechanicalproperties.Sulfurcanseverelyreducethehotworkabilityofsteels,becauseoftheformationofironsulfide,unlesssufficientmanganeseispresenttopreventsuchformation.Atroomtemperature,themechanicalpropertiesofresulfurizedsteelsdependontheorientationofthedeformedmanganesesulfideinclusions(anisotropy).Rephosphorizedsteelsaresignificantlylessductile,andareproducedsolelytoimprovemachinability.Aluminumisgenerallyveryeasytomachine,althoughthesoftergradestendtoformabuilt-upedge,resultinginpoorsurfacefinish.Highcuttingspeeds,highrakeangles,andhighreliefanglesarerecommended.Wroughtaluminumalloyswithhighsiliconcontentandcastaluminumalloysmaybeabrasive；theyrequirehardertoolmaterials.Dimensionaltolerancecontrolmaybeaprobleminmachiningaluminum,sinceithasahighthermalcoefficientofexpansionandarelativelylowelasticmodulus.Berylliumissimilartocastirons.Becauseitismoreabrasiveandtoxic,though,itrequiresmachininginacontrolledenvironment.Castgrayironsaregenerallymachinablebutare.Freecarbidesincastingsreducetheirmachinabilityandcausetoolchippingorfracture,necessitatingtoolswithhightoughness.Nodularandmalleableironsaremachinablewithhardtoolmaterials.Cobalt-basedalloysareabrasiveandhighlywork-hardening.Theyrequiresharp,abrasion-resistanttoolmaterialsandlowfeedsandspeeds.Wroughtcoppercanbedifficulttomachinebecauseofbuilt-upedgeformation,althoughcastcopperalloysareeasytomachine.Brassesareeasytomachine,especiallywiththeadditionpflead(leadedfree-machiningbrass).Bronzesaremoredifficulttomachinethanbrass.Magnesiumisveryeasytomachine,withgoodsurfacefinishandprolongedtoollife.Howevercareshouldbeexercisedbecauseofitshighrateofoxidationandthedangeroffire(theelementispyrophoric).Molybdenumisductileandwork-hardening,soitcanproducepoorsurfacefinish.Sharptoolsarenecessary.Nickel-basedalloysarework-hardening,abrasive,andstrongathightemperatures.Theirmachinabilityissimilartothatofstainlesssteels.Tantalumisverywork-hardening,ductile,andsoft.Itproducesapoorsurfacefinish；toolwearishigh.Titaniumanditsalloyshavepoorthermalconductivity(indeed,thelowestofallmetals),causingsignificanttemperatureriseandbuilt-upedge；theycanbedifficulttomachine.Tungstenisbrittle,strong,andveryabrasive,soitsmachinabilityislow