ManufacturingEngineeringandTechnologyMachiningSeropekalpakjianStevenR.Schmid20.9MACHINABILITYHHHThemachinabilityofamaterialusuallydefinedintermsoffourfactors:1、Surfacefinishandintegrityofthemachinedpart；2、Toollifeobtained；3、Forceandpowerrequirements；4、Chipcontrol.Thus,goodmachinabilitygoodsurfacefinishandintegrity,longtoollife,andlowforceandpowerrequirements.Asforchipcontrol,longandthin(stringy)curedchips,ifnotbrokenup,canseverelyinterferewiththecuttingoperationbybecomingentangledinthecuttingzone.Becauseofthecomplexnatureofcuttingoperations,itisdifficulttoestablishrelationshipsthatquantitativelydefinethemachinabilityofamaterial.Inmanufacturingplants,toollifeandsurfaceroughnessaregenerallyconsideredtobethemostimportantfactorsinmachinability.Althoughnotusedmuchanymore,approximatemachinabilityratingsareavailableintheexamplebelow.20.9.1MachinabilityOfOOOSteelsBecausesteelsareamongthemostimportantengineeringmaterials(asnotedinChapter5),theirmachinabilityhasbeenstudiedextensively.Themachinabilityofsteelshasbeenmainlyimprovedbyaddingleadandsulfurtoobtainso-calledfree-machiningsteels.ResulfurizedandRephosphorizedsteels.Sulfurinsteelsformsmanganesesulfideinclusions(second-phaseparticles),whichactasstressraisersintheprimaryshearzone.Asaresult,thechipsproducedbreakupeasilyandaresmall；thisimprovesmachinability.Thesize,shape,distribution,andconcentrationoftheseinclusionssignificantlyinfluencemachinability.Elementssuchastelluriumandselenium,whicharebothchemicallysimilartosulfur,actasinclusionmodifiersinresulfurizedsteels.Phosphorusinsteelshastwomajoreffects.Itstrengthenstheferrite,causingincreasedhardness.HarderHHHsteelsresultinbetterchipformationandsurfacefinish.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“lowcarbon,”aconditionthatimprovestheircorrosionresistance.)However,HHHbecauseleadisawell-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.Chattercanbeaproblem,necessitatingmachinetoolswithhighstiffness.However,HHHferriticstainlesssteels(also300series)havegoodmachinability.Martensitic(400series)steelsareabrasive,tendtoformabuilt-upedge,andrequiretoolmaterialswithhighhothardnessandcrater-wearresistance.Precipitation-hardeningstainlesssteelsarestrongandabrasive,requiringhardandabrasion-resistanttoolmaterials.TheEffectsofOtherOOOElementsinSteelsonMachinability.Thepresenceofaluminumandsiliconinsteelsisalwaysharmfulbecausetheseelementscombinewithoxygentoformaluminumoxideandsilicates,whicharehardandabrasive.Thesecompoundsincreasetoolwearandreducemachinability.Itisessentialtoproduceandusecleansteels.Carbonandmanganesehavevariouseffectsonthemachinabilityofsteels,dependingontheircomposition.Plainlow-carbonsteels(lessthan0.15%C)canproducepoorsurfacefinishbyformingabuilt-upedge.Caststeelsaremoreabrasive,althoughtheirmachinabilityissimilartothatofwroughtsteels.Toolanddiesteelsareverydifficulttomachineandusuallyrequireannealingpriortomachining.Machinabilityofmoststeelsisimprovedbycoldworking,whichhardensthematerialandreducesthetendencyforbuilt-upedgeformation.OtherOOOalloyingelements,suchasnickel,chromium,molybdenum,andvanadium,whichimprovethepropertiesofsteels,generallyreducemachinability.Theeffectofboronisnegligible.GaseousGGGelementssuchashydrogenandnitrogencanhaveparticularlydetrimentaleffectsonthepropertiesofsteel.OxygenOOOhasbeenshowntohaveastrongeffectontheaspectratioofthemanganesesulfideinclusions；thehighertheoxygencontent,thelowertheaspectratioandthehigherthemachinability.Inselectingvariouselementstoimprovemachinability,weshouldconsiderthepossibledetrimentaleffectsoftheseelementsonthepropertiesandstrengthofthemachinedpartinservice.Atelevatedtemperatures,forexample,leadcausesembrittlementofsteels(liquid-metalembrittlement,hotshortness；seeSection1.4.3),althoughatroomtemperatureithasnoeffectonmechanicalproperties.Sulfurcanseverelyreducethehotworkabilityofsteels,becauseoftheformationofironsulfide,unlesssufficientmanganeseispresenttopreventsuchformation.Atroomtemperature,themechanicalpropertiesofresulfurizedsteelsdependontheorientationofthedeformedmanganesesulfideinclusions(anisotropy).Rephosphorizedsteelsaresignificantlylessductile,andareproducedsolelytoimprovemachinability.20.9.2MachinabilityofVariousOtherOOOMetalsAluminumisgenerallyveryeasytomachine,althoughthesoftergradestendtoformabuilt-upedge,resultinginpoorsurfacefinish.HighHHHcuttingspeeds,highrakeangles,andhighreliefanglesarerecommended.Wroughtaluminumalloyswithhighsiliconcontentandcastaluminumalloysmaybeabrasive；theyrequirehardertoolmaterials.Dimensionaltolerancecontrolmaybeaprobleminmachiningaluminum,sinceithasahighthermalcoefficientofexpansionandarelativelylowelasticmodulus.Berylliumissimilartocastirons.Becauseitismoreabrasiveandtoxic,though,itrequiresmachininginacontrolledenvironment.Castgrayironsaregenerallymachinablebutare.Freecarbidesincastingsreducetheirmachinabilityandcausetoolchipp