外文翻译材料的可机加工性.docx
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外文翻译材料的可机加工性.docx
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外文翻译材料的可机加工性
原文:
20.9MACHINABILITY
Themachinabilityofamaterialusuallydefinedintermsoffourfactors:
1、Surfacefinishandintegrityofthemachinedpart;
2、Toollifeobtained;
3、Forceandpowerrequirements;
4、Chipcontrol.
Thus,goodmachinabilitygoodsurfacefinishandintegrity,longtoollife,andlowforceAndpowerrequirements.Asforchipcontrol,longandthin(stringy)curedchips,ifnotbrokenup,canseverelyinterferewiththecuttingoperationbybecomingentangledinthecuttingzone.
Becauseofthecomplexnatureofcuttingoperations,itisdifficulttoestablishrelationshipsthatquantitativelydefinethemachinabilityofamaterial.Inmanufacturingplants,toollifeandsurfaceroughnessaregenerallyconsideredtobethemostimportantfactorsinmachinability.Althoughnotusedmuchanymore,approximatemachinabilityratingsareavailableintheexamplebelow.
20.9.1MachinabilityOfSteels
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“lowcarbon,”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.
20.9.2MachinabilityofVariousOtherMetals
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,althoughitgreatlyimprovesatelevatedtemperatures.
Zirconiumhasgoodmachinability.Itrequiresacoolant-typecuttingfluid,however,becauseoftheexplosionandfire.
20.9.3MachinabilityofVariousMaterials
Graphiteisabrasive;itrequireshard,abrasion-resistant,sharptools.
Thermoplasticsgenerallyhavelowthermalconductivity,lowelasticmodulus,andlowsofteningtemperature.Consequently,machiningthemrequirestoolswithpositiverakeangles(toreducecuttingforces),largereliefangles,smalldepthsofcutandfeed,relativelyhighspeeds,and
propersupportoftheworkpiece.Toolsshouldbesharp.
Externalcoolingofthecuttingzonemaybenecessarytokeepthechipsfrombecoming“gummy”andstickingtothetools.Coolingcanusuallybeachievedwithajetofair,vapormist,orwater-solubleoils.Residualstressesmaydevelopduringmachining.Torelievethesestresses,machinedpartscanbeannealedforaperi
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