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microstructure,grainsize,heattreatment,chemicalcomposition,fabrication,hardness,yieldstrength,andtensilestrength.360毕业设计网
Microstructure:
Themicrostructureofametalreferstoitscrystalorgrainstructure
asshownthroughexaminationofetchedandpolishedsurfacesunderamicroscope.
Metalswhosemicrostructuresaresimilarhavelikemachiningproperties.Buttherecan
bevariationsinthemicrostructureofthesameworkpiece,thatwillaffectmachinability.
GrainSize:
Grainsizeandstructureofametalserveasgeneralindicatorsofits
machinability.Ametalwithsmallundistortedgrainstendstocuteasilyandfinisheasily.
Suchametalisductile,butitisalso‘gummy’.Metalsofanintermediategrainsize
representacompromisethatpermitsbothcuttingandfinishingmachinability.Hardness
ofametalmustbecorrelatedwithgrainsizeanditisgenerallyusedasanindicatorof
machinability.
HeatTreatment:
Toprovidedesiredpropertiesinmetals,theyaresometimesput
throughaseriesofheatingandcoolingoperationswheninthesolidstate.Amaterialmay
betreatedtoreducebrittleness,removestress,toobtainductilityortoughness,toincrease
strength,toobtainadefinitemicrostructure,tochangehardness,ortomakeotherchanges
thataffectmachinability.
ChemicalComposition:
Chemicalcompositionofametalisamajorfactorindetermining
itsmachinability.Theeffectsofcompositionthough,arenotalwaysclear,
becausetheelementsthatmakeupanalloymetal,workbothsinglyandcollectively.
Certaingeneralizationsaboutchemicalcompositionofsteelsinrelationtomachinability
canbemade,butnon-ferrousalloysaretoonumerousandvariedtopermitsuchgeneralizations.
Fabrication:
Whetherametalhasbeenhotrolled,coldrolled,colddrawn,cast,or
forgedwillaffectitsgrainsize,ductility,strength,hardness,structure-andtherefore-its
machinability.Theterm‘wrought’referstothehammeringorformingofmaterialsintopremanfacturedshapeswhicharereadilyalteredintocomponentsorproductsusingtraditionalmanufacturingtechniques.Wroughtmetalsaredefinedasthatgroupofmaterialswhicharemechanicallyshapedintobars,billets,rolls,sheets,platesortubing.
Castinginvolvespouringmoltenmetalintoamoldtoarriveatanearcomponentshapewhichrequiresminimal,orinsomecasesnomachining.Moldsfortheseoperationsaremadefromsand,plaster,metalsandavarietyofothermaterials.
Hardness:
Thetextbookdefinitionofhardnessisthetendencyforamaterialtoresistdeformation.HardnessisoftenmeasuredusingeithertheBrinellorRockwellscale.Themethodusedtomeasurehardnessinvolvesembeddingaspecificsizeandshapedindentorintothesurfaceofthetestmaterial,usingapredeterminedloadorweight.ThedistancetheindentorpenetratesthematerialsurfacewillcorrespondtoaspecificBrinellorRockwellhardnessreading.Thegreatertheindentorsurfacepenetration,thelowertheultimateBrinellorRockwellnumber,andthusthelowerthecorrespondinghardnesslevel.Therefore,highBrinellorRockwellnumbersorreadingsrepresentaminimal
amountofindentorpenetrationintotheworkpieceandthus,bydefinition,areanindicationofanextremelyhardpart.
TheBrinellhardnesstestinvolvesembeddingasteelballofaspecificdiameter,usingakilogramload,inthesurfaceofatestpiece.TheBrinellHardnessNumber(BHN)isdeterminedbydividingthekilogramloadbythearea(insquaremillimeters)ofthecirclecreatedattherimofthedimpleorimpressionleftintheworkpiecesurface.Thisstandardizedapproachprovidesaconsistentmethodtomakeparativetestsbetweenavarietyofworkpiecematerialsorasinglematerialwhichhasundergonevarioushardeningprocesses.TheRockwelltestcanbeperformedwithvariousindentorsizesandloads.SeveraldifferentscalesexistfortheRockwellmethodorhardnesstesting.Thethreemostpopularareoutlinedbelowintermsoftheactualapplicationthetestisdesignedtoaddress:
RockwellTesting
ScaleApplication
AFortungstencarbideandotherextremelyhardmaterials&
thin,hardsheets.
BFormediumhardnesslowandmediumcarbonsteelsintheannealedcondition.
CFormaterials>
thanRockwell‘B’100.
Intermsofgeneralmachiningpractice,lowmaterialhardnessenhancesproductivity,sincecuttingspeedisoftenselectedbasedonmaterialhardness(thelowerthehardness,thehigherthespeed).Toollifeisadverselyaffectedbyanincreaseinworkpiecehardness,sincethecuttingloadsandtemperaturesriseforaspecificcuttingspeedwithparthardness,therebyreducingtoollife.Indrillingandturning,theaddedcuttingtemperatureisdetrimentaltotoollife,sinceitproducesexcessheatcausingacceleratededgewear.Inmilling,increasedmaterialhardnessproduceshigherimpactloadsasinsertsenterthecut,whichoftenleadstoaprematurebreakdownofthecuttingedge.
YieldStrength:
Tensiletestworkisusedasameansofcomparisonofmetalmaterialconditions.Thesetestscanestablishtheyieldstrength,tensilestrengthandmanyotherconditionsofamaterialbasedonitsheattreatment.Inaddition,thesetestsareusedtocomparedifferentworkpiecematerials.Thetensiletestinvolvestakingacylindricalrodorshaftandpullingitfromoppositeendswithaprogressivelylargerforceinahydraulicmachine.Priortothestartofthetest,twomarkseithertwooreightinchesapartaremadeontherodorshaft.Astherodissystematicallysubjectedtoincreasedloads,themarksbegintomovefartherapart.Amaterialisintheso-called‘elasticzone’whentheloadcanberemovedfromtherodandthemarksreturntotheirinitialdistanceapartofeithertwooreightinches.Ifthetestisallowedtoprogress,apointisreachedwhere,whentheloadisremovedthemarkswillnotreturntotheirinitialdistanceapart.Atthispoint,permanentsetordeformationofthetestspecimenhastakenplace.
Yieldstrengthismeasuredjustpriortothepointbeforepermanentdeformationtakesplace.Yieldstrengthisstatedinpoundspersquareinch(PSI)andisdeterminedbydividingtheloadjustpriortopermanentdeformationbythecrosssectionalareaofthetestspecimen.Thismaterialpropertyhasbeenreferredtoasacondition,sinceitcanbealteredduringheattreatment.Increasedparthardnessproducesanincreaseinyieldstrengthandtherefore,asapartbecomesharder,ittakesalargerforcetoproducepermanentdeformationofthepart.Yieldstrengthshouldnotbeconfusedwithfracturestrength,crackingortheactualbreakingofthematerialintopieces,sincethesepropertiesarequitedifferentandunrelatedtothecurrentsubject.
Bydefinition,amaterialwithhighyieldstrength(forcerequiredperunitofareatocreatepermanentdeformation)requiresahighlevelofforcetoinitiatechipformationinamachiningoperation.Thisimpliesthatasamaterial’syieldstrengthincreases,strongerinsertshapesaswellaslesspositivecuttinggeometriesarenecessarytocombattheadditionalloadencounteredinthecuttingzone.Materialhardnessandyieldstrengthincreasesimultaneouslyduringheattreatment.Therefore,materialswithrelativelyhighyieldstrengthswillbemoredifficulttomachineandwillreducetoollifewhencomparedtomaterialswithmoremoderatestrengths.
TensileStrength:
Thetensilestrengthofamaterialincreasesalongwithyieldstrengthasitisheattreatedtogreaterhardnesslevels.Thismaterialconditionisalsoestablishedusingatensiletest.Tensilestrength(orultimatestrength)isdefinedasthemaximumloadthatresultsduringthetensiletest,dividedbythecross-sectionalareaofthetestspecimen.Therefore,tensilestrength,likeyieldstrength,isexpressedinPSI.Thisvalueisreferredtoasamaterialconditionratherthanaproperty,sinceitsleveljustlikeyieldstrengthandhardness,canbealteredbyheattreatment.Therefore,basedonthematerialselected,distincttensileandyieldstrengthlevelsexistforeachhardnessreading.
Justasincreasedyieldstrengthimpliedhighercuttingforcesduringmachiningoperations,thesamecouldbesaidforincreasedtensilestrength.Again,astheworkpiecetensilestrengthiselevated,strongercuttingedgegeometriesarerequiredforproductivemachiningandacceptabletoollife.
3.3PhysicalPropertiesofWorkMaterials
Physicalpropertieswillincludethosecharacteristicsincludedintheindividualmaterialgroups,suchasthemodulusofelasticity,thermalconductivity,thermalexpansionandworkhardening.
ModulusofElasticity:
Themodulusofelasticitycanbedeterminedduringatensiletestinthesamemannerasthepreviouslymentionedconditions.However,un