機(jī)械制造工程-開卷習(xí)題答案

QUESTIONS

1.1

Explainthedifferencebetweenaunitcellandasinglecrystal.

Answer(frompage#40):

TheSmallestgroupofatomsshowingthecharacteristiclatticestructureofaparticularmetalisknownasaUnitCell.Itisthebuildingblockofacrystal,andasinglecrystalcanhavemanyunitcells.

1.2

Intablesoncrystalstructures,ironislistedashavingbothabccandafeestructure.Why?

Answer(frompage#41):

Samemetalmayformdifferentstructures,becauseofalowerenergyrequirementatthattemperature.Forexample,ironformsabccstructure(alphairon)below912°C(1674°F)andabove1394°C(2541°F),butifformsanfccstructure(gammairon)between912°Cand1394°C.

1.3a.Defineanisotropy,andwhatmaterialscanyouthinkofotherthanmetalsthatexhibitanisotropicbehavior?

Answer(frompage#42):

Becausetheb/aratioisdifferentfordifferentdirectionswithinthecrystal,asinglecrystalhasdifferentpropertieswhentestedindifferentdirections.Wesaythatasinglecrystalisanisotropic.Where“a”isthespacingoftheatomicplanesand“b”isinverselyproportionaltotheatomicdensityintheatomicplanes.

Acommonexampleofanisotropyiswovencloth,whichstretchesdifferentlywhenwepullitindifferentdirections,orplywood,whichismuchstrongerintheplanardirectionthanalongitsthicknessdirection(itsplitseasily).

1.4

Whateffectsdoesrecrystallizationhaveonpropertiesofmetals?

Answer(frompage#50):

Recrystallizationdecreasesthedensityofdislocations,lowersthestrength,andraisestheductilityofthemetal.

1.5Whatisstrainhardening?Andwhateffectsdoesithaveonthepropertiesofmetals?

Answer(frompage#45):

Theeffecto9fanincreaseinshearstressthatcausesanincreaseintheoverallstrengthofthemetalisknownastheworkhardeningorstrainhardening.

Theeffectonthepropertiesofmetalsisthegreaterthedeformation,thegreaterthenumberofentanglements,henceanincreaseinthemetal’sstrength.

1.6

Explainwhatismeantbystructuresensitiveandstructureinsensitivepropertiesofmetals.

Answer(frompage#43,44):

Mechanicalandelectricalpropertiesofmetals,suchasyield,fracturestrength,andelectricalconductivity,areadverselyaffectedbythesedefects;theseareknownasstructure-sensitiveproperties.Ontheotherhand,theirphysicalandchemicalpropertiessuchasmeltingpoint,specificheat,coefficientofthermalexpansion,andelasticconstants(e.g.,modulusofelasticityandmodulusofrigidity)arenotsensitivetothesedefects;theseareknownasstructure-insensitiveproperties.

1.7Makealistofeachofthemajorkindsofimperfectioninthecrystalstructureofmetals,anddescribethem.

Answer(frompage#4):

Linedefects,calleddislocations

Pointdefects,suchasvacancy(missingatom),andinterstitialatom(extraatominthelattice),oranimpurity(foreignatom)thathasreplacedtheatomofthepuremetal.

Volumeorbulkimperfections,suchasvoidsorinclusions(nonmetallicelementssuchasoxides,sulfides,andsilicates);

Planarimperfection,suchasgrainboundaries.

1.8Whatinfluencedoesgrainsizehaveonthemechanicalpropertiesofmetals?

Answer(frompage#46):

Grainsizesignificantlyinfluencesthemechanicalpropertiesofmetals.Atroomtemperature,alargegrainsizeisgenerallyassociatedwithlowstrength,lowhardness,andlowductility.Largegrains,particularlyinsheetmetals,alsocausearoughsurfaceappearanceafterthematerialhasbeenstretched.

1.9Whatistherelationshipbetweenthenucleationrateandthenumberofgrainsperunitvolumeofametal?

Answer(frompage#45):

Thenumberandsizeofthegrainsdevelopedinaunitvolumeofthemetaldependsontherateatwhichnucleation(theinitialstageofformationofcrystals)takeplace.

1.10Whatisaslipsystem,andwhatisitssignificance?

Answer(frompage#42):

Thecombinationofaslipplaneanditsdirectionofslipareknownasaslipsystem.Ingeneral,metalswithslipsystemsof5oraboveareductile,whereasthosewithslipsystemsbelow5arenot.ItalsodeterminestheprobabilityofShearstress.

1.11Explainthedifferencebetweenrecoveryandrecrystallization.

Answer(frompage#49):

Recovery,whichoccursatacertaintemperaturerangebelowtherecrystallizationtemperatureofthemetal,thestressesinthehighlydeformedregionsarerelived.Subgrainboundariesbegintoform(aprocesscalledpolygonization),withnoappreciablechangeinmechanicalpropertiessuchashardnessandstrength.

Recrystallizationistheprocessinwhich,atacertaintemperaturerange,newequiaxedandstrain-freegrainsareformed,replacingtheoldergrains,incalledrecrystallization.

1.12Whatishotshortness,andwhatisitssignificance?

Answer(frompage#47):

Hotshortnessiscausedbyelocalmeltingofaconstituentoranimpurityinthegrainboundaryatatemperaturebelowthemeltingpointofthemetalitself.Whensubjectedtoplasticdeformationatelevatedtemperatures(hot-working),thepieceofmetalcrumblesanddisintegratesalongthegrainboundaries.Examplesareantimonyincopper,leadedsteels,andleadedbrass.

1.13Explainthedifferencesbetweencold,warm,andhotworkingofmetals.

Answer(frompage#51):

Cold-workingreferstoplasticdeformationthatisusually,butnotnecessarily,carriedoutatroomtemperature.Whenthedeformationiscarriedoutabovetherecrystallizationtemperature,itiscalledhot-working.Asthenameimplies,warm-workingiscarriedoutatintermediatetemperatures.Thuswarmworkingisacompromisebetweencold-andhot-working.

1.14Describewhattheorangepeeleffectis.

Answer(frompage#50,51):

Largegrainsproducesaroughsurfaceappearanceonthesheetmetals,calledorangepeel,whentheyarestretchedtoformapart,orwhenapieceofmetalissubjectedtocompression(suchasinforgingoperations).

1.15Whycan'tsomemetalssuchasleadbecomestrongerwhencoldworked?

Answer(frompage#50):

Recrystallizationdecreasesthedensityofdislocations,lowersthestrength,andraisestheductilityofthemetal.Leadrecrystallizationataboutroomtemperature;asaresult,whencold-worked,theydonotworkharden.Recrystallizationdependsonthedegreeofpriorcoldwork(workhardening):themorecoldwork,thelowerthetemperaturerequiredforrecrystallizationtooccur.Thereasonisthat,astheamountofcoldworkincreases,thenumberofdislocationsandtheamountofenergystoredindislocations(storedenergy)alsoincrease.Thisenergysuppliestheworkrequiredforrecrystallization.

1.16Describethedifferencebetweenpreferredorientationandmechanicalfibering.

Answer(frompage#48,49):

PreferredOrientationisalsocalledcrystallographicanisotropy.Whenametalcrystalissubjectedtotension,theslidingblocksrotatetowardthedirectionofthepullingforce.Asaresult,slipplanesandslipbandstendtoalignthemselveswithdirectionofdeformation.

MechanicalFiberingresultsformthealignmentofimpurities,inclusions(stringers),andvoidsinthemetalduringdeformation.

1.17Inmicroscopy,itiscommonpracticetoapplyachemicaletchanttoametalinordertohighlightgrainboundariesandmicrostructures.Explainwhy.

Answer(frompage#46):

Grainboundariesaremorereactivethanthegrainsthemselves,becausetheatomsalongthegrainboundariesarepackedlessefficientlyandaremoredisordered.Asaresult,theyhavehigherenergythantheatomsinorderlylatticewithinthegrains.Forthisreason,apolishedandetchedsurfacecanbecomerougher

1.18Whatistwinning?Howdoesitdifferfromslip?

Answer(frompage#42):

Thesecondmechanismofplasticdeformationistwinning,inwhichaportionofthecrystalformsamirrorimageofitselfacrosstheplaneoftwinning.Whereasinsliptheplasticdeformationcanonlytakeplacealongplanesofmaximumatomicdensityor,inotherwords,thatsliptakesplaceincloselypackeddirection.

=======================EndofHomeworkAssignment

QUESTIONS

2.1Distinguishbetweenengineeringstressandtruestress.

Answer(frompage#__57,60___):

Theengineeringstressornominalstressisdefinedastheratiooftheappliedloadptotheoriginalcross-sectionalareaAofthespecimen:

EngineeringStress,?=P/A

Truestressisdefinedastheratioofthelocalptotheactual(instantaneous,hencetrue)cross-sectionalareaAofthespecimen:

Truestress,?=P/A

2.2Describetheeventsthatoccurwhenaspecimenundergoesatensiontest.Sketchaplausiblestressstraincurve,andidentifyallsignificantregionsandpointsbetweenthem.Assumethatloadingcontinuesuptofracture.

Answer(frompage#_56,57____):

Whentheloadisfirstapplied,thespecimenelongatesinproportiontotheload.Thiseffectiscalledlinearelasticbehavior.Iftheloadisremoved,thespecimenreturnstoitsoriginallengthandshape,inanelasticprocesssimilartostretchingarubberbandandreleasingit.Astheloadisincreased,thespecimenbegins,atsomelevelofstress,toundergopermanent(plastic)deformation.Beyondthatlevel,thestressandstrainarenolongerproportional,astheywereintheelasticregion.Thestressatwhichthisphenomenonoccursisknownastheyyieldstress,y,ofthematerial.Ifthespecimenisloadedbeyonditsultimatetensilestrength,itbeginstoneckorneckdown.Finallyspecimenfracturesattheneckedregionorknowasbreakingorfracturesstress.

2.3Whatisductility,andhowisitmeasured?

Answer(frompage#_59____):

Ductilityistheextentofplasticdeformationthatthematerialundergoesbeforefracture.Therearetwocommonmeasuresofductility.

Thefirstisthetotalelongationofthespecimen:

Elongation=(lf–lo)/(lo)*100

Thesecondmeasureofductilityisthereductionofarea.

Reductionofarea=(Ao–Af)/(Ao)*100

2.5Whatisstrainratesensitivity,andhowisitmeasured?

Answer(frompage#___64,65__):

Whenincreasingthestrainrateincreasesthestrengthofthematerial(strain-ratehardening).Theslopeofthesecurvesiscalledthestrain-ratesensitivityexponent,m.Thevalueofmisobtainedfromlog-logplots,providedthattheverticalandhorizontalscalesarethesame.Aslopeof45°would,therefore,indicateavalueofm=1.Therelationshipisgivenbytheequation.

?=C?^m

WhereCisastrengthcoefficient.TheconstantChastheunitsofstress;?isthetruestrainrate,definedasthestrainthatthematerialundergoesperunittime.

Thesensitivityofstrengthtostrainrateincreasewithtemperature.

2.6Whattestcanmeasurethepropertiesofamaterialundergoingshearstrain?

Answer(frompage#___67__):

Torsiontest

2.7Whattestingprocedurescanbeusedtomeasurethepropertiesofbrittlematerials,suchasceramicsandcarbides?

Answer(frompage#_67____):

DiskTest

2.8Describethedifferencesbetweenbrittleandductilefracture.

Answer(frompage#__76,78___):

Ductilefractureischaracterizedbyplasticdeformationwhichprecedesfailureofthepart.Brittlefractureoccurswithlittleornogrossplasticdeformation.

2.9Explainthedifferencebetweenstressrelaxationandcreep.

Answer(frompage#_73,75____):

Creepisthepermanentelongationofacomponentunderastaticloadmaintainedforaperiodoftime.Itisaphenomenonofmetalsandofcertainnonmetallicmaterials,suchasthermoplasticandrubbers,anditcanoccuratanytemperature.

StressRelaxationiscloselyrelatedtocreep.Instressrelaxationthestressesresultingfromaloadingofastructuralcomponentdecreaseinmagnitudeoveraperiodoftime,eventhoughthedimensionsofthecomponentremainconstant.

2.10Describethedifferencebetweenelasticandplasticbehavior.

Answer(frompage#__57___):

Anelasticbehavioriswhentheloadisfirstapplied,thespecimenelongatesinproportiontotheload,andiftheloadisremoved,thespecimenreturnstoitsoriginallengthandshape.

APlasticbehavioriswhentheloadisincrease,thespecimenbegins,atsomelevelofstress,toundergopermanent(plastic)deformation.Beyondthatlevel,thestressandstrainarenolongerproportional,astheywereintheelasticregion.

2.11Explainwhatuniformelongationisintensiontesting.

Answer(frompage#__58___):

Underincreasingload,toelongatebeyondY,itscross-sectionalareadecreasespermanentlyanduniformlythroughoutitsgagelength.Ifthespecimenisunloadedfromastresslevelhigherthantheyieldstress,thecurvefollowsastraightlinedownwardandparalleltotheoriginalslope.Thestress-straincurveisoffsetbya0.002,or0.2%elongation.

2.12Describethedifferencebetweendeformationrateandstrainrate.Whatunitdoeseachonehave?

Answer(frompage#__64___):

Deformationrateisdefinedasthespeedatwhichatensiontestisbeingcarriedout,inunitsof,saym/sorft/min.Thestrainrate,ontheotherhand,isafunctionofthespecimenlength.Ashortspecimenelongatesproportionatelymoreduringthesametimeperiodthandoesalongspecimen.Thestrainratesareusuallystatedintermsofordersofmagnitude,suchas10^2s^-1,10^4S^-1,andsoon.

2.13Describethedifficultiesinvolvedinmakingacompressiontest.

Answer(frompage#__66___):

Thecompressiontest,inwhichthespecimenissubjectedtoacompressiveload,givesinformationusefulfortheseprocesses.Thistestisusuallycarriedoutbythecompressingasolidcylindricalspecimenbetweentwoflatdies(platens).Becauseoffrictionbetweenthespecimenandtheplatens,thespecimen’scylindricalsurfacebulges;thiseffectiscalledbarreling.Frictionpreventsthetopandbottomsurfacesfromexpandingfreely.

Becausethecross-sectionalareaofthespecimennowchangesalongitsheight,beingmaximuminthemiddle,obtainingstress-straincurvesincompressionisdifficult.Furthermore,frictiondissipatesenergy,sothecompressiveforceishigherthanitotherwisewouldbe,inordertosupplytheworkrequiredtoovercomefriction.Witheffectivelubrication,frictioncanbeminimized,andareasonablyconstantcross-sectionalareacanbemaintainedduringthetest.

2.14WhatisHooke'sLaw?Young'sModulus?Poisson'sratio?

Answer(frompage#_58,59____):

Theratioofstresstostrainintheelasticregionisknownasthemodulusofelasticity,EorYoung’smodulus.

Modulusofelasticity,E=?/e

ThislinearrelationshipisknownasHooke’slaw.Themodulusofelasticityisessentiallyameasureoftheslopeoftheelasticportionofthecurveandhencethestiffnessofthematerial.Thatbecauseengineeringstrainisdimensionless,Ehasthesameunitasstress.ThehighertheEvalue,thehighertheloadrequiredtostretchthespecimentothesameextent,andthusthestifferthematerial.

TheabsolutevalueoftheratiointhespecimenofthelateralstraintothelongitudinalstrainisknownasPoisson’sratio.

2.15Whatisnecking?

Answer(frompage#_58____):

Ifthespecimenisloadedbeyonditsultimatetensilestrength,itbeginstoneck,orneckdown.Thecross-sectionalareaofthespecimenisnolongeruniformalongthegagelengthandissmallerintheneckedregion.

2.16Whatisthereasonthat“ayield”strengthisdefinedasa0.2%offsetstrength?

Answer(frompage#__58___):

Forsoftandductilematerials,itmaynotbeeasytodeterminetheexactlocationonthestress-straincurveatwhichyieldingoccurs,becausetheslopeisofthestraight(elastic)portionofthecurvebeginstodecreaseslowly.ThereforeweusuallydefineYasthepointofthestress-straincurvethatisoffsetbyastrainof0.002,or0.2%elongation.

2.17Whydoesthefatiguestrengthofaspecimenorpartdependonitssurfacefinish?

Answer(frompage#__80,81___):

TheSurfacefinishcanhelpreducedecarburization’surfacepits(duetocorrosion)thatactasstressraisers’hydrogenembrittlement’galvanizing,andelectroplating.Insimplywaytoputit,itreducesStress-Corrosionandstresscracking.

2.18Ifstriationsareobservedundermicroscopicexaminationofafracturesurface,whatdotheysuggestregardingthemodeoffracture?

Answer(frompage#_____):80

2.19Explainthedifferencebetweentransgranularandintergranularfracture.

Answer(frompage#_79____):

Inpolycrystallinemetals,thefracturepathsmostcommonlyobservedaretransgranular(transcrystallineorintragranular)’thatis,thecrackpropagatesthroughthatgrain.Intergranularfracture,wherethecrackpropagatesalongthegrainboundaries,generallyoccurswhenthegrainboundariesaresoft,containabrittlephase,orhavebeenweakenedbyliquid-orsolid-metalembrittlement.

=======================EndofHomeworkAssignment

QUESTIONS

3.1Listreasonswhydensityisanimportantmaterialpropertyinmanufacturing.

Answer(frompage#91,92_____):

AsignificantrolethatDensityplaysisinthespecificstrengthstrength-to-weightratio)andspecificstiffness(stiffness-to-weightratio)ofmaterialsandstructure.Densityisanimportantfactorintheselectionofthematerialsforhigh-speedequipment.Forexample,theuseofmagnesiuminprintingandtextilemachinery,manycomponentsofwhichusuallyoperateatveryhighspeeds.Inordertoavoidvibrationsandpartfailurebecauseoftheirlowdensity,Densityisconsideredtobeasanimportantfactor.

3.2Whyisthemeltingpointofamaterialanimportantfactorinmanufacturingprocesses?

Answer(frompage#_94____):

Themeltingpointofthematerialshasanumberofindirecteffectsonmanufacturingoperations.Thechoiceofthematerialforthehigh-temperatureapplicationsisthemostobviouseffect.Becauseoftherecrystallizationtemperaturesofametalisrelatedtoitsmeltingpointoperationssuchasannealing,heattreating,andhotworkingrequiredaknowledgeofthemeltingpointofthematerialinvolved.

3.3Whatadverseeffectscanbecausedbythermalexpansionofmaterials?

Answer(frompage#_95____):

Thethermalexpansionofthematerialhastheseveralsignificanteffects.Generally,thecoefficientofthermalexpansionisinverselyproportionaltothemeltingpointofthematerial.Shrinkfitsutilizethermalexpansionandcontraction.Apartinwhichaholeistobeinstalledoverashaft,suchasaflangeoraleverarm,isheatedand

Thenitisslippedoveracoolshaftorspindle.Whenitislaterallowedtobecool,thepartshrinksandtheassemblybecomeseffectivelyintegral.

3.4Whatisthepiezoelectriceffect?

Answer(frompage#_96____):

Thepiezoelectriceffect(piezofromGreek,meaningtopress)isexhibitedbysomematerial,suchasquartzcrystalsandsomeceramics,inwhichthereisareversibleinteractionbetweenanelasticstrainandanelectricfield.Thispropertyisutilizedinmakingtransducers,whicharedevicesthatconvertthestrainfromanexternalforceintoelectricalenergy.

3.5Whatfactorsleadtothecorrosionofametal?

Answer(frompage#_98____):

Corrosioncanactinindirectways.Stress-corrosioncrackingisanexampleoftheeffectofacorrosiveenvironmentontheintegrityoftheproductthathasmanufacturedhadaresidualstressinit.ToolandDiematerialsalsocanbesusceptibletochemicalattackancancausecorrosion.

3.6Whatispassivation?Whatisitssignificance?

Answer(frompage#__98___):

Theusefulnessofoxidationisexhibitedthecorrosionresistanceofaluminum,titanium,andstainlesssteel.Aluminumdevelopsathin(afewatomiclayers),strongand,adherenthardoxidefilmsthatbetterprotectsthesurfaceformfurtherenvironmentalcorrosion.Titaniumdevelopsafilmtitaniumoxide.Asimilarphenomenonoccursinstainlesssteel,which,becauseofthechromiumpresentdevelopsaprotectivefilmonthesurfaces.Theseprocessesareknownaspassivation.

3.7Whatisthedifferencebetweenthermalconductivityandspecificheat?

Answer(frompage#__94___):

Amaterial'sspecificheatistheenergyrequiredtoraisethetemperatureofaunitmassofitbyonedegree.Alloyingelementshavearelativelyminoreffectonthespecificheatofthemetals.Thermalconductivityindicatestherateatwhichheatflowswithinandthroughamaterial.Metallicallybondedmaterialsgenerallyhavehighthermal

Conductivity,whileironicallyorcovalentlybondedmaterialshavepoorconductivitybecauseofthelargedifferenceintheirthermalconductivities,alloyingelementshaveasignificanteffectonthethermalconductivityofalloys

3.8Whatisstresscorrosioncracking?Whyisitalsocalledseasoncracking?

Answer(frompage#__98___):

Stresscorrosioncrackingisanexampleoftheeffectofacorrosiveenvironmentontheintegrityofaproductthat,asmanufactured,hadresidualstressesinit.Itiscalledaseasoncrackingbecausetheenvironmentaffectsit

3.9Whatisthedifferencebetweenasuperconductorandasemiconductor?

Answer(frompage#__96___):

Superconductivityisthephenomenonofalmostzeroelectricalresistivitythatoccursinsomemetalsandalloysbelowacriticaltemperatures.Thetemperatureinvolvedarebelowzero(0kto-273kor-460F);thehighesttemperatureatwhichsuperconductivityhasbeenexhibitedis150k.theelectricalpropertiesofSemiconductor

=======================EndofHomeworkAssignment

QUESTIONS

4.1Describethedifferencebetweenasoluteandasolvent.

Answer(frompage#104):

Soluteistheminorelement(suchassaltorsugar)thatisaddedtothesolvent,whichisthemajorelement(suchaswater).Intermsoftheelementinvolvedinametalcrystalstructure,thesolute(composedofsoluteatoms)istheelementthatisaddedtothesolvent(composedofhostatoms).

4.2Whatisasolidsolution?

Answer(frompage#104):

Whentheparticularcrystalstructureofthesolventismaintainedduringalloying,thealloyiscalledasolidsolution.

4.3Whataretheconditionsforobtaining(a)substitutionand(b)interstitialsolidsolutions?

Answer(frompage#104):

Twoconditions(knownasHume-Rotheryrules,afterW.Hume-Rothery1899-1968)aregenerallyrequiredtoformcompletesubstitutionsolidsolution.

Thetwometalsmusthavesimilarcrystalstructure.

Thedifferenceintheiratomicradiishouldbelessthan15%.

Twoconditionsnecessaryforforminginterstitialsolutions

Thesolventatommusthavemorethanonevalence

Theatomicradiusofthesoluteatommustbelessthan59%oftheatomicradiusfortheatom.

4.4Whatisthedifferencebetweenasinglephaseandatwophasesystem?

Answer(frompage#_105):

Aphaseisdefinedasaphysicallydistinctandhomogeneousportioninamaterial;eachphaseisahomogeneouspartofthetotalmassandhasitsowncharacteristicsandproperties.

Solidphaseinwhichtheelementsareuniformlydistributedthroughoutthesolidmass.Suchasystemislimitedbysomemaximumconcentrationofsoluteatomsinthesolvent-atomslattice,justasthereisasolubilitylimitforsugarinwater.Mostalloysconsistoftwoormoresolidphasesandmayberegardedasmechanicalmixture.Wecallasystemwithtwosolidphasesatwo-phasesystem.

4.5Explainwhatismeantby"secondphaseparticle."

Answer(frompage#106):

Alloyingwithfinelydispersedparticles(second–phaseparticles)isanimportantmethodofstrengtheningalloysandcontrollingtheirproperties.Intwo-phasealloysthesecond-phaseparticlespresentobstaclestodislocationmovementandthusincreasestrength.

4.6Describethefeaturesofaphasediagram.

Answer(frompage#106):

Theimportantfeaturesorthetypesofphasediagraminclude:completesolidsolution,eutectics,suchascastirons:andeutectoids,suchassteels

4.7Whatdotheterms"equilibrium"and"constitutional",asappliedtophasediagrams,indicate?

Answer(frompage#106):

Aphasediagram,alsocalledequilibriumaconstitutionaldiagram,showstherelationshipamongthetemperature,thecompositionandtheplacespresentinaparticularalloysystemunderequilibriumconditions.Equilibriummeansthatthestateofasystemremainsconstantoveranindefiniteperiodoftime.

Thewordconstitutionalindicatestherelationshipamongthestructure,thecomposition,andthephysicalmakeupofthealloy.

4.8Whatisthedifferencebetween"eutectic"and"eutectoid?"

Answer(frompage#108,112):

ThewordeutecticisfromtheGreekeutektos,meaningeasilymelted.Eutecticpointsareimportantinapplicationssuchassoldering,wherelowtemperaturesmayberequiredtopreventthermaldamagetopartsduringjoining.Eutectoidmeansthat,atacertaintemperatures,asinglesolidphase(austenite)istransformedintotwoothersolidphases(ferriteandcementite).Thestructureofeutectoidsteeliscalledpearlite,because,atlowmagnifications,itresemblesmother-of-pearl.

4.9Whatistempering?Whyisitdone?

Answer(frompage#119):

Temperingisaheatingprocessbywhichhardnessisreducedandtoughnessisimproved.Thebody-centeredtetragonalmartensiteisheatedtoanintermediatetemperature,typically150-650degreecentigrade,whereitdecomposestoatwo-phasemicrostructureconsistingofbodycenteredcubicalphaferriteandsmallparticlesofcementite.

4.10Explainwhatismeantbyseverityofquenching.

Answer(frompage#121):

Severityofquenchmeanstherateofcoolingofthealloy.

4.11Whatareprecipitates?Whyaretheysignificantinprecipitationhardening?

Answer(frompage#122):

Heattreatmentisatechniqueinwhichsmallparticles(ofadifferentphase,andcalledprecipitates)areuniformlydispersedinthematrixoftheoriginalphase.Inthisprocess,precipitateformsbecausethesolidsolubilityofoneelement(onecomponentofthealloy)intheotherisexceeded.

Heat-treatablealuminumalloys,copperalloys,Martensiticstainlesssteels,andsomeotherstainlesssteelsarehardenedandstrengthenedbyaprocesscalledprecipitationhardening.

=======================EndofHomeworkAssignment

QUESTIONS

5.1Whatarethemajorcategoriesofferrousalloys?

Answer(frompage#__137___):

Carbonandalloysteels,stainlesssteels,toolanddiesteels,castirons,andcaststeels.

5.2Listthebasicrawmaterialsusedinmakingironandsteel,andexplaintheirfunctions.

Answer(frompage#__138___):

ThebasicrawmaterialsareIronore,limestone,andcoke.

Theprincipal