METALLOGRAPHY LABORATORY EXPERIMENT MANUAL文档格式.docx
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METALLOGRAPHY LABORATORY EXPERIMENT MANUAL文档格式.docx
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2ndSementer
2008/2009
DepartmentofMetallurgyandMaterials
EngineeringFaculty
UniversityofIndonesia
INTRODUCTION
Metallographycanbedefinedasthevisualstudyoftheconstitutionandstructureofmaterials.Metallographicexaminationscanbebroadlyclassifiedintotwotypesnamely,macroscopicexaminationsandmicroscopicexaminations.MacroscopicexaminationsrefertotheobservationscarriedoutatamagnificationofX10ofless.Microscopicexaminations,ontheotherhand,refertotheexaminationofthestructureatamagnificationgreaterthanX10.Microscopicexaminations,dependingonthenatureofinformationtobeextracted,canbeaccomplishedusinganOpticalMicroscope(uptoX2000)orScanningElectronMicroscope(uptoX50000)oraTransmissionElectronMicroscope(uptoX500000).Formostoftheroutinepurposesinopticalmicroscopeisusedtoobtainfirsthandinformationonthegeometricarrangementofthegrainsandphasesinamaterial.Inordertoretaintheinformationvisualizedusingthemicroscope,microstructuraldetailsareoftenrecordedona35mmfilmoraPolaroidfilm.Thephotographthusobtained,revealingthemicrostructuraldetails,takenatamagnificationofgreaterthanX10isknownasaphotomicrograph.Maintainingarecordofthemicrostructuralstudiesintheformofphotomicrographsisacommonpracticeemployedbyresearchscholarsandleadinglaboratoriesallovertheworld.
Thestudyofmicrostructaraldetailsisimportantduetoitscorrelationwiththeensuingmechanicalpropertiesofthematerial.Asanexample,ifmaterialAexhibitsamorehomogeneousandrefinedmicrostructurethanmaterialB,itmayverywellbeanticipatedthatmaterialAwillexhibitbetterroomtemperaturepropertieswhencomparedtomaterialB.
Inordertometallographicallyexamineaspecimen,itisessentialtolearnaboutthevariousstepsthatarerequiredtoprepareit.Thefollowingsectionbrieflydescribethevariousstepsinvolvedinthemetallographicpreparationofthesamples.
METALLOGRAPHICPREPARATIONOFTHESPECIMENS
Thebasicoperationoutliningthemetallographicpreparationofthespecimensisasfollows:
SelectionoftheSizeoftheSpecimen:
Theselectionofthesizeofthespecimenisdependentonthenatureofmaterialandtheinformationtobegathered.Normally,thelineardimensionsmayvaryfrom5mmto30mmwhilethethicknessiskeptlowerthanthelineardimensions.
MountingtheSpecimen:
Mountingofthespecimenisnormallycarriedout,ifthespecimendoesnotpermitconvenienthandling.Plasticmountingisnormallycarriedoutbyplacingthespecimeninaplasticorrubbermoldfacedown,fillingthemoldwithmountinggradeofplasticandallowingittodryforafewhours.Theplasticmountingiscarriedoutsuchthatthesurfacetobeexaminedisexposedononesideoftheplasticmount.
RoughGrinding:
Roughgrindingiscarriedoutontheemerybeltsurfacerinordertoroundoffthecorners,ifnecessaryandtoremovedeepscratchesfromthesurface.
FineGrinding:
Finegrindinginvolvesrubbingofthespecimensurfaceagainstthesiliconcarbidepowdersbondedontospeciallypreparedpapers.Therearevariousgritsizesofsiliconcarbidepapersand.theonesnormallyusedare400grit,600gritand1000gritpapers.Thesepapersarenormallymountedonaflatsurface.Grindinginvolvesholdingthespecimensfacedownwardsontheabrasivepaperfollowedbyrubbinginforwardandbackwarddirectionsuntilthesurfaceiscoveredwithanevenpatternoffinescratches.Theprocessisrepeatedwithsuccessivelyfinergradepapers(increaseingritnumber).Witheachchangeofpaper,thespecimenshouldbeturnedthrough90tofacilitatetheobservationofthedisappearanceofthepreviousscratchmarks.Inaddition,ateverynewstagethespecimenandequipmentshouldbewashedofgritanddirtfromtheprecedinggrinding.
RoughPolishing:
Thisstageinvolvesthepolishingofthespecimensurfaceonarotatingwheelusingaluminaordiamondabrasivewithaparticlesizeofabout5microns.Polishingaidsincludediamondparticlesuspensionoraluminapowdersuspension.Inthepolishingstage,thespecimenismovedaroundthewheelinthedirectionoppositetothewheelitself.
Thisensuresauniformpolishingaction.
FinePolishing:
Thisstageinvolvestheremovalofveryfinescratchesandthethindistortedlayerremainingfromtheroughpolishingstages.Finepolishingisusuallycarriedonapolishingwheelusingfinealuminaparticleswithanaveragesizeoflessthan1micron(normally0.5micronsizeisused).Finepolishing,ifproperlycarriedout,yieldsascratchfreesurfacereadyforetching.
Etching:
Etchingiscarriedoutontheproperlydriedspecimenobtainedfromfinepolishingstep.Etchinginvolveschemicallytreatingthespecimensurfaceusingamildacidicoralkalinesolution.Theetchantdifferentiallyattacksvariousmicrostructuralfeaturesasaresultoftheirdifferentchemicalaffinities.Thisdifferentialattackleadstoanon-similarreflectionoflightintotheobjectivelensleadingtothegenerationofcontrastbetweenthevariousmicrostructuralfeatures.Afteretchingissuccessfullycarriedout,thespecimencanbetakentotheopticalmicroscopeformicrostructuralexamination.
I.THEORYOFWELDSTRUCTURE
Inthepresentlaboratorysession,wewillbeexaminingthemicrostructureoffusionweldedsteel.Inordertogainbetterunderstandingofthevariousfeatureswewillseeintheweldedarea,itisessentialtoknowaboutFe-Fe3Cmetastablephasediagramandthevariousmicrostructuralzonesthatarenormallyformedfollowingweldingoperation.Figure1showsatypicalFe-Fe3Cmetastablephasediagram.Thevariousphasesinthisphasediagramwhichmaybeofinteresttouscanbedefinedasfollows:
∙Ferrite(-iron):
asolidsolutionofcarboninironhavingamaximumcarboncontentofabout0.022%.
∙Austenite(-iron):
asolidsolutionofcarboninironformedathightemperaturescontainingamaximumcarboncontentofabout2.11%.
∙Cementite(Fe3C):
acompoundofcarbonandiron,havingachemicalformulaoFe3Candacarboncontent6.7%.
∙Pearlite(+Fe3C):
atwophasemixturethatexistswithanaveragecarboncontentof0.77%.Pearlitehasalamellarstructure.
Figure1.TheFe-Fe3Cphasediagram.(adaptedfromMetalsHandbook:
Metallography,StructuresandPhaseDiagrams,Vol.8,8thedition,
ASMHandbookCommittee,T.Lyman,Editor,ASM,1973,p.275).
AftergainingabasicinsightintotheFe-Fe3Cphasediagram,wewillinthefollowingsectiontrytogainknowledgeregardingfusionweldingandthemicrostructuralzonesthatareformedasaresultoffusionweldingprocess.
Afusionweldisnormallyproducedbytheelectricarcweldingprocess.Therearebasicallysixzonesthatcanbediscernedintheweldedareaandtheparentmetaladjacenttoit.Thesezonesarecreatedsincethevariousregionsoftheparentmetalandthewelditselfaresubjectedtodifferentdegreesofheattreatmentduringtheweldingprocess.Thesixbasiczonesare:
a)DepositedMetalZoneWeldMetalZone
b)FusionZone-
c)GrainGrowthZone
d)GrainRefinementZoneTheHeatAffectedZone
e)TransitionZone
f)UnaffectedZone
Figure2.Schematicdiagramshowingsixmicrostructuralzonesintheweldedsteel.
ThesesixzonesaredistributedintheweldedspecimenasshowninFigure2forhotrolledmetal.
TheDepositedMetalZoneistheportionwherethefillermetalwasdeposited.IntheFusionZone,theparentmetalwasheatedtothemeltingpointandsubsequentlycooled.ThesetwozonesaregenerallyindistinguishablefromeachotherandmakefortheWeldMetalZone.
TheWeldmetalzoneisgenerallycharacterizedbythepresenceoftwoimportantmicrostructuralfeaturesi.e.ColumnargrainsandWidmanstattenstructures.Assteelisagoodconductorofheat,theweldissubjectedtoveryrapidcoolingfromitsmoltenstateandtheresultisessentiallyachilledcastinghavingtheassociatedcolumnargrains.Ontheotherhand,aWidmanstattenstructureappearsasaresultoflargeaustenitegrainsbeingputthroughamoderatelyfastcoolingrate.
TheGrainGrowthZoneexhibitstheeffectofhightemperaturetreatmentattemperaturessomewhatlessthanitsmeltingpoint.Thehightemperatureexposureleadstoasignificantgrowthofaustenitegrains.Oncoolingtoroomtemperature,thiseffectisretainedasaregionofcoarseferritegrainsandWidmanstattenferriteandpearlite.Thedemarcationbetweengraingrowthzoneandweldmetalzoneisgenerallymoredistinctthanbetweentheotherzones.
IntheGrainRefinementZone,theparentmetalisheatedintothetemperaturerangecorrespondingtotheaustenitephaseintheFe-Fe3Cphasediagram.Becauseoftherelativelylowertemperatures,theaustenitegrainsbegantonucleateatmanypointstoformsmalleraustenitegrainswhichoncoolingwillresultinfineferriteandpearlitegrains.
IntheTransitionZone,theparentmetalduringweldingisheatedtotheregionwhereferriteandaustenitecoexist.Inotherwords,mostlyonlypearlitegrainshavetransformedtosmallaustenitegrains.Oncoolingveryfinepearlitegrainswithraggedlookingboundariesareformedamongthemostlyuntransformedoriginalferritegrains.
TheUnaffectedZonerepresentstheregionoftheparentmetalthatwasnotheatedbeyondtheeutectoidtemperature(727C)andthereisthereforenoobservablestructuralchange.
II.FERROUSANDNONFERROUSMICROSTRUCTURES
Thepropertyofmaterialstotallydependsontheirmicrost
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