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Solidification Process in Heat Treatment

Explore the principles and mechanisms of solidification in metals, differentiate between nucleation types, and grasp the formation of grain structures. Discover the significance and applications of controlled nucleation.

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Solidification Process in Heat Treatment

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  1. LECTURE1 SOLIDIFICATIONPROCESS

  2. DBT302–HeatTreatment Objectives •Studytheprinciplesofsolidificationastheyapply topuremetals. •Todiscusstheprocessofsolidificationand differentiatebetweenhomogenousand heterogeneousnucleation •Examinethemechanismsbywhichsolidification occurs. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  3. DBT302–HeatTreatment SolidificationProcess UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  4. DBT302–HeatTreatment SolidificationProcess •Allmetallicmaterials–ceramics,inorganic glasses,andtermoplasticpolymerareliquidor moltenatsomepointduringprocessing. •Forexample:Waterfreezestoice,molten materialssolidifyastheycoolbelowfreezing temperature. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  5. DBT302–HeatTreatment SolidificationProcess •SolidificationofCrystallineMaterialsrequired2 steps. –1stStep–Ultra fine Crystalline,knownasthenucleiof thesolidphase,formfromtheliquid. Theformationofstablenucleiinthemelt(nucleation) –2ndStep–Ultra fine Solid Crystallinebegintogrowas atomfromtheliquidareattachedtothenucleiuntilno liquidremains. Thegrowthofnucleiintocrystalsandtheformationof grainstructure. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  6. DBT302–HeatTreatment Variousstagesinthesolidificationofapolycrystallinematerial;thesquaregridsdepictunitcells. (a)Smallcrystallitenuclei. (b)Growthofthecrystallites;theobstructionofsomegrainsthatareadjacenttooneanotherisalso shown. (c)Uponcompletionofsolidification,grainshavingirregularshapeshaveformed. (d)Thegrainstructureasitwouldappearunderthemicroscope;darklinesarethegrainboundaries. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  7. DBT302–HeatTreatment Severalstagesinthesolidificationofmetals. (a)formationofnuclei, (b)growthofnucleiintocrystals, (c)joiningtogetherofcrystalstoformgrainsandassociated grainboundaries. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  8. DBT302–HeatTreatment GrowthofCrystal&Formationof GrainStructure Nucleusgrowintocrystalsindifferentorientations. Crystalboundariesareformedwhencrystalsjoin togetheratcompletesolidification. Solidifiedmetalcontainingmanycrystals- polycrystalline Crystalsinsolidifiedmetalsarecalledgrains. Grainsareseparatedbygrainboundaries. Morethenumberofnucleationsitesavailable,morethe numberofgrainsformed. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  9. DBT302–HeatTreatment GrowthofCrystal&Formationof GrainStructure •Numberofnucleationsitesavailableforfreezingmetal willaffectthegrainstructureofthesolidmetalproduced. •Fewnucleationsites-acoarseorlargegrainstructurewill produced. •Ifmanynucleationsites-afinegrainstructureresult. •2typesofgrainstructureusuallyproducedwhencasta puremetals: •Equiaxedgrains •Columnargrains UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  10. DBT302–HeatTreatment GrowthofCrystal&Formationof GrainStructure EquiaxedGrains: Crystals,smallerinsize,growequallyinalldirections. Formedatthesitesofhighconcentrationofthenuclie. ColumnarGrains Mold Example:-Coldmoldwall ColumnarGrains: Longthinandcoarse. Growpredominantlyinone direction. Formedatthesitesofslow coolingandsteeptemperature gradient. Example:-Grainsthatare EquiaxedGrains Knowledge.Sincerity.Exellence awayfromthemoldwall. UniversitiMalaysiaPerlis

  11. DBT302–HeatTreatment TechnologicalSignificance •Primaryprocessing-Processesinvolvingcasting ofmoltenmetalsintoingotsorsemi-finished usefulshapessuchasslabs. •Secondaryprocessing-Processessuchasrolling, extrusion,etc.usedtoprocessingotsorslabsand othersemi-finishedshapes. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  12. DBT302–HeatTreatment Nucleation •Nucleation-Thephysicalprocessbywhichanewphase isproducedinamaterial. •Criticalradius(r*)-Theminimumsizethatmustbe formedbyatomsclusteringtogetherintheliquidbefore thesolidparticleisstableandbeginstogrow. •Undercooling-Thetemperaturetowhichtheliquidmetal mustcoolbelowtheequilibriumfreezingtemperature beforenucleationoccurs. •Homogeneousnucleation-Formationofacriticallysized solidfromtheliquidbytheclusteringtogetherofalarge numberofatomsatahighundercooling(withoutan externalinterface). •Heterogeneousnucleation-Formationofacriticallysized solidfromtheliquidonanimpuritysurface. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  13. DBT302–HeatTreatment Aninterfaceiscreatedwhenasolidformsfromtheliquid. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  14. DBT302–HeatTreatment ApplicationofControlledNucleation Grainrefinement-Theadditionofheterogeneousnuclei inacontrolledmannertoincreasethenumberofgrains inacasting. Dispersionstrengthening-Increaseinstrengthofa metallicmaterialbygeneratingresistancetodislocation motionbytheintroductionofsmallclustersofasecond material. Solid-statephasetransformation-Achangeinphasethat occursinthesolidstate. Rapidsolidificationprocessing-Producingunique materialstructuresbypromotingunusuallyhighcooling ratesduringsolidification. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  15. DBT302–HeatTreatment GrowthMechanism Specificheat-Theheatrequiredtochangethe temperatureofaunitweightofthematerialonedegree. Solidificationfront-Interfacebetweenasolidand liquid. Planargrowth-Thegrowthofasmoothsolid-liquid interfaceduringsolidification,whennoundercoolingof theliquidispresent. Dendrite-Thetreelikestructureofthesolidthatgrows whenanundercooledliquidsolidifies. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  16. DBT302–HeatTreatment Whenthetemperatureoftheliquidisabovethefreezing temperatureaprotuberanceonthesolid-liquidinterfacewill notgrow,leadingtomaintenanceofaplanerinterface.Latent heatisremovedfromtheinterfacethroughthesolid UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  17. DBT302–HeatTreatment Figure(a):Iftheliquidisundercooled,aprotuberanceonthesolid-liquid interfacecangrowrapidlyasadendrite.Thelatentheatoffusionis removedbyraisingthetemperatureoftheliquidbacktothefreezing temperature. Figure(b):Scanningelectronmicrographofdendritesinsteel(x15). UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  18. DBT302–HeatTreatment SolidificationTime&DendriteSize Chvorinov’srule-Thesolidificationtimeofa castingisdirectlyproportionaltothesquareof thevolume-to-surfacearearatioofthecasting. Moldconstant(B)-Acharacteristicconstantin Chvorinov’srule. Secondarydendritearmspacing(SDAS)-The distancebetweenthecentersoftwoadjacent secondarydendritearms. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  19. DBT302–HeatTreatment Figure(a):Thesecondarydendritearmspacing(SDAS). Figure(b):Dendritesinanaluminumalloy(x50).(From ASMHandbook,Vol.9,Metallographyand Microstructure(1985),ASMInternational,Materials Park,OH44073-0002.) UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  20. DBT302–HeatTreatment SolidificationDefects Shrinkage-Contractionofacastingduring solidification. Microshrinkage-Small,frequentlyisolatedpores betweenthedendritearmsformedbytheshrinkagethat accompaniessolidification. Gasporosity-Bubblesofgastrappedwithinacasting duringsolidification,causedbythelowersolubilityof thegasinthesolidcomparedwiththatintheliquid. Sievert’slaw-Theamountofagasthatdissolvesina metalisproportionaltothepartialpressureofthegasin thesurroundings. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

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