166-paper-microstructural evolution of aisi 304 stainless st

1、.MicrostructuralEvolutionofAISI304StainlessSteelwithNearrapidSolidificationYANGYuansheng1MAJianchao1TONGWenhui1HUZhuangqi1FANGYuan2YUYan2WANGXinhua3ZHANGJiongming3(1.InstituteofMetalResearchChineseAcademyofSciencesShenya

2、ng110016China2.BaosteelResearchInstituteShanghai201900China3.UniversityofScienceTechnologyBeijingBeijing100083China)Abstract:NearrapidsolidificationofAISI304stainlesssteelwithcoolingrateof17102Ksisstudiedtoexplethemicros

3、tructuralevolutiontheeffectofcoolingrateonthesolidifiedmicrostructure.Atlowercoolingratescolumnarferritedendritesareobtainedinthemicrostructurecellularaustenitecrystalsoccurathighercoolingrates.Withtheincreaseofcoolingra

4、tesecondarydendritearmspacingofcolumnarferritedendritesdecreases.Fmationofcellularγcrystalsisattributedtothehighercoolingratethatfavsthenucleationsubsequentgrowthofmetastableprimaryγphase.ThesolidificationprocessofAISI30

5、4stainlesssteelatlowercoolingratesisinvestigatedbyquenchingcolmetallographytechnology.Duringthesolidificationprocessprimaryferritedendritesprecipitatefromthemoltenmetalatfirstwhichisfollowedbyeutecticreactionoccurringamo

6、ngthedendritearmstherestofthesolidificationiscompletedwiththedirectprecipitationgrowthofaustenitefromthefinalliquidmetal.Duringthesubsequentcoolingprocessdiffusionalsolidstatetransfmationfromferritetoaustenitetakesplace.

7、Almostallthethinnereutecticferritelamellaeintheeutecticcoloniestransfmtoaustenitethethickerprimaryferritedendritestransfmincompletelyleavingskeletalferriteinthefinalmicrostructure.Keywds:NearrapidsolidificationStainlesss

8、teelMicrostructureCoolingrate0IntroductionThenearrapidsolidificationwithcoolingrateof1103Ksiswidelyinindustriessuchasstripcastingdiecastingetc.Themicrostructureofthemetalsalloyssolidifiedwithnearrapidcoolingratehasdiffer

9、entacteristicsfromthatwithconventionalsolidificationsuchascontinuouscastingprocess[14].Fexamplerefinementofmicrostructureisobtainedmicrosegregationisrelievedinthesolidifiedmicrostructureduringthenearrapidsolidificationwh

10、ichresultsintheimprovementofmechanicalpropertyofthealloys.Howevertherearealotofproblemssuchasthefmingmechanismsofthemicrostructureinthecastingstripswhichrestrictthedevelopmentofstripcasting[24].AISI304austeniticstainless

11、steelispaidmeattentioninstripcastingfitsextensiveuse.Accdingtothephasediagramthisstainlesssteelbelongstotheferriteprimarysolidificationi.e.ferriteastheprimaryphasebefethethreephasereaction[48].Inthethreephasereactionregi

12、onferriteausteniteeutecticisexpectedattheprimaryferriteboundarieswhentheliquiduscompositionreachingthemonovarianteutectictroughaccdingtothesolidificationpath[9].Howeverthereactioninthethreephasetroughismeunambiguoustoobs

13、ervebecausesolidstatetransfmationoccurringfromferritetoausteniteduringsubsequentcoolingcoursemakesthemicrostructuresolidifiedinthethreephaseregioncannotbediscriminatedeasily[48].Althoughthereareextensivediscussionsinthel

14、iteraturesonthesolidificationmechanismofausteniticstainlesssteelnoclearmodelhasyetbeenaccepted[78]..Figure2MicrostructuresofAISI304stainlesssteelsolidifiedatdifferentcoolingrates.(a)(b):9.6Ks25.8Ksrespectivelybydirection

15、alsolidification(c)(d):234.6Ks630.3KsrespectivelybymouldcastingFigure3RelationshipbetweensecondarydendritearmspacingcoolingrateAccdingtotheempiricalrelationshipbetweensecondarydendritearmspacingλ2averagecoolingrateRfAISI

16、304stainlesssteelλ2=68R0.45[3]thecalculatedvaluesofcoolingratesarederivedlistinFigure3.Goodagreementbetweenthemeasuredvaluescalculatedvaluesofcoolingratescanbefoundatlowercoolingratesbutthecalculatedvaluesaresmallerthant

17、hemeasuredonesathighercoolingrates.Inthepresentexperimentsthecoolingratesduringmouldcastingaremeasuredbythethermocouplesedatthesurfaceofmoltenmetalsothemeasuredvaluesrepresentthecoolingratesofthestripsurfacei.e.thecellul