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The Influence of Microstructural Aspects on the Service Behaviour of Advanced Power Plant Steels

'The Influence of Microstructural Aspects on the Service Behaviour of Advanced Power Plant Steels'
ISIJInternational,Vol.39(1999), No.9, pp.874-888 Revie w TheInfluence of Advanced of M icrostructural PowerPlantSteels Aspects on theServiceBehaviour HorstCERJAK.PeterHOFERandBernhardSCHAFFERNAK [nstituteforMaterialsScience,WeldingandForming,TechnicalUniversity Graz,Kopernikusgasse24, A- 801OGraz,Austria, E-mail:cerjakCa)weld,tu-graz,ac.at (ReceivedonFebruary9.1999.?acceptedInfinalform onJune 18.1999) Thereis aworld-wide substantial demandtoincreasetheapplicationtemperatureanddesignstressesof advancedcreepresistant ferritic-martensitic9-12010Crsteels toincreasetheefficiencyofthermal power plants. Theyshow,comparedtoaustenitegrades,favourablephysicalproperties-like goodthermal conductivityandlowcoefficientofthermalexpansioncoupledwithbetterservicebehaviour,Intheframe workoftheEuropeanresearchaction COST501 intensiveinvestigationsofthese types ofsteels were performed.Thesealloys showduring serviceconditions pronouncedmicrostructuralchangesintheaged aswellas inthestressedconditions. Therefore anextrapoiation ofshort termcreepteststolongtimes-i,e. 1OOkh-using conventionalmethodsisnotsuccessfuibecauseofchangesintheacting creepmechanism governedbymicrostructuraidevelopment.Therefore an intensiveinvestigation andmodelling ofthe microstructureofW-andMo-alloyedadvanced9-120/0Crsteels,mainlyG-XI2CrMOVWNbN I0-1 -1 ,was applied tounderstandthemechanisms,Hardnesstests,opticalmicroscopy,SEM,TEMmethodsand thermodynamicequilibriumphasediagramcalculations wereperformed. Verypromising isthe use ofthe EFTEM(Energy filtering TEM)methodto quantifytheprecipitation sequenceof thedifferent types of precipitate: M23C6,MX,LavesPhaseandZ-Phaseasafunctionoftime,temperatureandstraining. Anewapproach(“GrazModel“)wasproposedfor theimprovementofthe creep resistantalioysin terms ofthec(/y-transformation andthe Curietemperaturerespectively,whichinfluencethemicrostructuralstabil ity, Bothwerefoundasimportantinfluencingfactors. Experimentally it wasfound, thatthealloysshowingthehighest creep resistance i.e. theJapanesegrade NF616/P92andthealloyB2, one ofthe COST501 activity, showthehighestdegreeofmicrostructural stabilityafter creepexposure. Furtherinvestigationsandmodelling is necessarytodevelopanoptimised alioycompositionofadvanced 9_120/0Crsteels. KEYWORDS: ferritic-martensitic chromiumsteels; creep resistance;microstructuralinvestigations; TEM; Energyfiltering TEM(EFTEM);calculated equilibriumphasediagrams;precipitation sequence; micro- structuraistability, l.Introduction There is a substantlalandgrowinginterestin power plantswithrelativelyhigh processtemperature and/or pressuresto improvethethermal efficiency andreduce C02emissions. Materialswith ferritic/martensiticmi- crostructures are preferredbecauseoftheirfavourable physicalpropertiessuch asgoodthermal conductivity andlowcoefficientofthermalexpansioncoupledwith higherresistancetothermalshock.Theseare someof theadvantagesoveraustenitic stainlesssteels. Forthese reasons therehasbeenagreatdemandforhighstrength andheatresista
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TheInfluenceofMicrostructuralAspectsontheServiceBehaviourofAdvancedPowerPlantSteels
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