Study On The Microstructrual Evolution During The Hot-Working Process Of X12CrMoWVNbN Steel

X12CrMoWVNbN steel is used in ultra-supercritical steam turbine rotors.The localization of their production requires a clear understanding on the formation during material preparation.To meet up these needs,the present thesis prepares as-cast samples at different cooling rate using electric induction furnace. Investigatesystematically the as-cast microstructure,after forging microstructure,heat-treated microstruc-ture,precipitation and evolution of carbide using OM,SEM,TEM,XRD and thermal simulator,etc.The main conclusions are as follows:In the cooling rate interval of 0.05℃/s 5℃/s,the matrix of X12 steel at as-cast condition is ferrite+martensite.The typical microstructure after forging is lath-sharped martensite+retained austenite. With the decrease of the cooling rate,the lath of martensite after forging tends to more and more refining.The matrix after quenching is also lath-sharped martensite,Comparing with higher cooling rate(>0.5℃/s) condition,lower cooling rate(<0.1℃/s) condition is accessible to fine lath-sharpedmartensite. Tempered martensite+ferrite is the typical microstructure after tempering.The cooling rate is higher ,the matensite is thicker,the content of martensite is more and the content of ferrite is littleThe major type of carbide in grain boundary and within the grain at as-cast condition is M3C. With the decrease of the cooling rate,the carbide in grain boundary is coursing and with the grain is refining.When the cooling rate reduces to 0.05℃/s, nearly-sphere carbide within the grain appears.The type of carbide after forging and after quenching is also M3C. M₂₃C₆ is not observed at the processes of casting ,forging and quenching.Ater high tempering,polygon carbide M₂₃C₆ distributes in chains in grain boundary and M₂₃C₆,MX and Laves phase precipitate within the grain.Discuss the possible formation mechanism of carbide:the equilibrium mechanim of M3C which is prior to nucleate.In the process of high tempering,the formation of M₂₃C₆ involves insitu nucleation mechanism and independent nucleation mechan-ism...