A Study On The Precipitate Phases In A High Cr Ferritic/Martensitic Steel Before And After Creep

The operating conditions of the fourth-generation nuclear reactors put forward higher requirements for the properties of materials for reactors.9-12%Cr FM steels have better comprehensive properties and they are considered as candidate materials for the structure and cladding in the fourth-generation nuclear reactors.The fourth-generation nuclear reactors have higher inlet and outlet temperatures,which puts forward higher requirements for the high-temperature mechanical properties of materials for the reactors.Previous researches indicate that the types,size and thermal stability of the precipitates in the 9-12%Cr FM steels significantly affect their high-temperature creep properties.In this thesis,an 11%Cr FM steel was studied by optical microscope,scanning electron microscope and transmission electron microscope to understand the characteristics of microstructure and precipitate phases of the steel before and after creep.The influence of the precipitate phases on the creep properties of the steel was also analyzed in this thesis.The study results indicate that the microstructure of the 11%Cr FM steel consists of tempered martensite with a certain amount of bamboo-like?-ferrite before creep.The types of precipitate phases in the steel before creep mainly include Cr-rich M₂₃C₆,MX,and Fe-rich M₅C₂ phases,as well as MX phases consisting of Nb-rich/Ta-Nb-rich/V-rich MC carbides,and Nb-rich M?C,N?carbonitride.In addition,a small amount of sigma phases??-FeCr and?-FeCrW?and Nd-rich phase were aslo found in the steel.After a creep test at 600?at 150 MPa for 1100 h,the microstructure of the 11%Cr FM steel still consists of tempered martensite with a certain amount of bamboo-like?-ferrite.Compared with the normalized and tempered condition,the changes of precipitate phases in the steel after the creep are the following points:?1?Cr-rich M₂₃C₆precipitates were slightly coarsened,and their chemical composition was almost unchanged;?2?The Fe-rich M₅C₂ precipitates might be dissolved;?3?The Nb content slightly increased accompanied with a decrease of Ta and Cr contents in Nb-rich precipitates;?4?The size of spherical MX precipitates became smaller,and some of larger MX precipitates were dissolved;?5?Large-scale Fe-W-Cr-rich M₆C phase and?Fe,Cr?₂W Laves phase were formed during the creep;?6?The sigma phase and Nd-rich phase were not found in the steel after the creep.These two precipitate phases may not have been observed yet or may be dissolved during the creep.Preliminary reasons for the formation of sigma phase in the tempered steel and M₆C phase after in the crept steel are also analyzed in this thesis.The TEM results indicate that sigma phase was formed during the tempering,and its nucleation location is located at the boundaries between?-ferrite and martensite.The decrease in carbon content in the boundaries between?-ferrite and martensite resulted from the formation of M₂₃C₆ phase in this location,and the higher Cr/W contents in?-ferrite may be the reason for the formation of sigma phase at boundaries between?-ferrite and martensite.?-ferrite with higher W and Cr contents lead to a formation of the Fe-Cr-W-rich M₆C phase during the short-term creep.In the 11%Cr FM steel,the dissolution of MX phase and the formation of large size M₆C phase and Laves phase after creep lead to a reduction of precipitation strengthening.The formation of M₆C phase and Laves phase also consumed solutes Cr and W,thereby reducing the solution strength.The formation of the sigma phase and the M₆C phase is closely related to the?-ferrite,and the two phases have unfavourable effects on the creep properties of high Cr FM steels.Therefore,controlling the amount of?-ferrite in high Cr FM steels is of great significance for improving their creep properties.