Effect of Heat Treatment Process on Mechanical Properties and Microstructure of Modified CNS-Ⅱ F/M Steel

Ferritic/martensitic（F/M） steels have been recommended as one of the candidate materials for supercritical water cooled reactor（SCWR） in-core components use for its high thermal conductivity,low thermal expansion coefficient and inherently good dimensional stability under irradiation condition in comparison to austenitic steel.CNS-Ⅱ F/M steel which has good mechanical properties was one of the 9-12Cr F/M steels designed for SCWR in the previous work.In this study a modified CNS-Ⅱ F/M steel was used and it＇s ultimate tensile strength was 925 MPa at room temperature and 483 MPa at 600 ℃ after optimizing heat treatment parameter.The ductile to brittle transition temperature of modified CNS-Ⅱ F/M steel is-55 ℃.Those are at the same level or even higher than that of CNS-Ⅱ and some commercial F/M steels nominated for SCWR in-core component use.The transmission electron microscope（TEM） results showed that the mechanical properties of the tempered martensite was closely related to the decomposition stage of the martensite.