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 °C after optimizing heat treatment parameter. The ductile to brittle transition temperature of modified CNS-II F/M steel is −55 °C. 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.