Tensile and Charpy impact properties of reduced activation ferritic/martensitic steel with small amounts of Ta and Ti

Abstract

Reduced activation ferritic/martensitic (RAFM) steels such as Eurofer97 and F82H, which have been widely studied, contain a small quantity of Ta to improve their mechanical properties by fine MX precipitates. In this study, the effect of addition of Ti as well as Ta on the strength and impact toughness of 9Cr-1W RAFM steel was investigated. Ta and Ti added-RAFM steel (TaTi-RAFM) shows a superior balance of strength and impact toughness compared to Ta added-RAFM (Ta-RAFM), which is similar to the composition of Eurofer97. In particular, the ductile-brittle transition temperature (DBTT) of TaTi-RAFM decreased markedly. M23C6 carbides of smaller size, which act as crack initiation sites, were formed in TaTi-RAFM, thereby improving the DBTT of TaTi-RAFM. Furthermore, the trend of the DBTTs in Ta- and TaTi-RAFMs was well explained using yield strengths according to the temperature and the brittle fracture strengths calculated by the modified Griffith model with the maximum size of M23C6 carbides as microstructural factors for brittle crack initiation.