Influence of cyclic softening on high temperature material properties in Mod.9Cr-1Mo steel

The applicability of Modified 9Cr-1Mo steel (ASME Grade 91 steel) as the main structural material in advanced loop-type sodium cooled fast reactor has been explored to enhance the safety, the credibility and the economic competitiveness of fast reactor plants. It is well-known that the steel exhibits cyclic softening behavior. Decrease of tensile and creep strength in softened materials has been already reported by other researchers. This paper discusses the relationship between cyclic softening conditions and high temperature material properties. Grade 91 steel has softened by repeat of plastic strain. The softening behavior can be evaluated by the softening rate. Decrease of tensile and creep strength in softened materials can be evaluated by the softening rate and it depends on the cyclic softening conditions. As a result, the cyclic loading conditions which produce little tensile and/or creep strength degradation have been identified. (author)