Long-term creep-fatigue life property of Mod.9Cr-1Mo steel for steam generator material in FBR

Extensive research is being executed to establish creep-fatigue life evaluation methodology for Mod.9Cr-1Mo steel which is the principal candidate material of steam generators in Demonstration Fast Breeder Reactor. Previous studies showed that the features of the creep-fatigue life property of the steel are small life reduction by tensile strain hold and significant reduction in compressive hold tests due to oxidation effect. Most of the experiments, however, were conducted under relatively short-term hold conditions and the failure life properties under long-term hold conditions have not yet been clarified. In order to investigate the oxidation and long-term strain hold effects on the creep-fatigue life property, the crack propagation tests in vacuum and air conditions, and the long-term creep-fatigue tests, up to 10 hours hold time per cycle, were performed. As a result, it was found that the significant life reduction occurred by introducing the tensile strain hold longer than 1 hour because of increase of the grain boundary damage due to cavity nucleation, and that the life reduction in compressive hold was caused by acceleration of the crack propagation rate due to accumulation of the tensile strain in center of the specimen. (author)