Influence of Thermal Aging on Cryogenic Fracture Toughness of JN 1 Steel for Superconducting Magnet Structures of Fusion Reactors

Elastic-plastic fracture toughness tests were conducted at 77K and 4K to examine an influence of thermal aging on fracture properties of an austenitic stainless steel JN 1 at cryogenic temperature, which is a candidate material for superconducting magnet structures of fusion reactors. The tests were applied to solution-treated material and materials aged at 923-1073 K for 5 h. The experimental results showed that the fracture toughness JIC measured on the thermally aged materials were smaller than that on the solution-treated material and decreased with increasing aging temperature. This decrease in fracture toughness due to thermal aging was caused by precipitation and coarsening of grain boundary M23C6 carbides. Additionally, small punch (SP) tests were conducted at cryogenic temperatures to investigate the relationship between the obtained JIC and SP properties. It was found that the equivalent fracture strain eqf in SP specimens was linearly correlated with JIC irrespective of testing temperature. This correlation suggests that the SP testing technique provides a useful tool for evaluating JIC at cryogenic temperature.