Correlation between the Fracture Toughness of Austenite Stainless Steel and Stability of the Austenite Phase in Cryogenic State

Austenite stainless steel is used for liquid natural tanks and superconducting facilities since it has a face-centered cubic lattice, which is less likely to decrease its toughness at cryogenic temperatures. The structural materials of the ITER toroidal field coil structure (TFCS) are required to have high fracture toughness at cryogenic temperature (4 K) in order to prevent unstable fracturing by the huge electromagnetic force. Yield strength at 4 K can be accurately predicted pragmatically. However, the estimation method for fracture toughness at 4 K is not yet well developed. In this study, the authors investigated the correlation between several material properties and 4 K fracture toughness of actual sized ITER TFCS materials. As a result, there is a low correlation between 4 K fracture toughness and the parameters (i.e., 4 K yield strength, nitrogen content and grain size), which were thought to be well matched for fracture toughness as reported in previous studies. In contrast, 4 K tensile strength and Md30 are in good correlation with fracture toughness because local transformation into martensite occurring at the crack tip affects fracture toughness. Md30 is used as an index of stability in the austenite phase. The authors therefore established a new method that simplifies controlling 4 K fracture toughness of austenite stainless steel using Md30. In addition, it is demonstrated that this method is effective for actual TFCS materials. The views and opinions expressed herein do not necessarily reflect those of the ITER organization.