Critical Stress on Strain Increase under Constant Load for SUS316LN Austenitic Stainless Steel at Cryogenic Temperature

Investigation has been conducted on the following subject: low temperature creep deformation behavior of SUS 316LN austenitic stainless steel under constant load which is expected to be the material of superconducting toroidal coil case in Tokamak fusion reactor. Round bar specimens were extracted from SUS 316LN, and several tensile tests were carried out: monotonic loading tests at a room temperature, 77K and 4.2K, and constant loading tests at a room temperature and 77K. Increase in strain under constant load occurs over proportional limit of the material. Critical stress not to allow the increase in strain is 75-100% of 0.01% proof stress at the test temperature. This is the same value as 0.01% proof stress of extremely low rate monotonic loading test when no creep deformation occurs; thus, this is the same value as critical stress where plastic deformation initiates. Based on the result at 77K, the critical stress at 4.2K not to allow the increase in strain under constant load is estimated to be about 90% of 0.01% proof stress at 4.2K, which is about 75% of 0.2% proof stress at 4.2K.