Host: The Japan Society of Mechanical Engineers
Name : [in Japanese]
Date : November 25, 2025 - November 29, 2025
Nuclear power plants promise to play an essential role in achieving carbon neutrality. Under such circumstances, the Generation IV reactor currently under development uses liquid sodium as the coolant, and the operating temperature is 550°C, surpassing that of conventional reactors. Stainless steel SUS316LN, known for its high corrosion resistance to liquid sodium, is a potential material for the pressure vessel and piping in this reactor. However, the accelerated formation and accumulation of voids and dislocations near grain boundaries in high-temperature creep-loading conditions lead to significant deterioration of fracture life due to intergranular cracking. In this study, high-temperature creep tests of stainless steel SUS316LN were conducted to investigate the grain boundary damage process. The loading stress was set at 120 MPa, and creep tests were conducted at 675°C, 700°C, and 725°C for a certain period. The degradation process around grain boundaries was then evaluated using KAM (Kernel Average Misorientation) values obtained from EBSD analysis. The KAM values near the grain boundary increased monotonically with increasing loading time and test temperature, confirming the damage accumulation process near grain boundaries. The activation energy of the increase in KAM values caused by creep loading was evaluated using an Arrhenius plot, and the increase in KAM values was quantitatively explained by a modified Arrhenius equation that considers the stress-induced change in the activation energy of Fe atom diffusion. Therefore, the formation and accumulation processes of voids near grain boundaries under creep loading can be evaluated based on the atomic diffusion accelerated by the local strain field near grain boundaries caused by the superposition of a loading stress and lattice mismatch between grains.
