Isothermal and Thermomechanical Fatigue Behavior of 316LN Stainless Steel Under Torsional Loading

316LN austenitic stainless steel is extensively utilized within the domain of nuclear power, where its susceptibility to high-temperature fatigue and thermomechanical fatigue has emerged as a pivotal area of research for this material. Nevertheless, prior investigations have predominantly concentrated on axial loading outcomes, with a notable absence of studies examining its fatigue failure behavior under torsional loading conditions. The present study undertakes isothermal fatigue testing at temperatures of 450 °C, 550 °C, and 650 °C, along with thermomechanical fatigue testing across a temperature range of 350-550 °C, with strain amplitudes of 0.6%, 0.8%, and 1.2%. The findings reveal that secondary hardening observed under conditions of small deformation is primarily attributed to the enhancement of frictional stress, stemming from the accumulation of planar slip. Furthermore, as the temperature escalates, variations are observed in the intensity of the dynamic strain aging and the dislocation density within the material.