Influence of grain size on the low-cycle fatigue lives of austenitic stainless steels at high temperatures

The influence of grain size on the fatigue lives was investigated for eight kinds of austenitic stainless steels with the grain size numbers from 9 to 1. Fatigue tests were carried out at 600 and 700 °C under triangular wave shapes at strain rates of 6.7 × 10^-3/s and 6.7 × 10^-5/s, respectively, and under truncated wave shape with 30 m;n hold-time at tension side. When a strain rate was 6.7 × 10^-3/s at both 600 and 700 °C, the fracture modes were always transgranular, and the fatigue lives scarcely depended on the type of steels or the grain size. When a strain rate was 6.7 × 10^-5/s at 600 °C, the fracture modes changed from a dominantly transgranular mode to a completely intergranular one and the fatigue lives decreased with decreasing the grain size number. When a strain rate was 6.7 X 10^-5/sVs at 700 °C, grain size dependence of the fatigue lives was divided into two groups of the steels depending on the type of steel. The fracture modes of some types of the steel were completely intergranular, and others mixed. In hold-time tests, the grain size dependence of the fatigue lives was similar to that in the tests of triangular wave shape at a strain rate of 6.7 × 10^-5/s.