Influences of temperature and strain rate on low-cycle fatigue behavior of austenitic stainless steel SUS 321

A strain controlled low-cycle fatigue test was conducted on an austenitic stainless steel SUS 321 at 500 to 7000C under a fully reversed triangular wave form strain with a strain rate of 10-4 to 10-2 sec-1, to obtain fundamental information on the effects of temperature and strain rate on the low-cycle fatigue behavior of austenitic stainless steels at high temperatures. The test results were examined by observation of fracture surfaces and discussed from view-points of deformation mechanism and fracture morphologies by considering cyclic strain induced precipitation at high temperatures. The main results of the study are as follows: (1) The fatigue life is reduced with decreasing strain rate at a constant temperature and with increasing temperature at a constant strain rate. (2) At 500 and 5500C, the fatigue life N sub(f) can be well correlated with the strain aging parameter, (T/epsilon exp(-Q/RT), while not at 6500 and 7000C. (3) The fatigue strength and cyclic strain hardening properties of the material are affected by dynamic strain aging, by the precipitation of Ti(C, N), M23C6 and a σ-phase and by creep deformation. At 5500C, dynamic strain aging plays an important role on the reduction of fatigue life caused by the decrease in strain rate, whereas at 6500C the fatigue life is reduced mainly by creep deformation. (4) The failure mode of the specimens tends to change from transgranular type to an intergranular as the temperature becomes high, the strain range large, or the strain rate slow. (author)