Effects of Test Temperature on Fatigue Damage Process of Austenitic Steel

Abstract

The temperature dependence of fatigue damage process was investigated using three austenitic steels, namely Fe-30Ni alloy which is metastable austenite, Fe-38Ni alloy which is stable austenite at room temperature and Fe-25Ni-16Cr alloy which has a low stacking faults energy of austenite. The variatiou of plastic strain amplitude was measured at the temperatures of 100°C and 200°C, and surface morphology and substructures were observed by optical and transmission electron microscopes. The main results obtained are as follows;
1) The hardening tendency in metastable austenitic region results from the work hardening of austenite and formation of martensite, and the decrease of hardness is considered to be due to the tempering effect of martensite.
2) The higher the stacking faults energy of austenite is, the more the hardening tendency is found, that is, this is attributed to the formation of the dislocation cells.
3) Initial cracks are apt to occur mainly at grain boundaries in the case of high stress levels and at slip bands in the case of low stress levels. The rate of occurrence of cracks at slip bands is increased with the formation of surface striation.