Dependence of Temperature and Strain Rate on the Low-Cycle Fatigue Life of Type 321 Stainless Steel

Abstract

There are three effects, mechanical, environmental, and creep effects, on the low-cycle fatigue life of Type 321 stainless steel at high temperatures. At intermediate temperature such as 450°C, the strain rate dependence of the fatigue life is due mainly to the mechanical effect caused by the cyclic strain hardening, because the fracture mode is transgranular and creep and environmental effects seem to be negligible. At higher temperatures above 600°C, the strain rate dependence of the fatigue life is due mainly to the change of the fracture mode from a transgranular type to an intergranular type with decreasing the strain rate. This intergranular fracture mode, composed of smooth, grain boundary facets, is similar to the creep fracture mode of wedge type cracking. The wedge type crack seems not to propagate without the oxidation effect, because it is said that in vacuum test the strain rate dependence of the fatigue life is hardly observed and the fracture mode is rather transgranular. Therefore, in this case the main damaging factor is the environmental effect. On the other hand, in hold time test in tension intergranular facets covered with small dimples are observed. This intergranular fracture mode is similar to the creep fracture mode of void type cracking. The void type crack seems to propagate without being influenced by oxidation, because it is said that the hold time effect is observed even in vacuum test. In void type intergranular cracking, creep effect, that is, creep-fatigue interaction exists.