Low Cyclic Fatigue Softening of Austenitic Stainless Steels

Abstract

Low cyclic fatigue behavior of some 18Cr-8Ni and 25Cr-20Ni austenitic stainless steels has been studied. Cyclic softening was observed after the initial cyclic hardening in all the steels investigated. The formation of α' martensite during the cyclic deformation depressed the softening and introduced the second hardening. The softening was revealed remarkably by nitrogen addition, and solute carbon also produced large softening but to a lesser degree than nitrogen. The cyclic softening occured especially in the surface layer of the specimens, and more slip bands were observed at the surface of the specimens showing the remarkable softening. A planar structure of dislocations was produced and a cell structure was scarcely observed in all the steels showing the large softening, while dislocations tended to form a cell or a band structure in the steels showing the small softening. The planar structure might enhance the cyclic softening through increasing the Bauschinger effect, the density of mobile dislocation, and/or the number of the slip band. The tendency of dislocations to form the planar structure could not be explained only with the stacking fault energy. The large cyclic softening observed in this study seemed to increase the fatigue life. In the case of stress controlled tests, however, the lives of the steel showing the significant softening were shorter, when they were compared at about the same strength ratio of the applied stress to 0.2% proof stress or ultimate tensile strength.