FATIGUE DAMAGE IN AUSTENITIC-FERRITIC DUPLEX STAINLESS-STEELS

Fatigue damage in two austenitic-ferritic duplex stainless steels, with the structure of a natural composite and different levels of nitrogen content, was studied in low-cycle fatigue. Both steels show initial cyclic hardening followed by softening and a long stabilisation period. The cyclic stress-strain curve increases with the nitrogen content while Manson-Coffin curves of both steels intersect at medium fatigue lives. The study of the surface relief reveals intensive slip markings both in ferrite and in austenite. Their density is influenced by the nitrogen content. Both the intensity and density of the persistent slip band (PSB) markings are higher in the ferrite, Crack initiation was found to appear predominantly in PSBs in the ferritic grains at the low strain amplitudes, and in the ferritic and austenitic grains at the highest strain amplitudes. The level of the cyclic stress-strain response and the fatigue lives are discussed in terms of the cyclic strain localisation and of the effect of texture and nitrogen content on the strength and fatigue damage. The increased strength of the austenitic phase, due to high nitrogen alloying, results in cyclic slip localisation in the ferrite, and the decrease of fatigue life, compared with the steel with the lower nitrogen content.