FATIGUE DAMAGE IN AUSTENITIC-FERRITIC DUPLEX STAINLESS-STEELS

Fatigue damage in two austenitic-ferritic duplex stainless steels, wit h the structure of a natural composite and different levels of nitroge n 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 w hile Manson-Coffin curves of both steels intersect at medium fatigue l ives. The study of the surface relief reveals intensive slip markings both in ferrite and in austenite. Their density is influenced by the n itrogen content. Both the intensity and density of the persistent slip band (PSB) markings are higher in the ferrite, Crack initiation was f ound to appear predominantly in PSBs in the ferritic grains at the low strain amplitudes, and in the ferritic and austenitic grains at the h ighest strain amplitudes. The level of the cyclic stress-strain respon se and the fatigue lives are discussed in terms of the cyclic strain l ocalisation and of the effect of texture and nitrogen content on the s trength and fatigue damage. The increased strength of the austenitic p hase, due to high nitrogen alloying, results in cyclic slip localisati on in the ferrite, and the decrease of fatigue life, compared with the steel with the lower nitrogen content.