Short crack growth and fatigue behavior of an aged second generation duplex stainless steel

M BALBI; M AVALOS; A EL BARTALI; I ALVAREZ

Hyannis, MA

Conferencia; International Conference on Fatigue Damage of Structural Materials VII; 2008

Elsevier

New generation of duplex austenitic-ferritic stainless steels (DSSs) are advanced materials with increasing number of applications in a range of industries. The growth of long cracks in this material has been thoroughly studied [1]. The fatigue life curves were determined and studies on the growth of short fatigue cracks in the as-received duplex steels have been reported [2]. In the present work, the kinetics of short crack growth during cycling has been studied in a second generation duplex stainless steel in the as-received and aged conditions. The heat treatment of aging was carried out in the following thermal conditions 100hr. Cylindrical specimens with shallow notch were subjected to constant plastic strain amplitude loading in both thermal conditions. The notch area has been mechanically and electrolytically polished to facilitate the observation of crack initiation and growth. The crack growth was studied by plastic replicas in an optical microscope. The characteristic features of the surface damage and crack growth at both thermal conditions have shown striking differences in micro-crack density, nucleation place and propagation rate even though the fatigue life was not very different. Previous works have shown that a first generation DSS presents a remarkable difference in lifetime for both thermal conditions [3] while the third generation does not at all [4]. In the as-received specimens, microcracks density is very low and nucleates mainly at grain boundary or second phase particles Fig.1. In the aged condition, the panorama is completely different. Slips lines firstly appear heterogeneously distributed in the ferritic phase and are the preferred site for microcracks nucleation. The propagation of them takes place along slip bands of adjacent grains at least for crack length of approximately five grains (50 microns) Fig.2 . The comparison of the fatigue life and the relation parameters of the short crack growth law were established.