Role of the Near-surface Microstructure during Low-Cycle Fatigue in SAF 2507 Duplex Stainless Steel Plate

MG MOSCATO,; MC MARINELLI; A EL BARTALI; I ALVAREZ

Hyannis, MA, USA

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

Elsevier

When an undamaged metallic material is subjected to cyclic loading, surface microcracks nucleate in slip bands or at grain boundaries, and with continued cycling the microcracks can growth crossing barriers and form cracks longer than one-grain diameter. More than 80% of the low cycle fatigue lifetime consists of the development of multiple surface short cracks, i.e. cracks with size comparable to the grain size of the material [1]. The basic mechanisms of fatigue crack initiation have been extensively studied [2, 3] on fcc crystals and to a less extent on bcc metals [4, 5]. Little information has been found in the literature about those mechanisms in the austeno-ferritic duplex stainless steel. Recently, Alvarez-Armas et al. [6] and Marinelli et al. [7] have studied the near-surface microstructure associated with crack initiation in low and high nitrogen Duplex Stainless Steels (DSS), respectively. They have found that microcracks nucleate when the strain incompatibility between both phases is important generating high local stresses.  In the case of the high nitrogen DSS plates investigated in this work, the microstructural morphology is not completely anisotropic in the sense that there are regions with different scale morphology (heterogeneous width of the constitutive phases)  and average grain size of 10mm in the ferritic phase and 5ìm in the austenite (measured perpendicular to the rolling direction). In this material, the characteristic near-surface microstructure after strain controlled cyclic tests carried out at total strain ranges between 0.8 and 1.2% corresponds to refined dislocation micro-bands extended over several grains. Fig. 1 shows an incipient micro-band in an austenitic grain that is crossing to the adjacent grain. The present investigation shows the strong influence of micro-band formation on the microcrack nucleation as a result of a less resistant microstructure.