Phase Morphology and Fatigue Cracks Nucleation in UNS S32750 DSS Plate

R. E. BOLMARO; W SIGNORELLI, J; I ALVAREZ,; MARINELLI MC,

Grado

Congreso; Duplex 2007; 2007

The studied duplex stainless steel (DSS) was the standard type UNS S32750 DSS in the form of a hot rolled 3 mm thickness plate. The material has a comparable volume fraction (¦Á and ¦Ã ¡Ö 50%) of the constituent phases, which do not show a completely homogeneous morphology. Indeed, the lamellar structure of the austenite in the ferrite matrix, typical of rolling processes, is missing in certain areas of the plate. It is widely accepted that, for the typical rolling morphology of DSSs, and cyclic deformation at plastic strain ranges lower than 0.4%, microcracks initiate in the austenitic phase and propagate to the ferrite. However, for the present material, the inverse takes place in regions where the lamellar structure is not well developed. The alloy was tested in specimens cut parallel and perpendicular to the rolling direction (RD) to compare the lifetime and the evolution of the surface damage during the fatigue life.  Despite these results have shown that the fatigue life of the specimen cut parallel to the RD is 30% higher than in the other direction, the distribution of the surface damage is identical in both specimens, that is, it is localized mainly in the austenitic phase where the lamellar structure is well-developed and in the ferritic phase where this structure is partially absent. To evaluate this fact, the relative strength of phases was studied by texture measurements and polycrystalline plasticity modeling and the results were compared with similar alloys in the literature [1]. The dominant texture in the ferrite is mainly characterized by the component {001}<110>, while in austenite the main component is {110}<111>. From the starting texture, by using self-consistent polycrystalline plasticity models, the Taylor factors were calculated, being the ratio between austenite and ferrite equal to 2.18/2.07 along the RD and 2.19/2.08 along TD. All values are close and almost insensitive to some model assumptions (strain rate sensitivity, relative critical resolved shear stresses, hardening law, etc.). Preferential strain localization, and further development of short cracks, is apparently not due to differences in average critical resolved shear stress or anisotropic behavior but rather to changing local contiguity (topology and percolation) either in one or the other phase.