EBSD analysis of the microstructure during static and metadynamic recrystallisation of austenitic stainless steel

The importance of controlling the microstructure of stainless steels during industrial process is twofold: on one hand, it defines the key properties of the semi-finished products, and on the other hand it affects the strength and formability of materials during the production cycle. In this context the study of post-deformation recrystallisation kinetics and its microstructure evolution are fundamental. Static and metadynamic recrystallisation of an austenitic stainless steel AISI 304L was investigated at 1100 °C with a strain rate of 10-2 s-1. The kinetics of recrystallisation was determined through double hit compression tests. Two strain levels were selected for the first compression hit: 0.15 for static recrystallisation and 0.25 for metadynamic recrystallisation. Both the as deformed and the recrystallized microstructures were investigated through electron back-scattered diffraction (EBSD) technique, to investigate grain size evolution and distribution of the dislocation structures. EBSD results revealed that strain induced grain boundary migration appeared to be significant during deformation, producing a square-like grain boundary structure aligned along the directions of the maximum shear stresses in compression. Furthermore, in both recrystallisation conditions, significant grain growth rates dominated on the nucleation rates for most of the recrystallisation transformations that are driven by stored energy reduction. EBSD analysis revealed that the dislocation density was distributed heterogeneously in the deformed grains, also for 0.25, besides a high density of small recrystallized nuclei produced by dynamic recrystallisation. Grain size growth driven by surface energy reduction was also investigated, finding that this kind of growth rate was too slow to explain the experimental grain evolution. Based on microstructure evidences, it was concluded that saturation of the nucleation sites occurred in the first stages of recrystallisation, whilst grain growth driven by strain induced grain boundary migration dominated the subsequent stages of the transformations, reducing the deformation stored energy and, at the same time, increasing significantly the grain size.