Improved tensile properties, stable microstructures and isotropic deformation of nanocrystalline 304 stainless steel

Abstract

The tensile properties of nanocrystalline 304 stainless steel plate (NSSP-304) produced by severe rolling technology and its conventional polycrystalline 304 stainless steel (CPSS-304) counterpart were studied at room temperature. The microstructures of fractured NSSP-304 and CPSS-304 were characterized. NSSP-304 had enough ductility (elongation 34.5%), larger strengths, elastic strain and the reduction of area despite its smaller volume fraction of strain-induced α′-martensite (SIM) and no obvious work hardening. The deformation of NSSP-304 was more isotropic, its microstructures were more stable and homogeneous during tensile process due to the following results about fractured NSSP-304: (1) the narrower variation amplitude of surface micro-hardness along and perpendicular tensile directions; (2) the smaller void density and few surface micro-cracks; (3) the smaller surface roughness and roughness variation, and their narrower fluctuation ranges at three directions; (4) only micro-cracks without macro-cracks around NSSP-304 fracture; (5) its smaller volume fraction of SIM. The voids in CPSS-304 initiated around MnS inclusion, at grain boundaries, triple junctions, the boundaries of austenite and martensite phases, and in the two phases. They became micro-cracks within their original positions and macro-cracks by passing through different grains. The voids in NSSP-304 initiated at grain boundaries, triple junctions and the boundaries of the two phases. They became micro- and macro-cracks by passing through different grain boundaries. The MnS inclusion in NSSP-304 scarcely influenced its tensile deformation. The fracture mechanisms of NSSP-304 and CPSS-304 were the intergranular fracture, and the mixture of intergranular and transgranular fracture respectively.