Improvement of mechanical properties in SUS304L steel through the control of bimodal microstructure characteristics

The present work deals with achieving improvement in the mechanical properties of SUS304L stainless steel through the application of a unique microstructure design termed as 'Harmonic structure', and establishing a co-relationship between various microstructural characteristics and mechanical properties. Harmonic structure essentially means a bimodal grain size distribution with a specific periodic arrangement of coarse- and ultrafine-grain fractions. SUS304L stainless steel samples having such microstructure were fabricated by a powder metallurgy route involving the mechanical milling of pre-alloyed steel powder followed by spark plasma sintering. Due to these peculiar microstructural characteristics, the harmonic-structured SUS304L stainless steels demonstrated a winning combination of high strength, large uniform elongation, and large total elongation to failure, simultaneously. It was also found that the fraction of a shell area (a three-dimensional continuously connected network of ultrafine-grained structure) is an important parameter controlling the balance of the mechanical properties of the harmonic-structured SUS304L steel compacts.