Abstract
The high strain powder metallurgy (HS-PM) process, which is a novel and the most efficient non-equilibrium powder metallurgy process, is applied to an SUS 316L austenitic stainless steel. The HS-PM process is a powder metallurgy process combining mechanical milling, heat treatment and sintering processes, and enables one to produce an ultra-fine grain structure. In the case of the SUS 316L stainless steel, room temperature recrystallization and recovery of an austenite phase take place because of the increased high angle grain boundary area and the existence of excess vacancies, which are stored during the milling process. Very fine ferrite grains are formed in the early stage of the milling and an ultra fine (α+γ) microduplex structure is formed at the end. In the case of higher energy milling, almost fully ferritic nanograin structure with an average grain size of approximately 20nm is formed. The ultra fine (α+γ) microduplex structure in the HS-PM processed powder accelerates precipitation of σ phase in the sintering process. The sintered compacts with a very fine (γ+σ) microduplex structure show an extremely high strength, i.e., more than three times higher 0.2% proof stress than the annealed parent material, without any severe depression in the elongation.
