ROOM-TEMPERATURE RECRYSTALLIZATION AND UL TRA-FINE GRAIN-REFINEMENT OF AN SUS316L STAINLESS-STEEL BY HIGH-STRAIN POWDER-METALLURGY PROCESS

The high strain powder metallurgy (HS-PM) process. which is a;novel an d the most efficient non-equilibrium powder metallurgy process, is app lied to an SUS 316L austenitic stainless steel. The MS-PM process is a powder metallurgy process combining mechanical milling, heat treatmen t and sintering processes, and enables one to produce an ultra-fine gr ain structure. In the case of the SUS 316L stainless steel, room tempe rature recrystallization and recovery of an austenite phase take place because of the increased high angle grain boundary area and the exist ence 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 (alpha + gamma) microduplex structure is formed a t he end. In the case of higher energy milling, almost fully ferritic na nograin structure with an average grain size of approximately 20nm is formed. The ultra fine (alpha + gamma) microduplex structure in the HS -PM processed powder accelerates precipitation of a phase in the sinte ring process. The sintered compacts with a very fine (gamma+sigma) mic roduplex structure show an extremely high strength, i..e., more than t hree times higher 0.2% proof stress than the annealed parent material, without any severe depression in the elongation.