Nano-Grain Formation in a Fully Ferritic SUS 316L Austenitic Stainless Steel Produced by HS-PM Process

Abstract

In this paper, the high strain powder metallurgy (HS-PM) process was applied to an SUS 316L stainless steel powder, and the microstructural changes during the process are discussed. The HS-PM process is applied to an SUS 316L austenitic stainless steel. 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. Although the amount of the α' martensite is not so large even after 98% cold rolling of the parent material, the extremely high density of defects introduced by the milling process increased free energy of the austenite phase, and thereby stabilized the α phase. In the late stage of milling, or in the case of higher energy milling, fully ferritic nano grain structure with an average grain size of approximately 20nm is formed. During heating, the fully ferritic powder showed complex microstructural changes. The powder compact sintered at 1173K for 3.6ks showed (α'+ M 2 C) microduplex structure with an average grain size of 250nm.