2019 年 64 巻 3 号 p. 171-177
Austenitic stainless steels have excellent characteristics of non-magnetic and high corrosion resistant, but have relatively low hardness and strength, and therefore have limitations in application fields. Although the surface hardness improvement treatment is very effective, gas-nitriding or gas-carburizing/carbonitriding conducted under gas atmosphere at high temperature is generally difficult because of the presence of the surface passivation films composed of hydrated chromium ferrous/ferric oxides, and the deterioration of corrosion resistance or the inability to maintain non-magnetic. However, by the low-temperature carburizing in which the direct-current plasma and the active screen plasma methods are combined, surface hardening can be performed efficiently without affecting non-magnetism or corrosion resistance. The hardened layer is called S-phase (extended austenite phase) and is also excellent in corrosion resistance, wear- and fretting-resistance. Austenitic stainless steels that have been subjected to S-phase formation process have already been applied to chemical plants, the nuclear power industry, food and medical device, as well as biomaterials in Europe, and are expected to expand into the Japanese market. In the future, the development of mass-production equipments with excellent productivity will be required in accordance with the size and needs of the market.
