Low Temperature Direct Current Plasma Nitriding of Ferritic Stainless Steel with Metal Screen

Surface modification involves chemically or physically imparting a high functionality to the surface of materials, and has become increasingly important in recent years. The nitriding method is a surface modification method that hardens the surface of metallic materials by causing nitrogen to permeate and diffuse into the surface to form various nitrides or by supersaturating a solid solution of nitrogen in the metal. This is effective in improving hardness, corrosion resistance, and wear resistance. Plasma nitriding, a type of nitriding process, has advantages such as low energy consumption, short processing time, and low environmental impact. In contrast, the conventional plasma nitriding method forms plasma on the surface of the treated material, which may cause phenomena that lead to defects in the treated material. Therefore, the direct current plasma nitriding with screen (S-DCPN) method reduces these problems because plasma is formed not only on the treated material but also on the surface of the screen. Stainless steel has excellent corrosion resistance; however, nitriding treatment above a certain temperature reduces corrosion resistance owing to chromium nitride precipitation. In this study, S-DCPN treatment, a type of plasma nitriding method, was applied to form a thick nitrided layer without reducing corrosion resistance. S-DCPN treatment was performed using ferritic stainless steel SUS430 as the sample and austenitic stainless steel SUS304 as the screen material at treatment temperatures of 633 and 653 K, treatment times of 5 and 15 h, gas pressure of 200 Pa, and gas composition of 75% N₂ ­– 25% H₂. Consequently, the α_N phase with supersaturated nitrogen solid solution was identified under all conditions. Nitrogen diffusion increased with increasing treatment temperature and time, and the hardness also increased. In corrosion tests, corrosion resistance was improved under all conditions.