Effects of Screen Material and Open Area Ratio on Nitrogen Transfer during Screen-Assisted Plasma Nitriding of Ferritic Stainless Steel

Abstract

Screen-assisted direct-current plasma nitriding (S-DCPN) was applied to ferritic stainless steel SUS430 to clarify the effects of screen material and open area ratio on nitrogen transfer and nitrided-layer formation. Ni screens with open area ratios of 49 % and 60 % were used to evaluate screen geometry, while SUS304, SPCC, and Ni screens at 60 % were compared to isolate material effects. Plasma nitriding was performed at 633–723 K for 5 h under a 75 vol.% N ₂ and 25 vol.% H ₂ atmosphere. Expanded ferrite (α _N ) formed under all conditions. Compared with the 60 % Ni screen, the 49 % screen enhanced nitrogen transfer, resulting in thicker nitrided layers and earlier iron-nitride formation at elevated temperatures. In contrast, the 60 % screen formed persistent Ni-rich deposits that suppressed nitrogen transfer. Surface hardness increased with nitrogen uptake, whereas corrosion behavior depended on the distribution of iron nitrides and Ni-derived deposits. At the same open area ratio of 60 %, nitrogen transfer followed the order SUS304 > SPCC > Ni, demonstrating that screen chemistry strongly influences nitrogen transfer. These findings provide new insight into the nitrogen-transfer mechanism and practical guidelines for optimizing metallic screen design in industrial plasma nitriding.