The Impact of Nitriding Parameters on Evolution of Properties of Stainless-Steel Surface Plasma-Nitrided in Glow Discharge

Abstract

Plasma nitriding in a glow discharge is a relatively new and extremely promising method of steel surface hardening. Ion-nitrided samples show a tendency to a longer lifespan during severe wear and cyclic loading compared to the traditional gas nitriding. However, a significant disadvantage of plasma thermochemical processes is their relatively long duration of about 20–30 hours. As a result, the surface treatment becomes inefficient and too expensive. As known, the properties of the nitrided layer, its phase composition and growth rate depend strongly on the parameters of the plasma nitriding (temperature, time, gas pressure, composition of the gas mixture, etc.). At the same time, there is still an open question regarding the choice of optimal modes of hardly-nitrided stainless-steels plasma nitriding, which will ensure of high treatment efficiency and obtaining of high-hardness strengthened layers with a controlled structural–phase composition. As established, to prevent the formation of Сr–N metastable phases’ precipitations on the surface of stainless steels, the appearance of which leads to deterioration of their corrosion performance, the treating temperatures should be limited: ≤ 450 °C. Low-temperature plasma nitriding provides the formation of a homogeneous nitrided layer of ‘white’ phase with a nitrogen concentration of 15 at.% and more. This phase ensures the nitrided-surface hardness of 1500 HV, which is by 4-to-5 times greater than the hardness of the untreated one. The possibility of the plasma-nitriding intensification by increasing the gas pressure in the reactor in the range from 130 to 400 Pa is shown. This provides an increase in the thickness of the nitrided layer and its hardness from ≈ 8 µm up to ≈ 18 µm and ≈ 700 HV up to ≈ 1500 HV, accordingly. Significant growth dynamics of the nitrided layer on the stainless-steels’ surfaces is achieved by determining the optimal 3-component gas mixture with a content of 55%Ar–30%N2–15%H2 that ensures the obtaining of a hardened layer with a thickness of ≈ 30 µm and a surface hardness of 1100 HV.

Keywords

• glow discharge
• plasma nitriding
• thermochemical treatment
• ion surface modification
• stainless steels