The role of Argon in the AISI 420 Stainless-Steel Low-Temperature Plasma Nitriding

AISI 420 steel samples were subjected to three heat treatment conditions: annealed, hardened, and tempered at 400°C, followed by low-temperature nitriding using N2, H2, and varying Ar proportions in a pulsed DC glow discharge. The study aimed to investigate the impact of varying Ar content (10–50 vol.%) on glow discharge characteristics and surface properties of nitrided samples, using an 80% N2 + 20% H2 base gas mixture. The samples underwent characterization including optical microscopy, X-ray diffractometry, microhardness, and roughness measurements. Plasma characterization was conducted using optical emission spectroscopy. The results indicated that higher Ar concentrations led to increased thickness of the nitrided layer (up to 67%), as well as hardness (up to 14%) and surface roughness (up to 50%). These improvements stemmed from increased Ar-based species bombardment on the surface, enhancing the cleaning effect of surface oxides. This facilitated nitrogen adsorption onto the steel surface, increasing the atomic nitrogen concentration in the outermost layer of the steel. The increased nitrogen concentration facilitated diffusion, resulting in significant physical-chemical reactions at the surface-plasma interface. These reactions, including sputtering, molecule dissociation, and recombination, led to enhanced high-diffusivity pathways within the martensitic microstructures of both the as-hardened and 400°C-tempered samples.

Keywords:
AISI 420 stainless steel; low-temperature pulsed DC plasma nitriding; argon in nitriding gas mixtures; optical emission spectroscopy; glow discharge (plasma) diagnostics