Low-temperature nitrocarburizing by pulsed-DC discharge of N₂-H₂-C₂H₂ for surface engineering of austenitic stainless steel

Nitrocarburizing is widely applied for case hardening and improved corrosion resistance of steels. The operational temperature of the process is above 550 ^oC causing the formation of ε-Fe2-3N, γ'-Fe4N, carbides of Fe, and nitrides of alloying elements present in the steel which provide the surface with greater hardness and improved corrosion resistance. When the process is applied to stainless steels, the temperature causes Cr₂N, CrN, and Cr carbides to form, enhancing the surface hardness but compromising corrosion resistance. So low-temperature nitrocarburizing process utilizing plasma is developed at 400 ^oC which forms S-phase and no Cr compounds. Gas mixtures of N₂-H₂-C₂H₂ with varied % N₂ were utilized to create pulsed-DC discharges and the processing durations were varied. The plasmas and the time durations tailored the surface properties differently and a surface property characterization using Vicker's microhardness indentation, XRD, XPS, SEM, Glow Discharge Optical Emission Spectrometry (GDOES) to measure concentration-depth profiles along the S-phase layers, and potentiodynamic polarization testing for corrosion study are displayed in the presented work. Optical emission spectroscopy of the discharges showed the presence of N₂⁺, N₂^*, CH-, and CN- species.