On the Effects of H2 and Ar on Dual Layer Formed by Plasma Nitrocarburizing on Austenitic Stainless Steels

Plasma nitrocarburizing has been widely used to improve the hardness of stainless steels without affecting their good corrosion resistance property. The process is usually carried out by using gas discharge of N and C containing gases, with admixing H-2 or Ar gas or H-2-Ar in combination. The aim of the present work was to study and compare the effects of H-2, Ar and H-2-Ar gas in combination, admixed with N-2-C2H2 gas mixture on the surface properties of austenitic stainless steels (viz. AISI 304 and 304L). The process was carried out at 400 degrees C and 500 Pa for 5 h. The treated samples were analyzed for their structural, mechanical, and electrochemical properties using x-ray diffractometer, scanning electron microscope, Vickers microhardness tester and potentiodynamic polarization testing instruments. Nitride- and carbide-free S-phase dual-layers were observed after plasma nitrocarburizing and their thicknesses varied with the different gas mixtures. Also, optical emission spectra of the gas discharges were recorded during the process and it was seen that the intensities of CH radical (431.44 nm) and CN radical (388.43 nm) lines were greater for Ar containing discharges. Hardness and pitting corrosion resistance were observed to be better with hydrogen compared to the other gas mixtures. Moreover, AISI 304 exhibited greater improvements in the surface hardness and corrosion resistance properties compared to AISI 304L.