Comparison of expanded ferrite formation in AISI 409, AISI 430, and AISI 444 steels during low-temperature plasma nitriding treatment

Thermochemical treatments, particularly those performed at low temperatures, have proven to be highly significant for improving the surface properties of stainless steels, especially in applications where tribological performance plays a substantial role. This is due to the fact that low-temperature thermochemical treatments enhance material properties, without forming chromium compounds on the surface, thereby preserving the corrosion resistance of stainless steels. To evaluate the evolution of surface properties in ferritic stainless steels, samples of AISI 409, AISI 430 and AISI 444 steels were subjected to plasma nitriding. The gas ratio used (N 2 :H 2 ) was 80:20, with a treatment duration of 8 h, and the nitriding temperature varied between 340, 380, and 400 °C. The results indicated that expanded ferrite could be formed via diffusion in all cases, with the best outcomes achieved at 380 °C. At 400 °C, all steels exhibited the formation of a phase predominantly consisting of the ε phase (Fe 2 – 3 N) and Fe 8 N. The nitrided layer thicknesses measured were 3.5, 7, and 12 μm for AISI 409 steel; 0, 5, and 12 μm for AISI 430 steel; and 3, 6, and 32 μm for AISI 444 steel, corresponding to treatment temperatures of 340, 380, and 400 °C, respectively. The lattice expansion was measured by XRD at 1.16 % and 2.31 % for AISI 409, 0.11 % and 0.27 % for AISI 430, and 0.17 % and 0.58 % for AISI 444, at treatment temperatures of 340 °C and 380 °C, respectively.