Nitriding of stainless steels: Combined plasma-gaseous process

In this work a combined process for the nitriding of stainless steels was developed, applying a first plasma nitriding treatment, which acts as an "activator" of the surface, then followed by a gaseous nitriding step, to increase the diffusion of the nitrogen and therefore the penetration of the treatment. The substrates for the process are two different stainless steels, AISI 304 and AISI 422, subjected to the single treatment of plasma nitriding (NP) and to the combined treatment of plasma nitriding followed by gaseous nitriding (NPNZ). The samples obtained were characterized (and compared with the two non-treated materials, NT) by cross-section micro-hardness profiles, metallographic analysis (SEM), wear tests (pin-on-disk tribometer) and accelerated corrosion tests (polarizations in acidic environment, H2SO4, and neutral saline, NaCl). The results obtained confirm that the combined treatment (NPNZ) is able to significantly increase the hardening depths, without excessively compromising the corrosion resistance in chlorides of the tested steels.