Plasma nitriding of 410S ferritic/martensitic stainless steel: microstructure, wear and corrosion properties

In this work, plasma nitriding treatments were performed on ferritic/martensitic stainless steel type 410S at 400 ºC (Low-temperature Plasma Nitriding - LTPN) and 530 ºC (Conventional Plasma Nitriding - CPN). The treatments were carried out under a 75% N2 -25% H2 gas mixture for 20 hours. After LTPN, a layer composed of expanded ferrite/martensite with a minor fraction of iron nitrides was obtained. In CPN, the nitrided surface consists of an outermost compound layer 15 μm thick followed by a diffusion zone with a depth of 170 μm. After both, CPN and LTPN nitriding treatments, the surface hardness increases more than four times compared to the original hardness of the non-nitrided steel and results in a flat-type transverse hardening profile for CPN and a diffuse-type profile for LTPN. The friction coefficients in the CPN and LTPN conditions, evaluated by the progressive linear scratching test, are lower than the condition without nitriding. The wear rate after nitriding is half that found for the non-nitriding condition. The corrosion resistance evaluated by immersion test in an aqueous solution with 3% FeCl3 for 88 hours shows that the behavior after LTPN is the same as in the non-nitrided condition, while for the CPN, the mass loss is higher and, therefore, the corrosion resistance is smaller than that observed for the LTPN. The precipitation of chromium nitrides during CPM nitriding leads to chromium depletion in the metal matrix. The results show the feasibility of promoting surface hardening of type 410S stainless steel by nitriding at low temperatures and improving the tribological properties without compromising corrosion resistance.