Ultra-low-to high-temperature plasma-assisted nitriding: revisiting and going further on the martensitic stainless steel treatment

AISI 420 steel was plasma nitrided aiming to study the relationships between treatment temperature and the treated layer phase constitution and properties. Nitriding was carried out in temperatures varying from 200 to 600 °C for 4 h treatment time. Samples were characterized by OM, XRD and hardness measurements. For treatments carried out at 200 and 250 °C (here termed 'ultra-low' temperatures), it was observed the formation of N-expanded martensite ( ′ N phase) only. For low-temperatures (300 to 400 °C), the treated surfaces presented ′ N and epsilon-iron nitride ( -Fe2+xN) phases. At high temperatures (450 to 600 °C), -Fe2+xN and increasing amount of chromium nitrides (CrN and Cr2N phases) were verified in the nitrided layers with temperature rise. From 500 °C, the occurrence of gama'-iron nitride (γ′-Fe4N) was additionally noticed. Results also indicate that nitrided layer growth follows the Arrhenius law and presents two slopes in the Arrhenius plot. The activation energy for ultra-low and low temperatures is about two times smaller than that for high temperatures, suggesting two different growth mechanisms, probably one based on high-diffusivity paths (for the lower temperatures), and the other one on volume diffusion (for the higher temperatures), with activation energy of 64 16 and 119 12 kJ mol−1, respectively.