On the kinetics of plasma nitriding a martensitic stainless steel type AISI 420

Plasma nitriding was employed to treat martensitic stainless steel type AISI 420. The ability to remove the passive film from the surface is an important advantage in this process in order to guarantee a homogeneous surface treatment. The resulting nitrided surface shows the presence of a compound layer and a diffusion zone. The interface between the diffusion zone and the substrate is flat, as a consequence of the high chromium content on the alloy. The precipitation of chromium nitrides is also responsible for the high levels of hardening obtained. A value up to 1350 HV 0.025 constituting a maximum hardness was obtained. Diffusion depth increases with an increase of the nitriding temperature as measured by optical microscopy and verified by the hardness profiles. Calculated diffusion coefficients of nitrogen were very low compared to the data for alpha iron, and allow calculation of the Arrhenius behaviour during the process.