Characteristics of martensitic stainless steel nitrided in a low-pressure RF plasma

Martensitic grade stainless steel AISI 420 was nitrided in a low-pressure RF plasma using pure nitrogen. With a treatment time of 4.0 h at 400 °C, the nitrogen-rich layer on the sample was 20 μm thick and had a hardness of approximately 4.3 times higher than that of untreated material. The layer thickness was much greater than that obtained on AISI 304 under identical conditions, reflecting the different Cr content of the two alloys. Below the temperature at which CrN forms, the treated layer retains its martensitic structure, but with a larger lattice parameter than the bulk, a phase that can be termed ‘expanded martensite’, by analogy with the situation with austenitic stainless steel. Kinetics of the layer formation was also investigated which yielded an activation energy of 33.4 kJ/mol.