Sequential low-temperature plasma-assisted thermochemical treatments of the AISI 420 martensitic stainless steel

The application of low-temperature plasma carburizing (C), nitriding (N) and nitrocarburizing (NC) treatments in sequential steps on AISI 420 martensitic stainless steel was investigated through nine different treatments. Experiments were performed at 400 degrees C, being divided in three (8 h)-single treatments, namely C, N and NC, and six distinct (4 h + 4 h)-sequential treatments, namely C + N, N + C, C + NC, N + NC, NC + N, and NC + C, using appropriate gas mixtures for each treatment step. Sequential treatments promoted the formation of treated layers constituted of iron carbides/nitrides, chromium nitrides, and nitrogen/carbon-expanded martensite phases. Interesting results were obtained for both C + N and C + NC sequential treatments, whose application results in greater hardening depths (similar to 20% thicker) and leads to similar to 40% increase on the surface hardness, when compared to the obtained for the single C treatment keeping a smooth hardness profile. From the microstructural characterization, no significant sensitization was evidenced in the outer layers obtained for all treatments comprising the carburizing step, an indicative result that the interstitial carbon retards the chromium-rich particles precipitation kinetics. The combination of N + NC treatments produced thicker and harder treated cases compared to those produced by the respective single treatments. In brief, results showed that the appropriate selection of thermochemical treatments' sequence can promote increased treatments efficiency concerning the treated layers thickness or effective surface hardness. This work also contributes to the validation of the presented processing approach, proving its advantages over single thermochemical treatments.