Effect of bias plasma on active screen nitrocarburising response of AISI 420 martensitic stainless steel

Abstract

This study addresses the potential of active screen plasma nitrocarburising (ASPNC) treatment applying a carbon active screen for short-time low-temperature diffusion treatment of AISI 420 martensitic stainless steel in terms of varying the bias plasma power directly at the steel surface. Without the application of a bias plasma the nitrocarburising effect was reduced due to cementite formation near the surface. The application of bias plasma in ASPNC processes resulted in the formation of expanded α′-martensite and ε-carbonitride and inhibited cementite formation. Thickness and phase composition of nitrocarburised layers produced by means of ASPNC were comparable to those that can be generated by conventional plasma nitrocarburising (CPNC), but with reduced plasma impact on the steel surfaces implying considerably lower thermal stress. Surfaces treated by ASPNC with bias plasma showed a tendency towards improved pitting resistance in a 3.5 wt% sodium chloride electrolyte compared to those generated by CPNC.