Influence of processing parameters on the characteristics of surface layers of low temperature plasma nitrocarburized AISI 630 martensitic stainless steel

Abstract

Plasma nitrocarburizing was performed on solution-treated AISI 630 martensitic precipitation hardening stainless steel samples with a gas mixture of H₂, N₂, and CH₄ with changing temperature, discharge voltage and amount of CH₄. When nitrocarburized with increasing temperature from 380 °C to 430 °C at fixed 25% N₂ and 6% CH₄, the thickness of expanded martensite (α'_N) layer and surface hardness increased up to 10 μm and 1323 HV_0.05, respectively but the corrosion resistance decreased. Though the increase of discharge voltage from 400 V to 600 V increased α'_N layer thickness and surface hardness (up to 13 μm and 1491 HV_0.05, respectively), the treated samples still showed very poor corrosion behavior. Thus, to further improve the corrosion resistance, the influence of variation of the amount of CH₄ in the nitrocarburizing process was investigated. Increasing the CH₄ percentage aided higher corrosion resistance, although it decreased the α'_N layer thickness. The most appropriate conditions for moderate α'_N layer thickness, high surface hardness and better corrosion resistance than the solution-treated bare sample were established, which is plasma nitrocarburizing at 400 °C with 400 V discharge voltage and containing 25% N₂ and 4% CH₄.