Hardness profile improvement of plasma nitrided high speed steel by glow discharge heating

Abstract

The hardness of a plasma nitrided high speed steel surface is extremely high at the surface because of the formation of nitrides and the diffusion layer. It decreases steeply to the value of the substrate. Such a brittle surface and steep change in hardness are not favourable for tool applications because of tool chipping. This paper investigated the improvement of such a hardness profile of plasma nitrided high speed steel using glow discharge heating as a post-nitriding diffusion treatment. High speed steel was plasma nitrided at 500°C under 667 Pa for 2 h in a gas mixture of nitrogen and hydrogen. A diffusion process was then conducted in the same chamber by changing the gas to hydrogen or argon. The plasma nitrided high speed steel was also diffusion treated by furnace heating. The microstructures and the hardness profiles of these two post-nitriding treated samples were compared. Diffusion treatment by glow discharge heating lessens the hardness gradient from steep to gentle more effectively than fur...