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 furnace heating does. Furthermore, the hard nitride layer was eliminated by glow discharge processing, but the layer remained after furnace heating. Results showed that the effect of a post-nitriding diffusion treatment by glow discharge heating results from the sputtering effect and the acceleration of diffusion by ion bombardment.

Keywords:

• Tool steel
• Plasma bombardment
• Hardness depth profile
• XRD analysis

This work was supported by KAKENHI 1181204400. The authors also greatly appreciate the support of Dr Kazuo Murata of Industrial Technology Support Institute of Higashiosaka City.