Effect of cathode workpiece surface emissivity on temperature uniformity in plasma nitriding

The goal of this paper is to demonstrate that glow discharge plasma heating of the cathode workpiece is different from radiant heating under vacuum, given the same pressure and gas composition, and that the final temperature obtained by the workpiece and the rate of heating depend on the workpiece emissivity. Towards this goal, several glow discharge heating tests were carried out using a number of geometrically identical carbon and austenitic steel samples, each prepared so as to generate differing surface emissivity values. These tests clearly demonstrate that samples with lower emissivity heat faster and to a higher temperature than do samples with higher emissivity values. Furthermore, several observations are made about the change in workpiece emissivity. For example, in situ measurements show that emissivity increases during plasma nitriding, and the emissivity change is most rapid during ramping to final temperature. It is further observed that the effect of plasma nitriding on emissivity is stronger for austenitic stainless steel than for carbon steel, and that powder metal samples, nitrided at a higher temperature than the other samples, achieve the highest emissivity values.