Abstract
This study reports on conditions of D. C. plasma nitriding with nitrogen and hydrogen mixed gas in order to get a smooth and clean surface without compound layers after nitriding steel. Tool steels of SKH 51, SKD 11 and SKD 61 were D. C. plasma nitrified at 773K for 3600sec in a nitrogen and hydrogen mixed gas atmosphere of 532Pa. The composition of nitrogen in the nitriding gas was changed from 10 to 90vol%. The steels were nitrided and hardened by a nitrogen diffusion layer both with and without compound layers in the steels.
The surfaces of the plasma nitrified steels kept metallic brightness and smoothness like a mirror without compound layers when they were nitrided with less then 20vol% nitrogen gas. In 30 to 90vol% nitrogen gas, the tool steels were nitrided with compound layer and their surfaces lost brightness and smoothness because they became rough and changed to a gray color.
SEM micrographs showed some differences between the shapes of the surfaces of the tool steels after the plasma nitriding. The surface roughnesses of plasma nitrided tool steels were unrelated to the thickness of compound layers.
The tool steels were plasma bright nitrided with less then 20vol% nitrogen gas. In this study of the tool steels, SKH 51 was the most suitable steel for plasma bright nitriding.
