BRIGHT NITRIDING IN BELL-TYPE FURNACE WITH CONTINUOUS RECIRCULATION OF ATMOSPHERE.

The bright-nitriding process recently developed by U. S. Steel is a gas-phase nitrogenation treatment for producing case-hardened ferrous-alloy parts free of white layer (iron nitride). This process evolved from the discoveries that a cyanide contaminant generated during the nitriding treatment can severely impair the nitriding rate, and that for case-hardenable steels, there are significantly different critical nitrogen activities below which no white layer will form. The objectives sought in the development were to minimize cyanide concentration in the nitriding atmosphere and to optimize nitriding rates while operating slightly below the critical activity so that white layer and network damage would be avoided. Bright-nitriding experiments and equipment are described. Significance of nitrogen activity and control of nitrogen activity are explained.