Abstract
High Cr steels are hardly nitrided in on ammonia gas atmosphere at usual nitriding temperatures, whereas they are easily nitrided when a small amount of oxygen is added to the ammonia atmosphere (called an oxynitriding atmosphere). Hence the protective oxide film of the steels is reduced by adding oxygen to the ammonia. This paradoxical phenomenon has not been explained. With respect to the reducing behavior of the NH~3~ο£ΏO~2~ gas mixture on the oxide film, the role of nitrogen oxide (NO) formed by a reaction between the NH~3~ and the O~2~ was noted. In the oxynitriding operation, the NO was detected in the exhaust gases. Based on this fact, Type 304 stainless steel was nitrided in NH~3~ο£ΏNO gas mixtures at elevated temperatures, and the action of the NO in NH~3~ as a reductant for the oxide film of the steel was discovered. Furthermore, nitriding in the NH~3~ο£ΏNO gas mixture resulted in a very high surface hardness (as high as Hv 1300β1700) for the steel. On the other hand, the oxynitrided surface was always of lower hardness than the caseβhardened layers formed in the steel. It is believed that NH~3~ο£ΏNO atmosphere nitriding will become a new nitriding method for steels.