A Study on the Nitriding of Metallic Chromium

The chemical changes of the metallic chromium during heating in nitrogen or in air were studied. The results obtained were as follows: (1) When metallic chromium was heated in extremely pure nitrogen, a rise of the heating temperature caused a continuous increase in the amount of nitrogen absorbed. The effect of the heating temperature, however, became less notable at higher temperatures, e.g. 1,250°C and upwards. (2) It was found that both oxidation and nitrization took place on the metallic chromium during heating in air. Although nitriding was prevented remarkably by the oxygen present at relatively low temperatures, e.g. 1,000∼1,200°C, it became gradually predominant with the rise of the heating temperature and consequently the increase in weight due to nitriding was about equivalent to that due to oxidation at 1,300°C. (3) The nitrides formed during heating at 1,000°C in nitrogen was identified as Cr2N and CrN by X-ray diffraction analyses, whereas CrN could not be found at 1,300°C. After heating in air, Cr2N was the only phase observed to be formed, whatever the heating temperature might be. (4) No marked difference in the amount of nitrogen absorbed by heating in nitrogen was found with the two different samples of chromium employed, i.e. of electrolytic chromium and of thermit chromium. On the contrary, if they were heated in air, both oxidation and nitriding occurred in a greater degree in the former than in the latter.