Binary and ternary interstitial alloys - II. The iron-carbon-nitrogen system

Chemical and X-ray investigation of the reaction of carbon monoxide with iron nitrides and of the reaction of ammonia with iron carbides discloses the existence of iron carbonitrides- a series of new ternary interstitial alloys containing iron, carbon and nitrogen. $\zeta $-phase carbonitrides, with structures similar to those of $\zeta $-iron nitrides, have a range of homogeneity extending approximately from Fe$_{8}$N$_{4}$ to Fe$_{8}$C$_{3}$N. The latter is isomorphous with Fe$_{2}$N. $\epsilon $-phase carbonitrides, which are isomorphous with $\epsilon $-iron nitrides, have a composition range of approximately 25 to 33 atomic% nitrogen plus carbon, i.e. from Fe$_{3}$X to Fe$_{2}$X, in which the higher carbon concentration limit is not less than 16 atomic%. Probable phase fields for part of the iron-carbon-nitrogen system are given on a ternary diagram. Prolonged reaction of carbon monoxide with iron nitrides results in complete elimination of nitrogen. Below 500 degrees C the product is a carbide of iron, now called iron percarbide, the narrow composition range of which includes Fe$_{20}$C$_{9}$. Above 500 degrees C the product of the same reaction is cementite.