Diferro carbide, Fe₂C

Combined Carbon in Iron.

By heating pure iron in a graphite crucible in contact with carbon, in an electric vacuum furnace, Ruff and Goecke have been able to determine the solubility of carbon in liquid iron at temperatures ranging from 1220° to 2626° C., and to prepare a diferro carbide, Fe₂C, the existence of which was suspected as resulting when cementite is subjected to prolonged annealing below 600° C. The results were obtained by suspending the crucible in the furnace, and when the desired temperature had been reached and maintained for a sufficient length of time the crucible was released by a mechanical device and dropped into ice water. The metal was thus rapidly quenched without fear of dissociation due to slow cooling. The combined carbon was then estimated, the results being as follows: plotted in the form of a curve (see fig.), exhibit a break at 1837° C., corresponding to the composition of cementite, Fe₃C. A maximum occurs at 2220° C., with 9.60 per cent, of combined carbon, which corresponds to diferro-carbide, Fe₂C. Up to this temperature it was observed that the molten mass remained uniformly liquid, with a bright reflecting surface. Above this temperature graphite began to separate out, but the temperature had to be raised very carefully, as otherwise the whole mass became suddenly ejected from the crucible, in consequence of the violent decomposition of the diferro carbide into iron and free graphite, with evolution of heat, the compound being endothermic.

Both carbides, Fe₃C and Fe₂C, are thus endothermic compounds at temperatures above 700° C., and the diminished solubility of graphite in iron above 2220° C. is due to dissociation of the diferro carbide. Below 2220° C. the reaction

3Fe₂C ⇔ 2Fe₃C + C obtains, whilst at 1837° C. the cementite dissociates: -
Fe₃C ⇔ 3Fe + C.