2922. RUFF, O. [Equilibrium Diagram of Iron-Carbon Alloys.] Metallurgie, vol. 8, 1911, pp. 456-464, 497-508; Chem. Abs., vol. 5, 1911, p. 3793.
It is concluded from experiments that besides cementite, Fe3C, which has been called triferrocarbide, a diferrocarbide exists, Fe3C. The equilibrium concentrate of the triferrocarbide is always less than the saturation concentrate. Above 700° the carbides are endothermic compounds and at 2,220° the diferrocarbide is quite strongly dissociated into Fe and C. As the temperature is lowered, the diferrocarbide breaks down into Fe3C and C, and eventually the Fe3C breaks down into Fe and C. In the metastable system, the solidification takes place in 4 periods: (1) The time up to that point when the melt reaches the equilibrium concentrate; (2) the time during which the melt remains between the equilibrium concentrate and the saturated concentrate; (3) the time during which the carbide eutectic is solidifying; (4) the time during which the solid system is cooling. The amount of solid carbide formed from the liquid phase is determined chiefly by the decomposition velocity of the carbide and the difference between the equilibrium concentrate and the saturation concentrate and also is affected by the velocity of solidification. IN the solid phase, the decomposition velocity of the carbide is always less than in the liquid phase. Thus, when the carbide is once formed as solids, it remains more stable in proportion as the temperature at which it is first deposited is low. Moreover, the more finely divided its condition, the more likely the carbide is to remain in metastable equilibrium with the surrounding solid solution. The reaction velocity of the carbide decomposition at 1,135° is such that about 50% of the carbide will decompose in 2 sec. Approximation formulas are developed for the results to be expected in the solidification of Fe-C alloys during the second and third periods.