586.    COHN, E. M., AND HOFER, L. J. E.  Mode of Transition From Hägg Iron Carbide to Cementite.  Jour. Am. Chem. Soc., vol. 72, 1950, pp. 4662-4664.

      Reaction of Hägg carbide, Fe₂C, to yield cementite, Fe₂C, and free C is a discontinuous or heterogeneous precipitation.  Hägg carbide and cementite have narrow ranges of composition and apparently do not form intermediate metastable solid solutions.  The reaction 3Fe₂C-Hägg→2Fe₃C+C becomes appreciable only at about 450°, while the reaction Fe₂C-Hägg+Fe→Fe₃C starts below 300°.  The unstable or labile carbide of Kölbel and coworkers is simply Hägg carbide.