166.    BAUKLOH, W., AND KNAPP, B.  [Progress of the Reaction Between Hydrogen and Carbon in Iron.]  Arch. Eisenhüttenw., vol. 12, 1939, pp. 405-411; Iron and Steel Inst. (London); Carnegie Schol. Mem., vol. 27, 1938, pp. 149-164; Chem. Abs., vol. 33, 1939, p. 3732.

                  Basic reaction Fe₃C+2H₂=3Fe+CH₄ is theoretically discussed.  At first, independently of temperature and C content, H dissolves C present on the Fe surface in cementite or y-solid solution.  The Fe formed in this stage obstructs further direct access of H to C, so the reaction can continue only after diffusion of H.  As long as diffusion is the slower process, the progress of the reaction is determined by the diffusion and, thus, depends on temperature, C content, and H₂ pressure.  The CH₄ equilibrium is another factor.  Beginning at 850° the migration velocity of C is higher than the reaction velocity of H and C, so the reaction becomes the slower process and is determined by the equilibrium conditions.  Three types of marginal decarburization zones are distinguished:  (1) Purely ferritic, up to 850°; (2) transition; and (3) hypereutectoid, above 850° within the 2-phase range of austentite + cementite.  Experiments, which corroborate the theoretical deduction, are described in detail.  10 refs.