3440.    ---------------.  [TRILLAT, J. J., AND OKETANI, S.]  [Electron-Diffraction Study of the Cementation of Iron.  II.  Action of CO+H₂ Mixtures.]  Compt. rend., vol. 232, 1951, pp. 1116-1118.

        Supplementary studies to those performed with pure CO (see abs. 2719) and at slightly raised temperatures of 350°-400° have shown under the experimental conditions that only cementite Fe₃C and oxide Fe₃O₄ are formed.  In the presence, however, of a mixture of equal parts CO+H₂ different results are noted.  At 350° and for the duration of the reaction (1-2 hr.), the presence is indicated of a-Fe, a little cementite Fe₃C, and a compound characterized by a diagram different from that of cementite and sometimes appearing in the pure state.  Detailed examination identifies it as the percarbide Fe₂C or Hägg carbide.  At 400°, after a 1-hr. treatment by CO+H₂, only cementite Fe₃C and a-Fe appear.  The percarbide may however be seen if the reaction is prolonged to 2-3 hr.  Above 400° only cementite exists.  The results thus seem to indicate that the percarbide ceases to be stable above 400° while the cementite exists up to at least 600°.  In the stable regions of percarbide, the following reactions probably occur:

        (1)  3Fe+2CO→Fe₃C+CO₂,

        (2)  2Fe₃C+2CO→3Fe₂C+CO₂.

The formation of percarbide follows that of cementite, which contains less C; up to 400° reaction (2) may occur but not above.  The presence of H₂ accelerates the speed of the reaction considerably.  Further studies will be made of the action of CO+H₂ at temperatures below 350° and above 600°.