2704.     PINGAULT, P.  [Formation and Decomposition of Cementite.]  Compt. rend., vol. 191, 1930, pp. 1007-1008; Chem. Abs., vol. 25, 1931, p. 1193.

        Pure cementite, prepared by the action of NaCN upon Fe filings at 650°, was heated in a vacuum of various temperatures and subjected to the attack of HNO₃ (d., 1.2).  After 10 hr. at 1,000° only faint traces of graphite could be found.  After 1 hr. at 1,075° the cementite was agglomerated, and distinct races of graphite were found.  After 1 hr. at 1,175° the cementite was transformed partly into metallic globules having the microscopic characteristics of gray-cast Fe and partly into a powder, containing much graphite.  Below 1,000°, therefore, the pure cementite may be considered very stable, but, as soon as the melting temperature of cast Fe is approached, the decomposition of the cementite becomes rapid.  In forming cementite from pure Fe and NaCN in the absence of air, it was found that the cementation of the Fe was negligible at temperatures of 600°-650° and 850°-900°.  The same results were obtained in pure and in commercial NaCN.  Different results obtained in the presence of air are believed to be due to the carbonizing action of the decomposition products of the NaCN.