472.    CHARPY, G., AND PINGAULT, P.  [Conditions Required for Formation of Cementite.]  Compt. Rend., vol. 187, 1928, pp. 554-556; Chem. Abs., vol. 23, 1929, p. 79.

                  Review of previous experiments, showing that Fe could be carburized at 600°-650° with the formation of a layer of cementite separated from the original metal by a narrow transition zone and that cementite could decompose at 700°.  Pure electrolytic Fe filings, free from H, were carburized by cyanide or hydrocarbons, and the physical properties were studied.  The density, decreased gradually with absorption of C to a limit of 7.40 at 0°.  The magnetic attraction of 1 gm. of Fe filings decreased, on carburizing from 7 gm. to a constant value of about 4 gm.  The filings carburized to constant density and magnetic attraction contained 6.75% C and left no trace of insoluble C when dissolved in HNO₃.  This product did not suffer any change in properties after gradual heating to 1,000° during 10 hr.  Slight separation of graphite in previous e3xperiments of this nature may be attributed to oxidation.  Fine Fe wire was transformed entirely into cementite, with no trace of graphite, after heating to 850° in cyanide.  Pyrometer protection tubes of Fe 1 mm. thick from cementation furnaces were also found to be completely transformed into cementite.  Carburation of solid Fe below 1,000° tends therefore to form cementite, and the distribution of the C beneath the surface is determined only by the rate of solution and diffusion of cementite in solid Fe.