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 1623.    JOHANSSON, A., AND VON SETH, R., AND ELFSTROM, N.  [Investigations on the Surface Decarburization of Steel.]  Jernkontorets Ann., vol. 116, 1932, pp. 565-655; Chem. Abs., vol. 27, 1933, p. 3429.

        Equilibrium isotherms have been determined for the systems 3Fe+2CO→Fe3C+ CO2 (1) and 3Fe+CH4→Fe3C+2H2 (II) between 680 and 1,100.  The isotherm for (1) at 710 shows a decreasing CO2 pressure with increasing C content, a result which is inconsistent with the assumption that C is insoluble in ferrite below the Ac1 point.  Above 900, FeO cannot be reduced to metallic Fe free from C.  At 680 the reaction proceeded too slowly for establishing equilibrium, but a mutual solubility of cementite and ferrite was indicated.  From 900-1,100 the activity coefficient of C was found to be nearly unity in cementite and Mn steels.  By varying the CO2:CO ratio in carburetted gas, either carburizing or decarburizing conditions may be established.  At 1,250 and 1,320 the solubility of C in molten Fe was 4.4 and 4.56%, respectively.  In (II) the rate of reaction was found to be a maximum at 950 but was noticeable at 600, and falling again above 1,000.  Ni, Mn, W, and Cr steels gave about the same rate as unalloyed steel.  No decarburization was found with N2 and none with air at 750, 710, or 650, probably because a protective oxide layer prevents diffusion of gas to the Fe surface.  26 refs.