2719.     POMEY, J.  [Cementation With Carbon and With Nitrogen.]  Rev. Mét., vol. 47, 1950, pp. 637-657, 727-738; Chem. Abs., vol. 45, 1951, p. 1929.

        Comprehensive review of the latest knowledge of cementation phenomena, giving a review of theories explaining the equilibrium of γ-iron in mixtures of CO, CO₂, CH₄, H₂ in different concentrations followed by their individual analysis.  It is held that H₂ is adsorbed during preheating, becomes dissociated on the surface of γ-iron, and decomposes CO, forming nascent C, which alloys with Fe.  Influence of type of C on the carburization phenomena was investigated by placing low-C steel specimens embedded either in graphite or in amorphous C in a furnace filled with a carburizing atmosphere and heating at a carburizing temperature.  Specimens placed in amorphous C were properly carburized, and those in graphite did not show any case.  Formation of graphite in carburized case should be avoided on this account, and curves are given to show temperature leading to formation of graphite and amorphous C.  Sugar and cellulose do not produce any graphite on carburization, but polynuclear aromatic compounds such as naphthalene, anthracene, or picene produce macromols on carbonizing, carrying on their periphery C atoms partly composed of graphite.  The mechanism of graphite formation is illustrated.  Aliphatic hydrocarbons dissociate on pyrolysis into H₂ and free radicals which can recombine with each other or with H₂, C₂H₄ being the principal radical.  A maximum C concentration of 0.85% produced by CH₄ can be increased to 1.15% C by using less stable C₂H₆ or C₂H₄ in concentration under 5% or, preferably, 0.5-3.5% C₃H₈.  Diffusion phenomena are extensively investigated mathematically, and an equation is developed.