328.    BRAMLEY, A., AND LAWTON, G.  Gaseous Cementation of Iron and Steel.  III.  Influence of Hydrocarbons on the Carburization of Iron and Steel.  Iron and Steel Inst. Carnegie Schol. Mem., vol. 16, 1927, pp. 35-100; Chem. Abs., vol. 22, 1928, p. 207. 

                   From a study of photomicrographs, it is concluded that the cementation produced when a carburetted gas (such as CO saturated with toluene at 18°) is passed over a specimen of Fe or steel in 1 direction only is much more uneven than when CO is used alone.  Obstructions to the free circulation of carburizing gas cause uneven cementation.  Alternate passing of the gas, first in 1 direction and then in the opposite, favors even cementation. The fact that the carburizing gas is often of such a nature as to be decomposed by the heat of the furnace must be taken into account if even cementation is desired.  Variations from this source will depend upon the temperature and size of the various parts of the furnace and upon the decomposition.  When a tubelike furnace is used and gas is passed in from both ends the cementation is often considerably weaker in the center of the bars than at the ends.  This is because the carburizing agent becomes exhausted too quickly and indicates either too slow a flow of gas or excessive decomposition of the latter.  Under certain conditions the decomposing gas deposits a sheath of C over the ends of the bars.  This sheath prevents contact of gas and metal and further carburization at these points is difficult.  Such bars show a greater cementation along their centers.  The authors prove that the liquidation theory of cementation as put forth by Giolitti is unsound.  They find that the amount of redistribution of the C in steel, while the latter is cooling down, is inappreciable.  The depth of carburization is greater the longer the period of cementation and the higher the temperature.  The depth increases with the temperature according to a linear law.  As the period of cementation is increased in geometrical progression, the depth of penetration increases in the same way but with a different common ratio.  The diffusivity constants for C diffusing into steel are calculated for the several carburizing mixtures and temperatures employed.