Abstract
An accurate consideration is given on the carburizing reaction which has heretofore been wrongly represented thus:
3Fe+2CO=Fe3C+CO2.
The reaction should be denoted as follows:
(Solid sol.)+2CO (Solid sol.)+CO2.
The concentration of carbon dissolved in iron increases as the partial pressure of CO increases until the solid solution becomes saturated with respect to carbon. Since Fe3C is unstable, it does not separate out even when the solid solution is saturated with carbon. The equilibrium existing over the saturated solid solution is,
2CO C+CO2
in which carbon is no longer dissolving in iron. The concentration of carbon varies with the ratio CO/CO2, the relation being shown in Fig. 4. In this case, the concentration of CO2 has a larger influence on the carbon content than CO. On the other hand, the reaction C+CO2→2CO, ingenearl, does not reach an equilibrium because of the comparatively inert nature of carbon. It is due to this fact that carburization with carbon alone is relatively feeble; that is, the atomosphere in contact with carbon is apart from the equilibrium 2CO C+CO2.
The influence of alkali or alkali-earth carbonates (except CaCO3) in promoting the carburizing reaction, which has hitherto been erroneously considered as a catalytic action, may also be explained on the ground that the equilibrium 2CO C+CO2 is established in the presence of these carbonates. On beeing heated, they easily react with carbon producing CO and oxy-carbonates or solution of oxides and carbonates. The CO evolved immediately dissociates maintaining the equilibrium 2CO C+CO2. Therefore, the surface of iron in contact with this mixture becomea saturated with carbon, which further promotes the diffusion of carbon into the mass of iron. In the ease of CaCO3, however, the temperature at which the decomposition pressure is equal to 1atm., is 780°Therefore, in a vessel open to air and above this temperature a complete decomposition of CacO3 will take place. Accordingly, CaCO3 cannot have influence upon the carburizing reaction.
