CEMENTATION EQUILIBRIUM AND CEMENTATION REACTION IN CO-GAS CURRENT

Abstract

In view of a wrong representation of the cementation equilibrium by formula 3Fe+2CO=Fe₃C+CO₂, the author has discussed that the cementation velocity can never exceed the saturation point of solid solution Agr and the cementation phenomena never occur below A1 point. for which some opposite opinions have been evoked. The adversary states that a converse experiment is possible; for instance, the cementation may be carried out in CO-gas current even beyond Agr point and free crystale of Fe3C can be produced. Such a contrary opinion is, however, insignificant. So far as the equilibrium such as 2CO C+CO₂ exists from the theory of cementation equilibrium, it is absolutely impossible to produce Fe3C, but under the circumstances such as pure CO which does not contain any CO₂ or as 2CO→C+CO₂ in the way of dissociation, CO might act with Fe and Fe3C might be produced. Consequently, there being possibility of the occurence of Fe3C in pure CO-gas current, the incidental occurrence of Fe₃C in an experiment is not contrary to the theory of equilibrium.
Infact, Fe₃C can never actually be produced in CO-gas current. The reason is that Fe is the most conspicuous catalyser which augments the dissociation of CO and at the moment of contact with Fe the equilibrium of 2CO C+CO₂ is attained with the instantaneous dissociation of CO; hence the actual action of pure CO and Fe is impossible even in CO-gas current. Fe₃C might, however, occur at a temperature below 500°C, as the dissociation velocity becomes small at a comparatively low temperature. The property of Fe which considerably augments the dissociation of CO plays an interesting role in the cementation reaction.
Generally the action of cementation in CO-gas current becomes weak when the current velocity becomes small. This is due to the diffusionayer made on the surface of specimen by CO₂ which is generated by the cementation reaction, i.e., for augmenting cementation some large current velocity is necessary so as to remove incessantly CO₂ generated whether or not the cementation is conspicuously made in gas current may be detected by the soot attached to the surface of specimoen; i.e., if the soot attached, the surface is proved to be at the saturation of cementation, and if not, it indicates the cementation is not saturated.