滲炭現象に關する理論及び實驗的研究

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An accurate consideration was made on the carburization of iron which had heretofore been wrongly represented : 3Fe+2CO=Fe_3C+CO_2. From the author's consideration, the reaction should be noted as follows : (Solid sol.)+2CO⇌C(Solid sol.)+CO_2 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 Fe_3C is unstable, it does not precipitate even when the solid solution is saturated with carbon. The equilibrium existing over the saturated solid solution is 2CO⇌C+CO_2, in which carbon is no longer dissolving in iron. That is, the content of carbon penetrated in iron does not exceed the solubility limits corresponding to the PQ and SE curves in the iron-carbon equilibrium diagram, where the solid solubility of oxygen may be neglected. From the above-mentioned consideration, it is possible to determine, by restoring to a method where thin iron sheets are carburized at constant temperature and the saturation content of carbon analyzed, the solubility curve of carbon in austenite, that is, the SE curve in the iron-carbon diagram. On the other hand, the solubility of carbon in α-iron is so small that below the entectoid temperature carburizing reaction can scarcely occur and the maximum solubility can only reach up to 0.03%. The velocity of the penetration of carbon in iron depends upon the concentration of carbon at the surface of iron which is, further, exceedingly influence by the CO_2 concentration in the atmosphere. In order to promote the carburization, it is necessary to remove the CO_2 evolved incessantly by the carburizing reaction from the surface of the iron. However, since the reducing reaction C+CO_2→2CO, in general, coes not take place readily, it can hardly reach an equilibrium. That is, because of the comparatively inert nature of carbon, the atmosphere in contact with carbon is usually apart from the equilibrium 2CO⇌C+CO_2. It is due to this fact that carburization with carbon alone, (especially with such inert carbon as coak or soot) is relatively feeble. The influence of alkali or alkaline-earth carbonates (except CaCO_3) in promoting the carburization, which has hitherto been erroneously considered as a catalytic action, may also be explained on the ground that the equilibrium 2CO⇌C+CO_2 is established in the presence of these carbonates. On being heated, they easily react with carbon producing CO and oxycarbonates or solution of oxides and carbonates. Since those systems consisting of carbon and carbonates are univariant, a definite decomposition pressure of CO and CO_2 evolved, corresponds to each temperature the equilibrium 2CO⇌C+CO_2 is maintained. Therefore, under a certain definite decomposition pressure, the surface of iron in contact with the mixture becomes saturated with carbon, which further promotes the diffusion of carbon into the mass of iron. In a vessel open to air the mixture has the greatest effect on the carburization at the temperatures where their decomposition pressure is equal to 1 atm. In the cases of Na_2CO_3 and BaCO_3, for example, such proper temperatures are above 860℃ and 930℃, respectively. As compared with them, the decomposition of CaCO_3 takes place completely above 780℃, so that CaCO_3 cannot have influence upon the carburization. Generally the carburization in CO-gas current becomes weak when the current velocity becomes small. This is due to the diffusion layer made on the specimen by CO_2 which is generated by the carburizing reaction, that is for promoting the carburization some large current velocity is necessary so as to remove incessantly CO_2 generated. It is of interest that Fe_3C is never actually produced in pure CO-gas current. This reason is that iron is the most conspicuous catalyzer which gives rise to the instantaneous dissociation of excessive CO at the moment of contact with iron ; hence the actual action of pure CO and iron is impossible in CO-gas current even when its velocity is large enough.
社団法人日本鉄鋼協会の論文
1942-07-25

著者

眞殿統
理化學研究所

滲炭現象に關する理論及び實驗的研究

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