ON THE CASE HARDENING OF STEELS BY MODIFIED GAS UTILIZING O2 (II)

In the Case of Using CO₂

Abstract

Steels of various kinds were heated in a closed vessel containing CO₂ with the cyanide salt of optimum amount, and both the powerful carburizing action of the formed atmosphere and the reaction between the cyanide salt and CO₂ were explained.
The result of the experiment showed that the carbonitriding power of the atmosphere became maximum, irrespective of the kind of steels, by the addition of about 16 gr K₄Fe(CN)₆ per litre CO₂, which coincided well with the theoretical estimation that the concentration of C₂N₂O became maximum by the addition of 16•E4 gr K₄Fe(CN)₆ per litre CO₂.
As to the case depth and the surface hardness in relation to the heating temperatures, the former generally became the larger but the latter the lower, the higher the heating temperature was raised. In specimens treated at higher temperatures, the maximum hardness was obtained at the inner portion slightly distant from the surface. This was presumably due to the fact (1) that, at the surface of such specimens, the matrix phase was too rich in the carbon and the nitrogen to be hardenable and moreover its amount was small; (2) that, at the inner portion slightly distant from the surface, the amount of the matrix, having optimum concentration of carbon and nitrogen for the good hardenability, became large and that of the proeutectoid constituents small, causing maximum hardness in the carbonitrided layer, and then (3) that, in the further inner portion the hardening effect became small, because the carbon and the nitrogen content decreased.