Microstructural, x-ray structural, and microspectral analyses indicate that internal oxidation occurs in steels 25KhGT, 25KhGM, and 25KhGNM carburized in an endothermic atmosphere.
The addition of 0.6% Mo to the steel does not eliminate or reduce the internal oxidation zone of chromium and manganese.
Internal oxidation to a depth of 0.03 mm is quantitatively observed by x-ray structural analysis in all steels investigated, although it is possible that oxides also occur at greater depths.
The oxides formed are primarily spinels of the types nFeO·Cr2O3 and MnO·Mn2O3.
With increasing cleanness of the surface the number of oxides formed and the depth at which they are formed decrease substantially.
Manganese and chromium do not occur together in spinels, while in some cases titanium and molybdenum oxides are encountered together with one of the two main types of oxides.
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Kal'ner, V.D., Yurasov, S.A. Internal oxidation during carburizing. Met Sci Heat Treat 12, 451–454 (1970). https://doi.org/10.1007/BF00668848