Conclusions
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1.
In the absence of decarburizing, incomplete hardening of carburized and carbonitrided steels in thin surface layers is induced by internal oxidation of alloying elements — chromium, manganese, and silicon.
Internal oxidation of carburized steel 30KhGT is accompanied by substantial reduction of the manganese and chromium concentrations in austenite.
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2.
The kinetics of internal oxidation with carbonitriding is substantially affected by the diffusion of alloying elements from the substrate. The diffusing ability of the elements depends on their concentration (increasing with the concentration).
The diffusion of these elements leads to reduction of their concentrations beneath the oxide layer and imcomplete hardening of the steel.
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3.
The nonmartensitic transformation of austenite at large depths in the carbonitrided case is associated with the formation of nitrogen compounds of chromium, manganese, and silicon.
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4.
Removal of the internally oxidized layer greatly increases the fatigue strength of carburized parts of steel 18Kh2N4VA.
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Literature cited
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Additional information
Central Scientific-Research Institute of Technology and Mechanical Engineering. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 40–44, June, 1972.