Thermodynamic analysis of carbide-layer formation in steel with microarc saturation by molybdenum

Abstract

Existing systems for diffusional saturation with molybdenum have numerous problems. The use of microarc treatment for this purpose is investigated. A steel sample is immersed in coal powder and heated by current transmission. Ammonium molybdate is the source of the molybdenum. The pyrolysis of coal is accompanied by the emission of hydrogen, methane, carbon dioxide, and carbon monoxide, which form a protective atmosphere and may reduce molybdenum to the atomic state. To identify the most likely chemical reactions, we calculate the standard change in Gibbs energy. The temperature ranges in which molybdenum trioxide is reduced to obtain molybdenum dioxide and atomic molybdenum are determined. The calculation results are verified experimentally. Simultaneous diffusion of carbon and molybdenum leads to the formation of a carbide coating (thickness 80–150 μm), with microhardness 13.5–15.0 GPa.