A Research on the Surface Oxidation During Gas Carburization of Cr-Mo-Si Steels for the Automotive Transmission Gears

Abstract

As global regulations of CO₂ emissions and fuel consumption efficiency become more strict, the automotive powertrain system has become more compact with lower lubrication viscosity. With the current trends in powertrain system improvement, the operating conditions of powertrain components such as gears are becoming more severe. As a result, it is increasingly important to improve the strength and durability of the materials used for the powertrain system by optimizing alloy design and heat treatment. Much research and development has focused on improving components by heat treatment, and especially carburization. Also, many different alloy concepts, such as Fe-Cr-Mo or Fe-Cr-Ni systems have been proposed for automotive components. Among various approaches, Cr and Si , and solid solution hardening, are known to effectively increase hardenability. However, it has been reported in many works that the carburized case depth can also be reduced by increasing the Cr/Si content, due to oxide film formation. The mechanism of Cr/Si oxide film formation during carburization has not yet been clearly determined. In the present work, therefore, the mechanism of surface oxide film formation during carburization was examined. Also, the microstructure, composition and crystal structure of different oxides were examined and compared in detail.