Study on surface oxidation of Cr Mo Si steel used in automobile gearbox during gas carburizing

With the increasingly strict global regulations on carbon dioxide emissions and fuel consumption efficiency, automobile powertrain systems become more compact and the viscosity of lubricating oil is lower. With the improvement trend of powertrain system, the operation conditions of gears and other powertrain components are more stringent. Therefore, in order to improve the strength and durability of powertrain materials, it is very important to optimize alloy design and heat treatment. In order to improve the composition, many research and development work are focused on heat treatment, especially carburizing treatment. In addition, many different alloy concepts are proposed for automobile parts, such as Fe Cr Mo or Fe Cr Ni system. Among various methods, Cr and Si addition and solution strengthening can effectively improve hardenability. However, it has been reported in many literatures that increasing Cr / Si content can also reduce the depth of carburized layer due to the formation of oxide film. Up to now, the formation mechanism of Cr / Si oxide film in carburizing process is not clear. Therefore, the formation mechanism of surface oxide film in carburizing process was studied, and the microstructure, composition and crystal structure of different oxides were investigated and compared in detail.

In recent years, with the aggravation of vehicle exhaust pollution to the atmosphere, the laws and regulations related to the environment, such as CO2 production, fuel consumption and so on, have become increasingly strict around the world. In order to comply with this trend, the field of automotive powertrain system is actively carrying out research, through the downsize technology to achieve lightweight, improve gear shape and lubrication optimization and improve transmission efficiency, so as to meet the requirements of environmental protection laws and regulations. Especially small displacement technology can improve the fuel efficiency and transmission efficiency most effectively by reducing the total weight of the transmission system. With the decrease of the size of the system components, even under the same torque condition, the stress and the number of revolutions transferred to each component will increase, which makes the driving environment of the components worse, thus shortening the service life of the components. In order to improve the durability and reliability of powertrain components, it is necessary to carry out research on improving performance at the same time, mainly including improving components of components or optimizing heat treatment conditions.

Exposed to the driving environment, the gears will have repeated bending and continuous contact, which are mainly treated by gas carburizing heat treatment to ensure the surface wear resistance and durability. Gas carburizing heat treatment is a kind of heat treatment process. Generally, at the temperature above 850 ℃, through the combustion of methane (CH4), propane (C3H8) and other hydrocarbon gases, the atmosphere of conversion gas (or “carrier gas”) is generated, and carbon molecules are penetrated into the surface of parts. Under the effect of carburizing, the surface hardening layer can ensure high hardness above 700hv. The deep non hardening layer is treated by quenched and tempered to achieve the best combination of strength and toughness, which can ensure high bending fatigue strength and wear resistance at the same time. Generally speaking, during carburizing, the following gas decomposition reaction will occur when carbon molecules penetrate into the surface of parts. This decomposition reaction is determined by the thermodynamic equilibrium state. Therefore, the accurate measurement and control of gas partial pressure in furnace is very important during carburizing.

Co → C (carburizing) + 1 / 2O2 (1)

2CO → C (carburizing) + CO2 (2)

CH4 → C (carburizing) + 2h2 (3)

However, in order to continuously improve the durability of parts, Cr, Mn, Mo, Si and other oxidation elements are added to the material, and the content of them is gradually increasing, and the sensitivity to oxidation / reduction reaction is also increasing during heat treatment. During carburizing, the oxidation / reduction reaction between metal elements and gas components will have a great influence on the quality of carburizing.

Since 1940, some scholars have been studying the high temperature oxidation of Fe and alloy steel, mainly from the perspective of oxidation kinetics and the structure of oxide film. First of all, for the oxidation characteristics of Fe, the research of Goursat and smeltzer confirmed that the oxidation rate of Fe is independent of oxygen partial pressure, but is proportional to √ T, and has double-layer structure oxidation film, which is composed of more than 95% inner layer of pyrite (Fe1 XO) and outer layer of hematite (Fe2O3) / magnetite (Fe3O4). In the research of zheludkevich, Trindade and pujilaksono and the review of Chen, there are also related research results on the oxidation behavior and film characteristics of Fe.

On the other hand, Simms and little et al. Studied the high temperature oxidation behavior of fe-2.25cr-1mo alloy steel, Hazan et al. Studied the high temperature oxidation behavior of AISI 4340 steel, bouhieda, Norden, nguyenet et al. Studied the selective oxidation behavior under oxygen partial pressure. In particular, G.C wood has well classified the oxidation behaviors among metal elements with different oxygen affinity in its review paper. It is pointed out that due to the different oxygen affinity of alloy elements, the diffusion coefficients in oxide film and matrix are also different. Therefore, the research on oxidation rate and oxide type needs to adopt a multi-level and comprehensive method. For this reason, the surface oxide film produced by carburizing is mainly investigated and its qualitative characteristics are studied for the high Cr Mo Si alloy steel which has been used more and more recently.

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