Automobile gears are usually made of low-carbon alloy carburized steel. Gear blanks generally need to go through multiple cold and hot processing processes such as forging ※ pre heat treatment ※ cutting ※ carburizing and quenching ※ finishing, so as to obtain high surface hardness and good core toughness, so that the finished gears have excellent properties such as wear resistance, fatigue resistance and corrosion resistance. At present, normalizing and isothermal annealing are mainly used in the pre heat treatment of automotive gear forging blanks in China, and the trend of using isothermal annealing instead of normalizing treatment is shown. The commonly used heat treatment processes mainly include carburizing, carbonitriding, nitriding and induction quenching.
The pre heat treatment of automotive gear forging blank is mainly to obtain the appropriate surface hardness of the forging and prepare the metallographic structure for the second heat treatment. Traditional pre heat treatment methods mostly adopt conventional normalizing treatment. The process has the advantages of simple equipment, low energy consumption and low process requirements, so it is widely used. With the improvement of the quality requirements of automobile gears, especially the diversification of steel materials for imported models, the microstructure of steel is uneven because the microstructure transformation of steel is carried out in a certain temperature range during ordinary normalizing; Moreover, during batch normalizing cooling, the forging stock is often cooled in air or blown by air. Its cooling speed is easily affected by its position in the pile and the surrounding environment, resulting in large hardness fluctuation (156 ~ 207hb) of the same batch of parts. At the same time, it will increase the deformation during carburizing and quenching, and even produce bainite structure for steel with high hardenability. Therefore, the isothermal annealing process is gradually adopted for pre heat treatment in China, and the isothermal temperature is usually in the range of 560 ~ 650 ℃, so as to ensure that the gear blank can obtain uniform structure and hardness (160 ~ 180hb) after pre heat treatment in modern mass production, which is of great significance to reduce the carburizing and quenching deformation of gears and reduce the noise during operation. In foreign countries, isothermal annealing treatment is widely used for automotive gear forgings, and different isothermal annealing processes are specified for different materials, such as German Volkswagen, Mercedes Benz, Italian Fiat and so on. Foreign production practice shows that the gear after isothermal annealing not only greatly improves the machining performance, but also significantly reduces the deformation after carburizing and quenching.
Of course, isothermal annealing still needs to reheat the forging blank from room temperature to a high temperature above 900 ℃, which will consume a lot of energy. As the final forging temperature of automobile gear forging is generally above 900 ℃, at this time, the workpiece is still in the state of austenite. If it is cooled evenly to the pearlite transformation zone below a R1 for isothermal transformation, the microstructure and hardness similar to the conventional isothermal annealing can be obtained, so as to greatly save energy, improve labor productivity and improve the quality of forging blank. In recent years, isothermal annealing using forging waste heat has been studied and applied in the automotive industry, but there are still some technical difficulties in mass production.