The heat treatment process of carburized steel includes preparatory heat treatment and carburizing and quenching process, in which heat treatment includes ordinary normalizing, isothermal normalizing, normalizing + tempering, and isothermal annealing. Carburizing and quenching mainly includes pre-cooling and direct quenching after carburizing, primary quenching after carburizing and air cooling after carburizing, or secondary quenching after carburizing, and tempering after carburizing and quenching.

At present, the commonly used standard for gear steel is JB/T7516-1994 “Gear gas carburizing heat treatment process and quality control”. In the carburizing process, the best carburized layer quality and the minimum deformation, improve the quality of the gear. Carburizing can only change the chemical composition of the surface of the part. To make the part hard on the outside and tough on the inside, it must be quenched and tempered at low temperature after carburizing heat treatment to improve the strength and toughness of the steel and stabilize the size of the part. According to the composition, shape and mechanical properties of the workpiece, the following heat treatment methods are often used after carburizing.

1) Direct quenching + low temperature tempering

The part is quenched directly from the heat treatment furnace, then tempered to obtain the required hardness of the surface. There are two conditions for direct quenching: the austenite grain size after carburizing heat treatment is above grade 5-6; there is no obvious network and massive carbide in the carburizing layer. Most steels such as 20CrMnTi are directly quenched after carburizing.

2) Pre-cooling direct quenching + low temperature tempering

The purpose of pre-cooling is to reduce the deformation of parts and reduce the residual austenite on the surface due to the precipitation of carbides. The surface hardness of pre-cooling and direct quenching increases slightly, but the grains do not change. The pre-cooling temperature should be higher than Ar3 to prevent the precipitation of ferrite in the core. Too high temperature will affect the precipitation of carbides during pre-cooling and the amount of retained austenite Increase, but also increase the quenching deformation.

3) One-time heating quenching + low temperature tempering

The carburized parts are quickly cooled to room temperature and then reheated for quenching and low-temperature tempering. It is suitable for parts that require higher strength and better toughness for the core after quenching.

4) High temperature tempering + quenching + low temperature tempering

After high-temperature tempering, the retained austenite decomposes, and the carbon and alloying elements in the carburized layer are precipitated in the form of carbides, which is easy to machine and reduce the retained austenite. It is mainly used for Cr-Ni alloy steel parts.

5) Secondary quenching + low temperature tempering

After the workpiece is cooled to room temperature, it is quenched twice, and then tempered at low temperature. This is a heat treatment method that ensures both the core and the surface to obtain high performance. The double quenching is beneficial to reduce the amount of retained austenite on the surface.

6) Secondary quenching + cold treatment + low temperature tempering

Also known as high-alloy steel heat treatment to reduce the amount of residual austenite on the surface, it is mostly used for gear and shaft parts.