Several heat treatment methods after carburizing

Carburizing can only change the chemical composition of the surface of the part, so that the part can obtain the outer hard and tough properties. After the carburizing heat treatment, it must be quenched and low temperature tempered to improve the toughness of the steel and the size of the stable parts. According to the composition, shape and mechanical properties of the workpiece, the following heat treatment methods are often used after carburizing.

Several heat treatment methods after carburizing
1) Direct quenching + low temperature tempering
The parts are taken out of the heat treatment furnace for direct quenching and then tempered to obtain the desired hardness of the surface. There are two conditions for direct quenching: the austenite grain size after carburizing heat treatment is above 5-6; there is no obvious network and massive carbide in the carburized layer. Steels such as 20CrMnTi are mostly directly quenched after carburizing.

2) Pre-cooling direct quenching + low temperature tempering
The purpose of pre-cooling is to reduce the deformation of the part and reduce the residual austenite on the surface due to the precipitation of carbides. The surface hardness of the pre-cooled direct quenching is slightly improved, but the crystal grains are not changed. The pre-cooling temperature should be higher than Ar3 to prevent the precipitation of ferrite in the core. The excessive temperature affects the precipitation of carbides during pre-cooling, and the amount of retained austenite. The increase also increases the quenching deformation.

3) One-time heating quenching + low temperature tempering
The carburizing part is cooled to room temperature and then reheated for quenching and low temperature tempering, and is suitable for parts with high strength and good toughness requirements after core quenching.

4) high temperature tempering + quenching + low temperature tempering
After high temperature tempering, the retained austenite decomposes. The carbon and alloying elements in the infiltrated layer are precipitated as carbides, which is easy to be machined and the retained austenite is reduced. 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 high performance of both the core and the surface. Two 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 retained austenite in the surface, mostly used in gears and shaft parts.