Common heat treatment methods after carburizing small fasteners

In order to improve the performance, quality, surface smoothness and aesthetics of small fasteners, fasteners are often surface treated - carburizing. In order to further obtain high quality, high hardness and wear resistance of fasteners, many manufacturers It will undergo heat treatment process after carburizing of fasteners.

After carburizing small fasteners, common heat treatment methods are as follows:

Direct quenching + low temperature tempering

The parts are removed from the heat treatment furnace, quenched directly, and then tempered to obtain the required surface hardness. There are two conditions for direct quenching: after carburizing heat treatment, the austenite grain size is above level 5-6; there are no obvious network and massive carbides in the carburized layer. Steels such as 20CrMnTi are mostly directly quenched after carburizing.

Pre-cooling direct quenching + low temperature tempering

The purpose of precooling is to reduce the deformation of the parts and reduce the residual austenite on the surface due to the precipitation of carbides. The surface hardness of precooling direct quenching increases slightly, but the grains do not change. The precooling temperature should be higher than Ar3 to prevent the precipitation of ferrite in the center. Excessive temperature affects the precipitation of carbides during the precooling process and the amount of retained austenite. Increase, while also increasing the quenching deformation.

One-time heating quenching + low temperature tempering

Quickly cool the carburized parts to room temperature and then reheat them for quenching and low-temperature tempering. It is suitable for parts that require higher strength and better toughness in the core after quenching.

High temperature tempering + quenching + low temperature tempering

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

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 high performance in both the core and the surface. Double quenching is beneficial to reducing the amount of retained austenite on the surface.

Secondary quenching + cold treatment + low temperature tempering

It is also known as the heat treatment of high alloy steel to reduce the amount of retained austenite on the surface. It is mostly used for gears and shaft parts.