What are the types of heat treatment processes for quenching equipment?

(1) Liquid quenching

Single-liquid quenching is a quenching operation method in which the austenitic workpiece is quickly immersed in a certain quenching medium and cooled to room temperature. The selection of single liquid quenching cooling medium is based on the fact that the cooling rate of the workpiece in this medium must be greater than the critical cooling rate of the workpiece steel, and the workpiece should not be quenched and cracked. Single liquid quenching media include water, brine, alkaline water, oil, and some specially formulated water-based quenching agents.

(2) Double liquid quenching

In order to overcome the shortcomings of single-liquid quenching and make the quenching and cooling of the workpiece as close to the ideal situation as possible, two media with different cooling capacities can be used together, that is, the heated workpiece is quenched into the first medium with large cooling capacity, and cooled to a slightly lower temperature. Above Ms temperature (about 300), then immediately transferred to a second medium with less cooling capacity to cool to room temperature. This quenching cooling method is called double liquid quenching. For some workpieces, in order to further slow down the cooling rate below Ms, water quenching air cooling or oil quenching air cooling can also be used, and air can also be used as a cooling medium.

(3) Staged quenching (martensite staged quenching)

The characteristic of this cooling method is that the workpiece is first immersed in a molten pool with a temperature slightly higher than Ms, and then kept in the molten pool until the surface and center of the workpiece cool to the temperature of the molten pool, and then taken out for air cooling. The bath temperature is generally 10 to 20. The medium in the bath consists of nitrate bath, alkali bath and neutral salt bath.

(4) Pre-cooling and quenching

After high-frequency quenching, the workpiece is not immersed in the cooling medium immediately, but is cooled in the air for a short time, and then immersed in the cooling medium after the workpiece is cooled to a certain temperature. This quenching method is called pre-cooling quenching or delayed quenching.

The key to pre-cooling is to control the pre-cooling time, and the effect of short pre-cooling time is poor. Long periods of time may reduce the quenching hardness of the workpiece (non-martensitic transformation). Due to the different materials, shapes and sizes of workpieces, as well as the influence of furnace temperature and ambient temperature, the pre-cooling time is difficult to calculate accurately and mainly depends on the skill and experience of the operator.

(5) Local quenching

Some workpieces only require one part to have a higher hardness, and other parts have no hardness requirements or require a lower hardness. In this case, a local quenching method can generally be used, that is, only a certain part of the workpiece is quenched. There are two main forms of local quenching: local heating and local cooling and bulk heating and local cooling. The former is mainly suitable for heating workpieces in salt bath furnaces, while the latter can be used in box furnaces and salt bath furnaces.

(6) Cold treatment

Cold treatment is a post-quenching operation in which the quenched steel is continuously cooled to a temperature below room temperature, so that the untransformed retained austenite at room temperature is continuously transformed into martensite.

For some parts with high dimensional stability, it is necessary to reduce the retained austenite in the quenched structure to the lowest temperature, so as to avoid failure due to changes in shape and size exceeding the accuracy requirements during use. That’s what cold processing is for. The cold treatment temperature is mainly determined according to the Ms point of the steel, combined with the technical requirements of the parts, process equipment conditions and other factors. After the quenched workpiece is cooled to room temperature, it must be cold-treated immediately, otherwise its effect will be affected. The cold treatment of small and medium pieces is generally maintained for 1 to 3 hours, and should be slowly heated in the air after treatment. When the workpiece is heated to room temperature, it should be tempered immediately, which can effectively prevent the workpiece from cracking.