According to the different cooling forms, the types of quenching mainly include single liquid quenching, double liquid quenching, graded quenching and isothermal quenching. Single liquid quenching. Single liquid quenching is a quenching operation method in which the austenitic workpiece is quickly immersed in a quenching medium and cooled to room temperature. The selection of cooling medium for single liquid quenching is based on the fact that the cooling speed of the workpiece in this medium must be greater than the critical cooling speed of the steel grade of the workpiece, and the workpiece should not be quenched and cracked. Single-liquid quenching medium includes water, salt water, alkaline water, oil and some specially prepared quenching agents. Double liquid quenching. In order to overcome the shortcomings of single liquid quenching and make the quenching and cooling of the workpiece as close as possible to the ideal situation, two media with different cooling capacities can be used together, that is, the heated workpiece is quenched in the first medium with large cooling capacity, cooled to a temperature slightly higher than Ms (about 300℃), and then immediately transferred to the second medium with small 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 adopted, and air can also be used as the cooling medium. Step quenching (martensite step quenching). This cooling method is characterized in that the workpiece is first immersed in a bath with a temperature slightly higher than Ms, and kept in the bath until the surface and center of the workpiece are cooled to the temperature of the bath, and then taken out for air cooling. The bath temperature is generally ms+( 10 ~ 20)℃. The medium in the bath is nitrate bath, alkali bath and neutral salt bath. Precooling and quenching. After quenching and heating, the workpiece is not immediately immersed in the cooling medium, but 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 precooling quenching or delayed quenching.

The key of precooling quenching is to control the precooling time, which is short and the effect is poor; Long time may reduce the quenching hardness (non-martensitic transformation) of the workpiece. Due to the different materials, shapes and sizes of workpieces, as well as the influence of furnace temperature and ambient temperature, it is difficult to accurately calculate the pre-cooling time, which mainly depends on the skills and experience of operators. Local quenching. Some workpieces have higher hardness in some parts, but no or lower hardness requirements in other parts. In this case, the method of local quenching can generally be adopted, 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 overall heating and local cooling. The former is mainly suitable for the workpiece heated by salt bath furnace, while the latter can be used for both box furnace and salt bath furnace. Cold treatment. Cold treatment is a subsequent quenching operation in which the quenched steel is continuously cooled to a certain temperature below room temperature, so that the untransformed retained austenite at room temperature continues to be transformed into martensite.