1. Single medium quenching
The workpiece is cooled in one medium, such as water quenching and oil quenching. The advantages are simple operation, easy mechanization and wide application. The disadvantage is that it is only one cooling speed.
2. Dual medium quenching
The workpiece is cooled to about 300 degrees Celsius in a medium with strong cooling capacity, and then cooled in a medium with weaker cooling capacity, such as water quenching and then oil quenching, which can effectively reduce the internal stress of martensitic transformation and reduce the tendency of workpiece deformation and cracking. It can be used for quenching workpieces with complex shapes and uneven cross-sections. The disadvantage of dual medium quenching is that it is difficult to grasp the moment of dual medium conversion. If the conversion is too early, it is easy to quench hard, and if the conversion is too late, it is easy to quench and crack. In order to overcome this disadvantage, the graded quenching method was developed.
3. Graded quenching
The workpiece is quenched in a low-temperature salt bath or alkali bath furnace. The temperature of the salt bath or alkali bath is near the Ms point. The workpiece stays at this temperature for 2-5 minutes and then is taken out for air cooling. This cooling method is called graded quenching. The purpose of graded cooling is to make the temperature inside and outside the workpiece more uniform, and at the same time to carry out martensitic transformation, which can greatly reduce quenching stress and prevent deformation and cracking. The grading temperature was previously set slightly higher than the Ms point. After the temperature inside and outside the workpiece is uniform, it enters the martensitic zone. Now it has been improved to a temperature grading slightly lower than the Ms point. Practice shows that the effect of grading below the Ms point is better. For example, high-carbon steel molds are graded quenched in an alkali bath at 160 degrees Celsius, which can be hardened and deformed less, so it is widely used.
4. Isothermal quenching
The workpiece is quenched in an isothermal salt bath. The salt bath temperature is at the lower part of bainite (slightly higher than Ms). The workpiece stays isothermally for a long time until the bainite transformation is completed and taken out for air cooling. Austempering is used for medium carbon steel and above to obtain lower bainite to improve strength, hardness, toughness and wear resistance. Low carbon steel is generally not subjected to austempering.
round steel tube
