The selection of CS seamless pipe (SMLS) quenching method is mainly based on obtaining martensite and reducing internal stress, workpiece deformation and cracking. Commonly used quenching methods for CS seamless pipes include single-medium quenching, double-medium quenching, graded quenching and austempering.
1) Single medium quenching
The workpiece is cooled in a medium, such as water quenching, oil quenching. The advantages are simple operation, easy mechanization and wide application. The downside is that it's just a cooling rate.
2) Dual medium quenching
The workpiece is first cooled to about 300 degrees Celsius in a medium with a strong cooling capacity, and then cooled in a medium with a weak cooling capacity, such as water quenching first and then oil quenching, which can effectively reduce the internal stress of martensitic transformation and reduce the deformation of the workpiece The tendency of cracking 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 shortcoming, a graded quenching method has been 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 it is taken out for air cooling. This cooling method is called step quenching. The purpose of staged cooling is to make the internal and external temperature of the workpiece more uniform, and at the same time carry out martensitic transformation, which can greatly reduce the quenching stress and prevent deformation and cracking. The classification temperature was previously set at slightly higher than the Ms point, and after the internal and external temperature of the workpiece was uniform, it entered the martensitic zone. It is now improved to a temperature classification just below the Ms point. Practice shows that the effect of grading below the Ms point is better. For example, high-carbon steel molds are quenched step by step in an alkali bath at 160 degrees Celsius, which can be hardened and deformed small, so it is widely used.
4) Austempering
The workpiece is quenched in an isothermal salt bath, and the temperature of the salt bath is at the lower part of the bainite (slightly higher than Ms). The workpiece isothermally stays for a long time until the transformation of the bainite is completed, and then it is taken out and air-cooled. Austempering is used for steels above medium carbon to obtain lower bainite to improve strength, hardness, toughness and wear resistance. Low carbon steel generally does not use austempering.
