Carburizing and quenching heat treatment

Introduction to various quenching methods of carburizing and quenching heat treatment

Carburizing and quenching heat treatment is widely used in modern machinery manufacturing industry. The important parts of machinery, especially the steel parts used in automobiles, airplanes and rockets, almost all have undergone carburizing and quenching heat treatment. In order to meet the different technical requirements of various parts, various quenching processes have been developed.

Single medium quenching is the workpiece cooling in a medium, such as water quenching, oil quenching. Advantages are simple operation, easy to achieve mechanization, wide application. The disadvantage is that quenching stress in water, the workpiece is easy to deformation and cracking; Quenching in oil, the cooling rate is small, the diameter of quenching is small, the large workpiece is not easy to quench.

Dual-medium quenching means that the workpiece is cooled to about 300℃ in a medium with strong cooling capacity, and then cooled in a medium with weak cooling capacity. For example, water quenching followed by oil quenching can effectively reduce the internal stress of martensite transformation, reduce the tendency of deformation and cracking of the workpiece, and can be used for quenching the workpiece with complicated shape and uneven section. The disadvantage is that it is difficult to grasp the time of double liquid conversion, conversion too early is easy to quench not hard, conversion too late and easy to quench. In order to overcome this shortcoming, the step quenching method was developed.

Step quenching means that the workpiece is quenched in a low-temperature salt bath or alkali bath furnace. The temperature of salt bath or alkali bath is near Ms point. The workpiece stays at this temperature for 2min~5min, and then is taken out for air cooling. The purpose of fractional cooling is to make the temperature inside and outside the workpiece more uniform, while martensite transformation, can greatly reduce the quenching stress, to prevent deformation and cracking.

The fractional temperature was previously set at a little higher than Ms point, and the workpiece entered the martensite area after the internal and external temperature was even. Now improved for temperature grading at just below Ms. Practice shows that the effect of grading is better below Ms point. For example, high carbon steel mold in 160℃ alkali bath step quenching, can be hardened, deformation and small, so it is widely used.

Another type of carburizing and quenching heat treatment is isothermal quenching. The workpiece is quenched in an isothermal salt bath. The temperature of the salt bath is at the lower part of the bainite zone (slightly higher than Ms). Isothermal quenching is used for steels above medium carbon in order to obtain lower bainite to improve strength, hardness, toughness and wear resistance. Low carbon steels are generally not isothermal quenched.

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