Quenching process and quenching medium

Quenching process is widely used in modern mechanical manufacturing industry. The important parts of machinery, especially the steel parts used in automobiles, airplanes and rockets, are almost quenched. This paper will briefly introduce the process and method of quenching and the types of commonly used quenching media.

Quenching process

The quenching process includes three stages: heating, heat preservation and cooling. Taking the quenching of steel as an example, the principle of selecting process parameters in the above three stages is introduced.

1. Quenching and heating

Based on the critical point of transformation of steel, fine and uniform austenite grains should be formed during heating and quenching, and fine martensite structure can be obtained after quenching.

2. Quenching and heat preservation

1) The time of quenching and heat preservation is determined by many factors, such as heating mode of equipment, size of parts, composition of steel, charging quantity and power of equipment. For the whole quenching, the purpose of heat preservation is to make the internal temperature of the workpiece uniform. For all kinds of quenching, the holding time depends on obtaining a good quenching heating structure in the area where quenching is required.

2) Heating and heat preservation are the important links that affect the quenching quality. The microstructure obtained by austenitizing directly affects the properties after quenching. -The austenite grain of general steel is controlled in 5-8 grades.

3. Quenching and cooling

1) In order to change the high temperature phase austenite into low temperature metastable phase martensite in the cooling process, the cooling rate must be higher than the critical cooling rate of steel. In the process of cold metal material processing, there is a constant difference in the cooling rate between the metal material processing surface and the center. If the difference is large enough, it may cause the part greater than the critical cooling rate to be transformed into martensite, while the part less than the critical cooling rate cannot be transformed into martensite.

2) In order to ensure that the whole section is transformed into martensite, it is necessary to select quenching medium with enough cooling capacity to ensure that the workpiece center has enough high cooling speed. However, due to the large cooling rate, the uneven expansion and contraction of the workpiece may cause internal stress, which may cause deformation or cracking of the workpiece. Therefore, the above two contradictory factors should be considered, and the quenching medium and cooling mode should be selected reasonably.

3) In the cooling stage, it is the key link of the quenching process to not only obtain the reasonable structure and achieve the required performance, but also to maintain the size and shape accuracy of the parts.

Quenching method

1. Single medium quenching

The workpiece is cooled in a medium, such as water quenching and oil quenching. The advantages are simple operation, easy to realize mechanization and wide application. The disadvantage is that the quenching stress in water is large, the workpiece is easy to deform and crack; the quenching speed in oil is small, the quenching diameter is small, and the large workpiece is not easy to harden.

2. Double medium quenching

1) The workpiece is cooled to about 300 ℃ in a medium with strong cooling capacity, and then cooled in a medium with weak cooling capacity, such as water quenching followed by oil quenching, which can effectively reduce the internal stress of martensitic transformation, reduce the tendency of deformation and cracking of the workpiece, and can be used for quenching of workpieces with complex shape and uneven section.

2) The disadvantage of double liquid quenching is that it is difficult to master the time of double liquid conversion. The conversion is too early and easy to harden, and the conversion is too late and easy to crack. In order to overcome this shortcoming, the classification quenching method has been developed.

3. Graded quenching

1) 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 2min-5min, and then it is taken out for air cooling. This cooling method is called graded quenching. The purpose of staged cooling is to make the temperature inside and outside the workpiece more uniform, and to conduct martensitic transformation at the same time, which can greatly reduce the quenching stress and prevent deformation and cracking.

2) Before the classification temperature was set at a point slightly higher than MS, the temperature inside and outside the workpiece was even and then entered the martensite region. Improved to a temperature classification slightly below Ms. The practice shows that the effect of grading below MS point is better. For example, the high carbon steel mould can be quenched in the alkaline bath at 160 ℃, which can not only harden, but also deform little, so it is widely used.

4. Isothermal quenching

The workpiece is quenched in an isothermal salt bath, and the temperature of the salt bath is at the lower part of the bainite region (slightly higher than MS). The workpiece stays in the isothermal state for a long time until the bainite transformation is over, and then it is taken out for air cooling. The purpose of Austempering for steel above medium carbon is to obtain lower bainite to improve its strength, hardness, toughness and wear resistance. Isothermal quenching is not generally used for low carbon steel.

5. Surface hardening

Surface quenching is a kind of local quenching method that hardens the surface layer of steel to a certain depth, while the core part remains in the unhardened state. During the surface quenching, the surface of the rigid part is heated rapidly to the quenching temperature, and it is cooled immediately before the heat penetrates into the center of the workpiece to realize the local quenching.

6. Induction hardening

Induction heating is to use electromagnetic induction to generate eddy current in the workpiece and heat the workpiece.