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What is cold treatment crack?

What is cold treatment crack?

In principle, cold treatment is carried out immediately after quenching. When the quenched parts are placed at room temperature, the residual austenite is stabilized. The transformation of residual austenite is very slow and it is difficult to martensite. However, if the cold treatment is carried out immediately after quenching, the same crack as the quenching crack will occur because the stress generated by quenching is superimposed with the stress generated by martensitization of residual austenite.

This kind of crack is a sub zero crack. The stress of small parts is small, so it is not easy to have cold treatment cracks. It can be said that cold treatment cracks must occur in large parts or thick wall parts. To prevent cold treatment cracks, it is best to temper slightly at 100 ~ 130  before cold treatment. Stabilize the retained austenite, and then carry out cold treatment after reducing the stress due to quenching. Although this can prevent the occurrence of cold treatment cracks, the stabilized retained austenite will remain intact. About 5% of the retained austenite is in a stabilized state and plays a buffer role, which is a great advantage. Before cold treatment, tempering in 100 boiling water is the key. Sometimes, due to the measurement of quenching hardness, cold treatment cracks will also occur at the indentation, which is the residual stress left after the hardness test, which will become the inducement of cracks.

20220306053640 43759 - What is cold treatment crack?

When the cold treatment temperature rises to room temperature, the common practice is to put the cold treated parts in air, water or hot water. This method is called cold treatment and rapid heating method (up-hill quenching). Cold treatment and rapid heating method is useful to eliminate the thermal stress caused by low-temperature rapid cooling. If the parts after cold treatment are placed in the air, cracks often occur (called cold treatment and temperature rise cracks), which can be prevented by cold treatment and rapid heating method.
There are two kinds of cold treatment: ordinary cold treatment with dry ice (- 78 ℃) and ultra cold treatment with liquid nitrogen (196 ℃). Among them, the effect of super cooling treatment is the best. However, the key problem is that no matter what kind of cold treatment, the parts must be tempered in 100C boiled water before cold treatment, and the parts shall be put into water or hot water for cold treatment and rapid heating after cold treatment. To prevent cold treatment cracks.

Why do cold treatment cracks occur?

There are large quenching stresses (thermal stress and transformation stress) and 10 ~ 30% residual austenite (AR) in the quenched steel. When such a steel is cold treated, the martensitization of retained austenite will produce transformation stress. The stress is superimposed with the existing quenching stress to cause the same crack as the quenching crack, which is the cold treatment crack. If decarburization occurs on the surface of the part (decarburization layer with specific carbon residue rate of C > 0.4%), the cold treatment crack of the mesh will also occur. In addition, cold treatment cracks often occur when it rises from low temperature to room temperature and is placed in air. There are two reasons for cold treatment cracks:

  • 1. Is the stress caused by rapid cooling.
  • 2. Martensitization of retained austenite.

This kind of crack is called cold treatment heating crack, and it can be concluded that it is the crack that appears when the quenched steel is cold treated.

When do cold treatment cracks occur?

When quenched steel parts are cold treated without other treatment, cold treatment cracks will occur during low-temperature cooling or low-temperature insulation. It is the same as quenching crack. Sometimes it rises from low temperature to room temperature and cracks at – 20 ~ – 30 ℃. Most of the cracks in low-temperature cooling can be heard from the sound.
How to prevent cold treatment cracks
Quenching stress increases due to the increase of cold treatment surface, resulting in cold treatment cracks. Therefore, the quenching stress should be reduced before cold treatment. Do not carry out cold treatment immediately after quenching. It is best to temper with 100 ℃ boiling water and keep it warm for about one hour before cold treatment. Tempering at low temperature of 100 ℃ can eliminate about 25% of quenching stress. However, it is not limited to tempering with 100 ℃ boiling water. Tempering at 200 ℃ can also be used. Tempering at 200 ℃ can eliminate 50% of quenching stress, so it is more effective. Some people worry that tempering before cold treatment will stabilize retained austenite. In fact, the stabilized retained austenite can play a buffer role, which is conducive to the improvement of properties.

The cold treatment and rapid heating method from low temperature into water or hot water can eliminate about 50 ~ 60% of the cold treatment stress and effectively prevent cold treatment cracks. If there is decarburization layer, cold treatment cracks are more likely to occur, so it is best to remove the decarburization layer before cold treatment. Of course, the prerequisite is that there is no decarburization layer during quenching and heating. In addition, due to the residual stress in the indentation left by the measurement of quenching hardness, cold treatment cracks often occur from the indentation. Therefore, it is necessary to avoid hardness before cold treatment. If the hardness has been hit and the indentation is left, the cold treatment must be carried out after the hardness is measured and tempered with 100C boiled water.

Source: Network Arrangement – China Pipe Fittings Manufacturer: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]

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