After quenching, there is still part of the subcooled austenite that has not been transformed into martensite, which remains in use and becomes retained austenite, which affects the performance of 40Cr seamless steel tube. If it is placed below zero to continue cooling, it can promote the martensite transformation of retained austenite. Therefore, the essence of cold treatment is to continue quenching. The quenching stress at room temperature and the quenching stress at zero are superimposed, and cold treatment cracks are formed when the stack response force exceeds the strength limit of the material.

Preventive measures: (1) After quenching the 40Cr seamless steel tube, placing the 40Cr seamless steel tube in boiling water for 30-60 minutes before cold treatment can eliminate 15%-25% of the internal quenching stress and stabilize the retained austenite before proceeding- Conventional cold treatment at 60℃, or deep cooling treatment at -120℃, the lower the temperature, the greater the amount of retained austenite transformed into martensite, but it is impossible to complete the transformation. Experiments show that there is about 2%-5% of retained austenite. The retained austenite can relax the stress and play a buffering effect if necessary to retain a small amount of retained austenite. Because the retained austenite is soft and tough, it can partially absorb the rapid expansion energy of martensitization and ease the transformation stress.

(2) After the cold treatment is completed, take out the 40Cr seamless steel tube and put it in hot water to heat up, which can eliminate 40%-60% of the cold treatment stress. After the temperature is raised to room temperature, it should be tempered in time, and the cold treatment stress will be further eliminated to avoid the formation of cold treatment cracks and obtain stable tissue performance, and to ensure that the 40Cr seamless steel tube is not deformed during storage and use.