How can cracks appear in 45# seamless steel pipe?
45# seamless steel pipes are mostly medium and high carbon steels. After heat treatment, some low-temperature ferrite is not transformed into austenite and becomes retained austenite in application, which endangers performance indicators. If it is placed below zero and cooled again, it can promote the change of retained austenite. Therefore, the essence of cold violence is heat treatment again. The in-situ stress of heat treatment under indoor temperature and that under zero temperature are accumulated, and cold violent cracks will occur when the superimposed in-situ stress exceeds the strength limit of the raw material.
Precautions:
(1) 45# seamless steel pipe is cooled after heat treatment. Before boiling 45# seamless steel pipe in boiling water for 30-60min, 15% - 25% heat treatment heat stress can be removed and residual austenite can be prevented from aging. Then basic cold violence at - 60 ℃ or deep cold violence at - 120 ℃ can be carried out. The lower the temperature, the more retained austenite will be transformed into austenite, but it is unlikely that all changes will be completed. Test instructions, about 2% - 5% of retained austenite is preserved. A small amount of retained austenite can relax the in-situ stress and play a regulatory role. Because the retained austenite is soft and tough, it can partially digest and absorb the austenitizing kinetic energy, greatly increase the kinetic energy and alleviate the change of in-situ stress;
(2) After the cold treatment, remove the 45# seamless steel pipe and put the funds into boiling water to raise the temperature, which can remove 40% - 60% of the cold violent in-situ stress. After the temperature is raised to the indoor temperature, it shall be quenched immediately, and the cold violent in-situ stress shall be further removed to prevent the cold violent crack, obtain the stable mechanism characteristics, and ensure that the 45# seamless steel pipe will not change during commodity storage and application.