Most seamless steel pipes are medium carbon and high carbon alloy steel. After quenching, some undercooled austenite does not change into martensite, and remains in service as residual austenite, which affects the service performance.
If it is cooled below zero, the martensitic transformation of retained austenite can be promoted. So the essence of cold treatment is to continue quenching. The room temperature quenching stress and zero quenching stress are superimposed. When the reactor response force exceeds the strength limit of the material, a cold treatment crack is formed.
(1) After quenching and cold treatment, the seamless steel pipe can be placed in boiling water for 30-60 minutes to eliminate 15% - 25% of the internal quenching stress, stabilize the residual austenite, and then carry out conventional cold treatment at - 60 ℃ or - 120 ℃ cryogenic treatment. The lower the temperature, the more residual austenite will be transformed into martensite, but it is impossible to complete the transformation. The experiment shows that the residual austenite is about 2% - 5%. It is necessary for the press to retain a small amount of residual austenite to relax the stress and play a buffering role. Because the residual austenite is soft and tough, it can partially absorb the energy of rapid expansion of martensite and relieve the transformation stress;
(2) After the cold treatment, take out the seamless steel pipe and heat it in hot water, which can eliminate 40% - 60% of the cold treatment stress. Temper in time after the temperature rises to room temperature. The cold treatment stress is further eliminated to avoid the formation of cold treatment cracks and obtain stable microstructure and properties. Ensure that seamless steel pipe products do not deform during storage and use.