The seamless steel tubes are mostly medium-sized, high-carbon alloy steels. After quenching, some of the supercooled austenite is not transformed into martensite, and remains in the used state to become retained austenite, which affects the performance. If it is cooled below zero, it can promote the martensite transformation of the retained austenite. Therefore, the essence of the cold treatment is that the quenching continues. The quenching stress at room temperature is superimposed on the quenching stress at zero, and the cold treatment crack is formed when the refolding stress exceeds the strength limit of the material.
Precautionary measures: (1) The seamless steel pipe is placed in boiling water for 30-60 minutes before quenching after quenching, which can eliminate 15%-25% quenching internal stress and stabilize retained austenite, then -60 °C Conventional cold treatment, or cryogenic treatment at -120 ° C, the lower the temperature, the more the retained austenite transforms into martensite, but it is impossible to complete the transformation. Experiments show that about 2%-5% retained austenite remains. Down, retain a small amount of retained austenite relaxation stress as needed, buffering effect, because the retained austenite is soft and tough, can partially absorb the martensite rapid expansion energy, and ease the phase transformation stress;
(2) cold treatment is completed After taking out the seamless steel pipe and putting it into the hot water to heat up, it can eliminate the 40%-60% cold treatment stress. After heating to room temperature, it should be tempered in time. The cold treatment stress is further eliminated, avoiding cold treatment crack formation, obtaining stable microstructure and ensuring seamless steel pipe products. No distortion occurs during storage and use.
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