The stainless steel seamless steel pipes
are mostly medium and high carbon and alloy steels. After quenching, some of the subcooled austenite has not been transformed into martensite, and it remains in the service state to become retained austenite, which affects the performance. 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 when the stack response force exceeds the strength limit of the material, cold treatment cracks are formed.
Cold treatment crack prevention measures:
(1) After the seamless steel pipe is quenched and cold treated, the seamless steel pipe is placed in boiling water for 30-60 minutes, which can eliminate 15%-25% of the internal quenching stress and stabilize the retained austenite, and then perform the conventional cold treatment at -60°C. Or perform cryogenic treatment at -120℃, the lower the temperature, the more the amount of retained austenite transformed into martensite, but it is impossible to complete the transformation. Experiments show that about 2%-5% of retained austenite remains. Press It is necessary to retain a small amount of retained austenite to relax the stress and play a buffering role. 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, the seamless steel pipe
is taken out and put 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. The cold treatment stress is further eliminated to avoid the formation of cold treatment cracks and obtain stable tissue performance. Ensure that seamless steel pipe products are not deformed during storage and use.