Analysis of cold treatment cracks of seamless steel pipes

For seamless steel pipe high-carbon alloy steel, some subcooled austenite has not been transformed into martensite after quenching, and it remains in use as retained austenite, which affects its 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 cold treatment cracks are formed when the stack response force exceeds the strength limit of the material.

Preventive measures: (1) After the seamless steel pipe is quenched, the seamless steel pipe is boiled in boiling water for 30-60 minutes before cold treatment, which can eliminate 15%-25% of the quenching internal stress and stabilize the retained austenite, and then carry out -60℃ Conventional cold treatment or 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% retained austenite remains Afterwards, retaining a small amount of retained austenite as needed can relax the stress and act as a buffer. Because the retained austenite is soft and tough, it can partially absorb the energy of the rapid expansion 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.