Analysis and preventive measures of stainless steel seamless pipe cold treatment cracks

Stainless steel seamless pipes are typically made of medium to high carbon and alloy steels. After quenching, some subcooled austenite may not fully transform into martensite, resulting in the presence of retained austenite. This retained austenite can negatively impact the pipe's performance. By further cooling below zero, the transformation of retained austenite into martensite is promoted, making cold treatment essentially an extension of quenching. However, the stresses from quenching at room temperature combined with those from sub-zero treatment can exceed the material’s strength limit, leading to cold treatment cracks.

Preventive Measures for Cold Treatment Cracks

- Post-Quenching and Cold Treatment:

After quenching and cold treatment, placing the stainless steel seamless pipes in boiling water for 30-60 minutes helps to relieve 15%-25% of internal quenching stress. This process stabilizes the retained austenite. Following this, cold treatment can be performed at -60°C or cryogenic treatment at -120°C. Lower temperatures promote greater transformation of retained austenite into martensite, though complete transformation is unattainable—approximately 2%-5% of retained austenite will remain. A small amount of retained austenite is beneficial, as its softness and toughness absorb the rapid expansion energy of martensitization, helping to alleviate transformation stress.

- Post-Treatment Heating:

Once cold treatment is complete, heating the pipe in hot water can eliminate 40%-60% of the remaining cold treatment stress. After the pipe returns to room temperature, it should be tempered promptly to further eliminate residual stress, thus preventing cold treatment cracks. This also ensures stable material properties, reducing the risk of deformation during storage and use.