There are many reasons for the longitudinal cracking of thick-walled seamless steel pipes, usually are material and process. For example, if the thick-walled seamless steel pipe has poor plasticity or poor annealing, the probability of longitudinal cracking will increase, but the fundamental reason is that the cold-drawn thick-walled seamless steel pipe has a large residual stress.

According to the concept of cracking mechanics, cracking of components is the result of the formation and propagation of internal microcracks. Longitudinal cracks of thick-walled seamless steel tubes are basically brittle cracks, and the cracks are open.

Generally, the existence of metal micro-cracks is inevitable, but under the same material conditions, the occurrence of cracks depends on the tensile stress of the component. The tensile stress that affects the component can be loaded outside the component, or it can exist in the residual stress of the component.

After cold drawing, especially after empty drawing, the residual stress is large, which is expressed as the tangential and axial tensile stress near the outer surface of the thick-walled seamless steel pipe, and its value can reach a large value. According to the above tensile cracking mechanism, it can be concluded that the tangential residual stress near the outer surface of the thick-walled seamless steel pipe plays a key role in the longitudinal cracking of the thick-walled seamless steel pipe.