What causes straight seam steel pipes to bend during production

Straightening of straight seam steel pipes is an important process in the production of straight seam steel pipes. Especially for oil casing and oil and gas pipes of API standard with high-quality requirements, and special pipes for mechanical equipment, these steel pipes not only have strict requirements on steel grade and weld quality but also have high requirements on the straightness of steel pipes.

The deviation of straightness is directly related to the processing and connection of pipe end threads and pipe clamps of oil casing and conveying pipes, as well as the distortion and deformation of steel pipes during use. There are two processing forms of pipe end threading-steel pipe rotation and tool rotation. Most threading processes use steel pipe rotation, which requires higher straightness of steel pipes.

So why do steel pipes bend during production? There are many reasons for the bending of steel pipes, such as the heat effect of welds during welding, eccentricity during forming, and unbalanced compression and bending forces. But fundamentally, bending is the effect of internal stress in steel pipes. Simply put, bending is a stress imbalance. So, does a straight steel pipe have no internal stress? No. Straight steel pipes also have internal stress, but the internal stress of straight steel pipes is smaller.

What is internal stress? Internal stress is a term in physical mechanics. Its essence is the interaction force between molecules when the material is deformed by temperature and external forces. When steel pipes are formed and welded, they will also be affected by external forces such as welding temperature and forming bending to produce internal stress. The cross-section of the steel pipe is a ring, and two basic stresses will be generated on this ring area: forces parallel to the ring and forces perpendicular to the ring. Parallel stress will make the steel pipe out of round; vertical stress will make the steel pipe bend. Therefore, there is a cold expansion process in the production process of straight seam steel pipes, the purpose of which is to eliminate the internal stress of the steel pipe and increase the use strength of the straight seam steel pipe.