What are the differences between straight seam steel pipes and spiral steel pipes

The production process of straight seam steel pipes is relatively simple, mainly involving high-frequency welded straight seam pipes and submerged arc welded straight seam pipes. Straight seam steel pipes have high production efficiency, low cost, and rapid development. Spiral steel pipes generally have higher strength than straight seam steel pipes. The main production process is submerged arc welding. Spiral steel pipes can be produced from steel billets of the same width but with different diameters, and can also be produced from narrow steel billets with larger diameters. However, compared to straight seam steel pipes of the same length, the welding length increases by 30% and 100%, respectively, resulting in lower production speeds. Therefore, large-diameter steel pipes are mostly welded using spiral welding, while small-diameter steel pipes are mostly welded using straight seam welding. In industrial production of large-diameter straight seam steel pipes, T-welding technology is used, which involves butt-jointing a small portion of straight seam steel pipes to meet the required length for a project. T-welding straight seam steel pipes has significantly increased defects, and the residual stress in the T-weld is relatively large. The weld metal is usually under triaxial stress, which increases the likelihood of cracking. In terms of welding technology, the welding methods for spiral welded steel pipes and straight seam welded steel pipes are the same. However, a large number of T-shaped welds are unavoidable, and the residual stress in these welds is very high. Therefore, the possibility of welding defects is also high. After improvement, the weld metal is usually under triaxial stress, which increases the possibility of cracking.

Furthermore, according to the technical specifications for submerged arc welding, each spiral welded steel pipe should undergo arc initiation and extinguishing treatments. However, this condition cannot be met when welding circumferential seams on each spiral welded steel pipe, thus potentially leading to more welding defects during the arc extinguishing process.

When a spiral welded steel pipe is subjected to internal pressure, two main stresses are typically generated on the pipe wall: radial stress and axial stress. The combined stress at the weld of a spiral welded steel pipe is denoted by α, where α is the helix angle of the weld.

The combined stress at the weld of a spiral welded steel pipe is the principal stress of a straight seam welded steel pipe. Under the same working pressure, the wall thickness of a spiral welded steel pipe of the same diameter is less than that of a straight seam welded steel pipe.

When parallel defects appear near the weld seam of a spiral welded steel pipe, the expansion risk of the spiral weld seam is lower than that of a straight weld seam in a straight seam welded steel pipe due to the smaller force exerted by the weld seam. Since radial stress is the maximum stress on a straight seam welded steel pipe, the weld seam bears the maximum load in the direction perpendicular to the stress direction. The joint bears the largest load, the circumferential weld bears the smallest load, and the spiral weld bears a load in between.

The development trend of welded steel pipes is towards larger diameters and higher strength. With the increase in the diameter and steel grade of welded steel pipes, the tendency for stable expansion at the ductile fracture tip is greater. Although spiral and straight seam welded steel pipes have the same grade, spiral welded steel pipes exhibit higher impact toughness.