The production process of straight seam steel pipe is relatively simple, mainly involving high-frequency welded straight seam pipes and submerged arc welded straight seam pipes. Straight seam pipes have high production efficiency, low cost, and rapid development. The strength of spiral steel pipes is usually higher than that of straight seam steel pipes. The main production process of spiral steel pipes is submerged arc welding. Spiral steel pipes can be produced with different diameters from the same width of steel plate, or larger diameter pipes can be produced from narrower steel plates. However, compared to straight seam pipes of the same length, the weld length increases by 30% to 100%, resulting in lower production speed. Therefore, large-diameter pipes are mostly produced using spiral welding, while small-diameter pipes are mostly produced using straight seam welding. When producing large-diameter straight seam pipes in the industry, T-shaped welding is used, which means small segments of straight seam pipes are butt-welded and connected to meet the required project length. The defects of T-shaped straight seam pipes increase significantly, and the residual welding stress at T-shaped welds is large. The weld metal is usually in a triaxial stress state, which increases the possibility of cracking.
In terms of welding technology, the welding methods of spiral steel pipes and straight seam steel pipes are the same, but a large number of T-shaped welds are inevitable, and the residual welding stress at T-shaped welds is very high. Therefore, the possibility of welding defects is also high. After improvement, the weld metal is usually in a triaxial stress state, which increases the possibility of cracking.
In addition, according to the technical requirements of submerged arc welding, each weld seam should be treated with arc striking and arc extinguishing, but each steel pipe cannot meet this condition when welding the circumferential seam, so there may be more welding defects during arc extinguishing.
When the pipeline is subjected to internal pressure, two main stresses are usually generated on the pipe wall: radial stress and axial stress. The comprehensive stress at the weld seam, where α is the spiral angle of the spiral steel pipe's weld.
The comprehensive stress at the spiral weld is the main stress in straight seam steel pipes. At the same working pressure, the wall thickness of a spiral steel pipe with the same diameter is less than that of a straight seam steel pipe.
When parallel defects appear near the spiral weld, due to the smaller acting force on the spiral weld, the risk of bulging is lower than that of the straight weld. Since the radial stress is the largest stress on the steel pipe, the weld bears the largest load in the vertical stress direction. The longitudinal weld bears the largest load, the circumferential weld bears the smallest load, and the spiral weld bears an intermediate load between the two.
The development trend of pipelines is towards large diameter and high strength. As the diameter and steel grade of steel pipes increase, the tendency for stable expansion at the crack tip of brittle fractures increases. Although spiral steel pipes and straight seam steel pipes have the same grade, spiral steel pipes have higher impact toughness.
