Comparison And Difference between Straight Seam Steel Pipe And Spiral Steel Pipe

The production process of straight seam steel pipes is relatively simple. The main production processes are high-frequency welded straight seam steel pipes and submerged arc welded straight seam steel pipes. Straight seam steel pipes have high production efficiency, low cost, and rapid development. The strength of spiral steel pipes is generally higher than that of straight seam steel pipes. The main production process is submerged arc welding. Spiral steel pipes can produce welded pipes of different diameters from billets of the same width, and can also produce welded pipes with larger diameters from narrow billets. However, compared with the straight seam steel pipe of the same length, the welding length is increased by 30% and 100% respectively, and the production speed is lower. Therefore, large-diameter steel pipes are mostly welded by spiral welding, and small-diameter steel pipes are mostly welded by straight seam welding. When producing large-diameter straight seam steel pipes in industry, T-shaped welding technology is used, that is, a small portion of straight seam steel pipes are butted and connected to meet the length required by the project. The defects of T-shaped straight seam steel pipes will be greatly increased, and the welding residual stress of T-shaped welds will be larger. Weld metal is often under triaxial stress, which increases the potential for 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 will inevitably appear, and the welding residual stress of the T-shaped welds will be very large. Therefore, the possibility of welding defects is also high. After modification, the weld metal is usually in a triaxial stress state, which increases the possibility of cracking.

In addition, according to the technical regulations of submerged arc welding, each weld should be subject to arc ignition and arc extinguishing. However, each steel pipe cannot meet this condition when welding girth seam welding, so more problems may occur during the arc extinguishing process. welding defects.

When a pipe is subjected to internal pressure, two main stresses are usually generated on the pipe wall, namely radial stress and axial stress. The comprehensive stress at the weld, where α is the weld helix angle of the spiral steel pipe.

The comprehensive stress at the spiral weld is the main stress of the straight seam steel pipe. Under the same working pressure, the wall thickness of spiral steel pipes of the same diameter is smaller than that of straight seam steel pipes.

When parallel defects occur near the spiral weld, the expansion risk of the spiral weld is less than that of the straight weld because the force of the spiral weld is smaller. Since radial stress is the maximum stress on the steel pipe, the weld bears the maximum load in the vertical stress direction. Seams bear the greatest load, circumferential welds bear the least load, and spiral welds bear loads in between.

The development trend of pipelines is large diameter and high strength. As the steel pipe diameter and steel grade increase, the tendency of stable expansion of the ductile fracture tip is greater. Although spiral steel pipes and straight seam steel pipes have the same grade, spiral steel pipes have higher impact toughness.