The welding seam of the spiral steel pipe is longer than that of the straight seam steel pipe. If the pipe length is L, the welding seam length is L/cos(θ). The vast majority of steel pipe defects are concentrated in the weld and heat-affected zone. The long weld means that the probability of defects is high. This is the main reason that restricts the wider application of spiral welded pipes for a long time. Straight seam steel pipe, especially the question of who is superior compared with UOE steel pipe.
With the development of spiral steel pipe manufacturing technology to this day, we should evaluate and compare comprehensively and correctly, and re-understand the problem of long welding seam of spiral steel pipe.
First of all, since the defect is parallel to the weld, for the spiral steel pipe, the defect of the weld is an "oblique defect". In the process of use, the principal stress direction of the steel pipe, that is, the equivalent defect length in the axis direction of the steel pipe is smaller than that of the straight seam steel pipe; secondly, because the pipeline steel is all rolled steel plate, the impact toughness has a large anisotropy, and it is along the rolling direction. The CVN value can be 3 times higher than the CVN value perpendicular to the rolling direction.
The principal stress of the straight seam steel pipe is just perpendicular to the direction of the impact resistance of the pipe, while the spiral steel pipe staggers the direction of the impact resistance of the pipe, which turns the disadvantage of the long weld of the spiral steel pipe into an advantage.