In terms of the welding process, the welding method of spiral welded pipe and straight seam steel pipe is the same, but the straight seam welded pipe will inevitably have many T-shaped welds, so the probability of welding defects is also greatly increased, and the welding residue at the T-shaped weld The stress is large, and the weld metal is often in a three-way stress state, which increases the possibility of cracks. Moreover, according to the technical regulations of submerged arc welding, each weld should have an arc starting point and an arc quenching point, but each straight seam welded pipe cannot meet this condition when welding the circumferential seam, so there may be arc quenching points at the arc quenching point. More welding defects.
When the pipe is subjected to internal pressure, two main stresses are usually generated on the pipe wall, namely radial stress δ and axial stress δ. The resultant stress δ at the weld, where α is the helix angle of the weld of the spiral welded pipe. The helix angle of the spiral welded pipe weld is generally degrees, so the resultant stress at the spiral welded seam is the main stress of the straight seam welded pipe. Under the same working pressure, the wall thickness of the spiral welded pipe of the same diameter can be reduced compared with that of the straight seam welded pipe.
Expanding technology of straight seam welded pipe:
1. In the preliminary rounding stage, the fan-shaped blocks are opened until all the fan-shaped blocks touch the inner wall of the steel pipe. At this time, the radius of each point in the inner circular pipe of the steel pipe within the step range is almost the same, and the steel pipe is initially rounded.
2. In the nominal inner diameter stage, the fan-shaped block starts to reduce the movement speed from the front position until it reaches the required position, which is the required inner circumference position of the finished pipe.
3. In the rebound compensation stage, the fan-shaped block starts to further reduce the speed at the position of the second stage until it reaches the required position, which is the inner circumference position of the steel pipe before the rebound required by the process design.
4. In the stable pressure holding stage, the inner circumference of the steel pipe remains stationary for some time before the sector block is rebounded. This is the stable pressure holding stage required by the equipment and the expansion process.
5. In the stage of unloading and returning, the fan-shaped block rapidly retracts from the inner circumference of the steel pipe before rebounding until it reaches the position of the initial expansion, which is the small shrinkage diameter of the fan-shaped block required by the expansion process.