Pre-welding and welding of straight seam steel pipe

Pre-welding of straight seam steel pipes: pre-welding includes seams and welds.
1. The joint (that is, the forming seam) has no misalignment or the misalignment is less than the specified value. Generally, the amount of misalignment is ≤ 8% of the plate thickness, and the maximum is not more than 1.5mm.
2. To ensure that the weld has a suitable penetration depth and deposition amount, it is necessary to ensure that there is no cracking after welding, no burn-through phenomenon, and the height of the weld must be controlled so that the reinforcement of the external weld will not be affected.
3. The weld bead is continuous and formed, to facilitate the external welding after the guarantee.
4. There are no defects such as welding deviation, air holes, cracks, slag inclusions, burn-through, and back welding bumps in the weld seam, and the center deviation of the weld seam is required to be ≤ 1mm.
5. There is no arc burn, or little splash, and does not affect the bevel and surface of the pipe end.
6. The weld seam matches the base metal, and the physical and chemical properties of the weld metal meet the requirements.

Straight seam steel pipe welding essentials:
The forming methods for manufacturing large-diameter longitudinally welded pipes include UOE forming, row-roll forming (CFE), CE forming, etc. For the last process of most forming methods, the full-length diameter expansion of the welded tube blank is performed again to improve the shape of the welded steel tube. Diameter expansion has become an important process to ensure the finished pipe in the production of large-diameter longitudinal welded pipe. Diameter expansion is a pressure processing process that uses hydraulic or mechanical means to apply force from the inner wall of the steel pipe to expand the steel pipe radially outward. Compared with the hydraulic method, the mechanical method has simpler equipment and higher efficiency. It has been adopted in several large-diameter straight seam welded pipe pipeline expansion processes around the world. The mechanical expansion uses the split fan-shaped block at the end of the expansion machine to expand radially. , the process of making the tube blank along the length direction in a stepwise manner to realize the plastic deformation of the full tube length in sections.

1. Preliminary full circle stage. The fan-shaped blocks are opened until all fan-shaped blocks touch the inner wall of the steel pipe. At this time, the radii of all points in the inner circular pipe of the steel pipe within the step range are almost the same, and the steel pipe obtains a preliminary full circle.
2. The nominal inner diameter stage. The fan-shaped block starts to reduce the moving speed from the front position until it reaches the required position, which is the required position of the inner circumference of the finished tube.
3. It is the recovery compensation stage. The fan-shaped block starts to lower the speed at the position of the second stage until it reaches the required position, which is the position of the inner circumference of the steel pipe before spring back is required by the process design.