Pre-welding of straight seam steel pipes: pre-welding includes joints and welds.

Introduction to pre-welding and welding essentials of straight seam steel pipes

1. The joint (also known as the forming seam) has no misalignment or the misalignment is less than the specified value. Generally, the misalignment is ≤8% of the plate thickness, and the maximum is no 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 or burn-through after welding, and to control the weld height, so as not to affect the excess height of the external weld.
3. The weld is continuous and formed to facilitate the external welding after guarantee.
4. The weld does not have defects such as welding deviation, pores, cracks, slag inclusions, burn-through, and back weld nodules. The center deviation of the weld is required to be ≤1mm.
5. There is no arc burn, small spatter, and no impact on the pipe end groove and surface.
6. The weld matches the parent material, and the physical and chemical properties of the weld metal meet the requirements.

Essentials of straight seam steel pipe welding: The forming methods for manufacturing large-diameter straight seam welded pipes include the UOE forming method, row roller forming method (CFE), CE forming, etc. For most forming methods, the latter process is to expand the full length of the pipe blank after welding to improve the shape of the welded steel pipe. Diameter expansion has become an important process to ensure the finished pipe in the production of large-diameter straight seam welded pipes. Diameter expansion is a pressure processing process that uses hydraulic or mechanical methods to apply force from the inner wall of the steel pipe to expand the steel pipe radially outward. The mechanical method is simpler and more efficient than the hydraulic method. The expansion process of several large-diameter straight seam welded pipe pipelines in the world is adopted. Mechanical diameter expansion uses the fan-shaped block at the end of the expander to expand radially, so that the pipe blank is stepped along the length direction and the plastic deformation of the entire pipe length is realized in sections.

1. Initial rounding stage. The fan-shaped blocks are opened until all the fan-shaped blocks are in contact with the inner wall of the steel pipe. At this time, the radius of each point in the inner round tube of the steel pipe within the step range is almost the same, and the steel pipe is initially rounded.
2. Nominal inner diameter stage. The fan-shaped blocks start to reduce the movement speed from the front position until they reach the required position, which is the required position of the inner circumference of the finished pipe.
3. Rebound compensation stage. The fan-shaped blocks start to further reduce the speed at the position of stage 2 until they reach the required position, which is the position of the inner circumference of the steel pipe before the rebound required by the process design.