According to the production process, longitudinal steel pipes can be divided into high-frequency longitudinal steel pipes and submerged arc welded longitudinal steel pipes. Submerged arc welded straight seam steel pipes are further divided into UOE, RBE, JCOE steel pipes according to their different molding methods. The following describes the most common molding processes for high-frequency longitudinal steel tubes and submerged arc welded longitudinal steel tubes.

Submerged arc welding process

1. Panel inspection: After the steel plate used to manufacture the large-diameter submerged arc welded longitudinal steel pipe enters the production line, the whole board ultrasonic inspection is first carried out.
2. Edge milling: double-sided milling the two edges of the steel plate by the edge milling machine to achieve the required plate width, parallelism of the edge and the shape of the groove.
3. Pre-bending edge: use the pre-bending machine to pre-bend the edge of the board so that the edge of the board has the required curvature.
4. Molding: on the JCO molding machine, first of all, half of the pre-bent steel plate is pressed into a "J" shape after several steps, and then the other half of the steel plate is also bent into a "C" shape, finally forming an open "O" shape.
5. Pre-welding: weld the longitudinal welded steel pipe together after forming and use gas shielded welding (MAG) for continuous welding.
6. Internal welding: welding by longitudinal multi-wire submerged arc welding (up to four wires) on the inside of a longitudinal steel pipe;
7. External welding: vertical multi-wire submerged arc welding is adopted to conduct welding on the outside of longitudinal submerged arc welding steel pipe.
8. Ultrasonic inspection Ⅰ: inside and outside of longitudinal  welded steel pipe weld and weld on both sides of the base of 100% of the check.
9. X-ray inspection I: 100% X-ray industrial TV inspection of internal and external welds, using image processing system to ensure the sensitivity of flaw detection.
10. Expanding diameter: Expanding the full length of the submerged arc welded straight seam steel pipe to improve the dimensional accuracy of the steel pipe and improve the distribution of stress in the steel pipe.
11. Hydraulic test: On the hydraulic testing machine, the steel pipe after the diameter expansion is tested one by one to ensure that the steel pipe meets the test pressure required by the standard. The machine has automatic recording and storage functions;
12. Chamfering: machining the pipe ends after qualified inspection to reach the required pipe end groove size.
13. Ultrasonic inspection II: Ultrasonic inspection is carried out again to check for defects that may occur after straightening and water pressure of longitudinal welded steel pipes.
14. X-ray inspection II: X-ray inspection and tube end weld imaging of the steel pipe after the expansion and hydrostatic test.
15. Tube end magnetic particle inspection: Perform this inspection to find the tube end defect.
16. Anti-corrosion and coating: qualified steel pipes shall be anticorrosion and coating according to the requirements of users.