Straight seam steel pipes are steel pipes with welds parallel to the longitudinal direction of the pipe. They are typically categorized as metric electric-welded steel pipes, electric-welded thin-walled pipes, transformer cooling oil pipes, and other types. The production process for straight seam welded steel pipes is simple, efficient, and cost-effective, leading to rapid development. Spiral-welded steel pipes generally offer higher strength than straight seam welded pipes. They can be produced from narrower billets, and welded steel pipes of varying diameters can be produced from billets of the same width. However, compared to straight seam steel pipes of the same length, the weld length increases by 30-100%, and production speeds are lower.
Straight Seam Steel Pipe Production Process: Straight seam steel pipes can be categorized by production process as high-frequency straight seam steel pipes and submerged arc welded straight seam steel pipes. Submerged arc welded straight seam steel pipes are further categorized by forming methods, such as UOE, RBE, and JCOE. The following describes the most common forming processes for high-frequency straight seam steel pipes and submerged arc welded straight seam steel pipes.
Submerged Arc Welding Process for Straight Seam Steel Pipes
1. Plate Inspection: After entering the production line, the steel plates used to manufacture large-diameter submerged arc welded straight seam steel pipes undergo full ultrasonic inspection.
2. Edge Milling: A milling machine performs double-sided milling on both edges of the steel plate to achieve the required plate width, edge parallelism, and groove shape.
3. Edge Bending: A pre-bending machine pre-bends the plate edges to achieve the required curvature.
4. Forming: On a JCO forming machine, one half of the pre-bent steel plate is first subjected to multiple step-punching cycles to form a "J" shape. The other half is then bent similarly to form a "C" shape, ultimately forming an open "O" shape.
5. Pre-Welding: The formed straight seam welded steel pipe is then joined and continuously welded using gas shielded arc welding (MAG).
6. Internal Welding: A longitudinal multi-wire submerged arc welding (up to four wires) is used to weld the inside of the straight seam steel pipe.
7. External Welding: Welding is performed on the outside of the LSAW steel pipe using longitudinal multi-wire submerged arc welding.
8. Ultrasonic Inspection I: 100% inspection of the internal and external welds and the parent metal on both sides of the welds of the LSAW steel pipe.
9. X-ray Inspection I: 100% inspection of the internal and external welds using industrial X-ray television, utilizing an image processing system to ensure flaw detection sensitivity.
10. Diameter Expansion: The entire length of the LSAW steel pipe is expanded to improve dimensional accuracy and internal stress distribution.
11. Hydrostatic Testing: Each expanded pipe is inspected on a hydrostatic testing machine to ensure it meets the required test pressure. The machine automatically records and stores the pressure.
12. Chamfering: Pipes that pass inspection undergo end processing to achieve the required bevel dimensions.
13. Ultrasonic Inspection II: Ultrasonic testing is performed on each pipe again to detect any defects that may have occurred after expansion and hydrostatic testing.
14. X-ray Inspection II: After expansion and hydrostatic testing, steel pipes undergo X-ray industrial television inspection and radiography of pipe end welds.
15. Magnetic Particle Inspection of Pipe Ends: This inspection is performed to detect defects on pipe ends.
16. Corrosion Protection and Coating: Qualified steel pipes undergo corrosion protection and coating according to user specifications.
High-Frequency Welding Process for Straight-Seam Steel Pipes: Straight-seam welded steel pipes are produced by rolling long steel strips of specified specifications into a round tube using a high-frequency welding machine and then welding them through the straight seam. The shape of the steel pipe can be round, square, or shaped, depending on the sizing process performed after welding. Welded steel pipes are primarily made of low-carbon steel, low-alloy steel with a σs ≤ 300 N/mm² or σs ≤ 500 N/mm², or other steel materials.
High-frequency welding of straight seam steel pipes: High-frequency welding utilizes the principles of electromagnetic induction and the skin effect, proximity effect, and eddy current heating of alternating current charges in a conductor to locally heat the steel at the weld edge to a molten state. Roller extrusion then creates intercrystalline bonding in the butt weld, achieving the desired weld. High-frequency welding, a type of induction welding (or pressure contact welding), requires no filler, produces no weld spatter, has a narrow heat-affected zone, offers aesthetically pleasing welds, and exhibits excellent mechanical properties. Therefore, it is widely used in steel pipe production. High-frequency welding of steel pipes utilizes the skin and proximity effects of alternating current. After roll forming, steel (strip) is formed into a circular tube with a broken cross-section. A resistor (or a group of resistors) rotates within the tube near the center of an induction coil. This creates an electromagnetic induction loop with the resistor and the tube opening. The skin and proximity effects generate a strong, concentrated heat effect at the edge of the tube opening, rapidly heating the weld edge to the required welding temperature. After compression by rollers, the molten metal achieves intergranular bonding, and after cooling, a strong butt weld is formed.
High-frequency welding of straight seam steel pipes: High-frequency welding utilizes the principles of electromagnetic induction and the skin, proximity, and eddy current heating effects of alternating current charges in a conductor. This locally heats the steel at the weld edge to a molten state. The rollers then compress the butt weld, achieving intergranular bonding and ultimately achieving the desired weld. High-frequency welding (HFW) is a type of induction welding (or pressure contact welding). It requires no filler, produces no weld spatter, has a narrow heat-affected zone, produces aesthetically pleasing welds, and exhibits excellent mechanical properties. Therefore, it is widely used in steel pipe production. HFW utilizes the skin and proximity effects of alternating current. After roll forming, the steel (strip) is formed into a circular tube with a broken cross-section. Inside the tube, one or a group of resistors (magnetic rods) rotate near the center of an induction coil. These resistors and the tube opening form an electromagnetic induction loop. The skin and proximity effects generate a strong, concentrated heat effect at the edge of the tube opening, rapidly heating the weld edge to the required welding temperature. After compression by the rollers, the molten metal achieves intergranular bonding, and after cooling, a strong butt weld is formed.
High-frequency welding unit for straight seam steel pipes: High-frequency welding of straight seam steel pipes is performed in a high-frequency welding unit. A high-frequency welding steel pipe unit is usually composed of roll forming, high-frequency welding, extrusion, cooling, sizing, flying saw cutting, and other components. The front end of the unit is equipped with a material storage loop, and the rear end of the unit is equipped with a steel pipe turning rack; the electrical part mainly consists of a high-frequency generator, a DC excitation generator, and an instrument automatic control device.
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