Longitudinally welded steel pipes can be categorized into high-frequency longitudinally welded steel pipes and submerged arc welded longitudinally welded steel pipes based on their production processes. Below are the forming processes of the most common types: high-frequency and submerged arc welded longitudinally welded steel pipes.
Submerged Arc Welding (SAW)
After entering the production line, steel plates intended for large-diameter submerged arc welded longitudinally welded steel pipes undergo full-plate ultrasonic inspection. Both edges of the steel plate are then double-sided milled by an edge milling machine to achieve the required plate width, parallelism of plate edges, and groove shape. Pre-bending is performed using a pre-bending machine to impart the required curvature to the plate edges. On a JCO forming machine, half of the pre-bent steel plate is pressed into a "J" shape through multiple step stamping operations, while the other half is similarly bent into a "C" shape, ultimately forming an open "O" shape.
The formed pipe is closed and continuously welded using gas metal arc welding (MAG). Subsequently, multi-wire submerged arc welding (up to four wires) is employed for welding inside the pipe, followed by the same process on the outside of the submerged arc welded longitudinally welded steel pipe. After welding, the pipe undergoes a series of inspections: the first ultrasonic inspection (primarily examining the weld seam and base material on both sides), the first X-ray inspection (ensuring flaw detection sensitivity), expansion, and a hydrostatic test (with automatic record keeping).
Qualified pipes are then processed to meet the required dimensions and undergo a second ultrasonic inspection, a second X-ray inspection, magnetic particle inspection of pipe ends, corrosion protection, and coating, completing the entire manufacturing process.
High-Frequency Welding (HFW)
High-frequency welding heats the steel at the weld edge to a molten state based on the principles of electromagnetic induction, skin effect, proximity effect, and eddy current heating effect of alternating current in conductors. The molten edges are then pressed together by rollers, achieving intercrystalline bonding of the butt weld. As an induction welding (or pressure contact welding) method, HFW requires no filler material, produces no welding spatter, and results in a narrow heat-affected zone, aesthetically pleasing welds, and excellent mechanical properties, making it widely used in steel pipe production.
In high-frequency welding of steel pipes, alternating current's skin effect and proximity effect are utilized. After roll forming, the steel (strip steel) forms a circular pipe billet with a discontinuous cross-section. Inside the pipe billet, near the center of the induction coil, one or a set of impedances (magnetic bars) rotate, forming an electromagnetic induction loop with the pipe billet's opening. Under the skin effect and proximity effect, a powerful and concentrated heat effect is generated at the edge of the pipe billet's opening, rapidly heating the weld edge to the required temperature. Upon compression by rollers, the molten metal achieves intercrystalline bonding, cooling to form a strong butt weld.
