In modern steel pipe manufacturing, longitudinally welded pipes (LSAW and ERW) are widely used for their high forming efficiency, straight weld seams, and stable structure. These pipes are essential in sectors like oil and gas transportation, structural support, bridge construction, and pile foundations. Depending on the application and specifications, different production methods are employed-primarily Submerged Arc Welding (SAW) and High-Frequency Welding (HFW). Below is a detailed explanation of both processes.
1. Submerged Arc Welding (LSAW) – Ideal for Large-Diameter, High-Strength Steel Pipes
1.1 Material Preparation and Pre-Treatment
The production of LSAW pipes starts with rigorous quality inspection of the steel plates. Each plate undergoes full ultrasonic testing to detect defects such as cracks or inclusions. After inspection, the plate edges are milled using an edge milling machine to meet the desired width, parallelism, and bevel shape, ensuring precise weld joint preparation.
1.2 Precision Forming and Welding
The milled plates are pre-bent to introduce curvature on both sides. These plates are then fed into a JCO forming press, where they are shaped in stages: first into a "J" on one side, then a "C" on the other, eventually forming an open "O"-shaped pipe body. The open pipe is tack welded using MAG (Metal Active Gas) welding, followed by multi-wire submerged arc welding-both internally and externally. This technique, using up to four wires simultaneously, ensures strong, uniform welds suitable for large-diameter, load-bearing pipes.
1.3 Quality Inspection and Finishing
After welding, a series of tests and treatments are carried out:
- First ultrasonic testing on the weld seam and adjacent base material
- X-ray inspection to verify internal weld integrity
- Diameter expansion to improve roundness and dimensional accuracy
- Hydrostatic testing to evaluate strength and tightness
- Sizing and cutting to meet customer specifications
- Second round of ultrasonic and X-ray testing, as well as magnetic particle testing at the pipe ends
- External anti-corrosion treatment, such as epoxy powder or 3-layer PE coating
LSAW pipes produced through this process offer high strength and reliability, making them suitable for high-pressure fluid transport and heavy-duty structural applications.
2. High-Frequency Welding (ERW) – Efficient and Best for Medium to Small-Diameter Pipes
2.1 Principle of High-Frequency Induction Welding
High-frequency welding is a type of induction welding (or pressure contact welding) that utilizes electromagnetic induction and the effects of skin effect, proximity effect, and eddy current heating. These principles rapidly heat the steel strip edges to a molten state. A set of pressure rollers then squeeze the heated edges together, causing grain boundary fusion and forming a strong, clean butt weld. This process doesn't require filler materials, has minimal spatter, and creates a narrow heat-affected zone with excellent mechanical properties.
2.2 Streamlined Process and Mass Production Suitability
ERW is mainly used for the production of medium and small-diameter steel pipes. The process includes:
- Uncoiling and leveling of steel strip
- Continuous forming into a round pipe blank via roll forming
- Induction heating of the weld edges
- Pressing the edges together using squeeze rolls
- Cooling, sizing, cutting to length
- Inline weld seam testing
Compared to LSAW, ERW is faster, more energy-efficient, and highly automated-ideal for projects requiring large production volumes and cost control. It is widely applied in structural tubing, fluid transport, automotive tubing, and mechanical applications.
Conclusion
LSAW and ERW represent the two dominant methods for manufacturing longitudinal welded steel pipes. LSAW is preferred for large-diameter, high-strength pipes in demanding environments, while ERW stands out for its production efficiency and cost advantages in smaller diameter applications. Choosing the right welding process based on project needs is key to balancing performance and economics.
