Welded pipes are categorized based on their forming process into spiral welded pipes and straight seam welded pipes. Additionally, they are classified by the welding method into high-frequency resistance welding (ERW) and submerged arc welding (SAW). Spiral welded pipes typically use submerged arc welding technology. For straight seam welded pipes, those produced using submerged arc welding are commonly referred to as UOE pipes, while those made with high-frequency resistance welding are known as ERW pipes.
High-frequency resistance welded steel pipes (ERW) differ from submerged arc welded pipes in their welding process. During ERW, no additional welding materials are used. The weld is formed through a recrystallization process rather than melting, resulting in a weld that has the same chemical composition as the base metal. After welding, the pipe undergoes annealing, which helps relieve cold working and welding stresses. As a result, ERW pipes generally exhibit superior mechanical properties compared to other types of welded pipes.
Straight seam submerged arc welded pipes (UOE) employ a post-weld cold expansion process to enhance dimensional accuracy. This ensures better joint quality when connecting pipes, and it also helps reduce internal stress. Furthermore, multi-wire welding techniques—such as three or four wire systems—are often used in the production of UOE pipes. This method reduces heat input, minimizing the impact on the heat-affected zone. The first welding wire can help eliminate welding-induced stress, thereby improving the overall mechanical performance of the pipe. Compared to spiral welded pipes, straight seam SAW pipes have shorter welds, leading to fewer welding defects and less susceptibility to damage. The base material of these pipes can be fully inspected using ultrasonic testing, making them suitable for high-pressure pipeline applications. However, despite their superior performance, UOE pipes tend to be more expensive, which may limit their use for projects with tight budgets.
Spiral welded pipes feature a helical weld pattern. The weld area includes the heat-affected zone, which typically has lower mechanical properties compared to the base material. Since the maximum internal stress in pressure pipes is aligned along the axial direction, spiral welded pipes may be weaker in this region. However, the helical arrangement helps avoid the direction of maximum stress, thus enhancing the pipe's overall performance. Another challenge with spiral welded pipes is the increased difficulty in applying external anti-corrosion coatings due to the shape and height of the weld. There may also be gaps between adjacent welds. To address this issue, some manufacturers now use horizontal winding techniques to apply three-layer polyethylene (PE) or two-layer PE coatings, effectively solving the corrosion problem associated with spiral welded pipes.
By understanding the differences between various welding technologies, engineers and designers can make informed decisions based on the specific requirements of their projects, such as pressure rating, cost, and environmental conditions.