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Introduction to welded pipe technology
**Introduction to Welded Pipe Technology**
Welded pipes are categorized into two main types based on their forming processes: spiral welded pipes and straight seam welded pipes. Depending on the welding technique, they are further classified into high-frequency resistance welding and submerged arc welding. All spiral welded pipes utilize submerged arc welding technology. For straight seam welded pipes, those produced through submerged arc welding are known as UOE pipes, while those made using high-frequency resistance welding are called ERW pipes.
ERW (high-frequency resistance welded) steel pipes differ from submerged arc welded pipes due to their unique welding process. Unlike submerged arc welding, the ERW process does not introduce additional welding materials. Instead, the weld formation occurs without reaching a fully molten state; rather, it involves a recrystallization process. As a result, the weld retains the same chemical composition as the base metal. Following welding, the steel pipe undergoes an annealing process, which helps relieve internal stresses caused by cold working and welding. Consequently, ERW steel pipes exhibit superior comprehensive mechanical properties compared to other types of welded pipes.
UOE (straight seam submerged arc welded) pipes undergo a post-weld cold expansion process to increase their diameter, ensuring more precise geometric dimensions. During docking, these pipes maintain excellent joint quality and welding integrity, helping to eliminate certain internal stresses. Additionally, UOE pipes often employ multi-wire welding techniques—such as three-wire or four-wire configurations—which minimize heat input and reduce the impact on the heat-affected zones of the base metal. This approach allows the first weld to mitigate residual stresses from the welding process, enhancing the overall mechanical properties of the pipe. Compared to spiral welded pipes, UOE pipes feature shorter weld lengths, leading to fewer welding defects and reduced stress concentrations. Moreover, the base material of high-pressure straight-seam pipes can be subjected to 100% ultrasonic inspection, satisfying stringent requirements for high-pressure pipeline applications. Despite their superior performance, however, UOE pipes come at a significantly higher cost, which may deter budget-conscious users.
Spiral steel pipes have their welds arranged in a helical pattern. Typically, the weld area encompasses the heat-affected zone, which generally exhibits inferior mechanical properties compared to the rest of the pipe. In pressure vessels, the maximum internal stress tends to distribute axially along the pipe. Since the spiral weld runs diagonally, it avoids this direction of maximum stress, thereby improving the pipe's overall performance. Furthermore, the unique shape and height of spiral welds make external corrosion protection more challenging. Gaps between consecutive welds can sometimes occur, creating potential weak points. To address this issue, some manufacturers now apply a horizontal winding method that coats the pipe with three layers of polyethylene (PE) or two layers of PE. This innovative approach effectively resolves the corrosion concerns associated with spiral steel pipes.
In conclusion, both ERW and UOE pipes offer distinct advantages depending on the application. While ERW pipes excel in terms of mechanical properties and cost-effectiveness, UOE pipes provide greater precision and reliability, albeit at a premium price. On the other hand, spiral welded pipes remain popular due to their versatility and affordability, especially when combined with advanced anti-corrosion solutions. Each type of welded pipe serves specific purposes, making them indispensable components in various industries such as construction, oil and gas, and water supply systems.