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Introduction to welded pipe technology
**Introduction to Welded Pipe Technology**
Welded pipes come in various forms, primarily categorized based on their forming processes and welding techniques. Depending on how they are formed, welded pipes can be either spiral welded pipes or straight seam welded pipes. As for the welding process, options include high-frequency resistance welding and submerged arc welding. Notably, all spiral welded pipes utilize submerged arc welding technology. On the other hand, straight seam welded pipes are further classified; those that use submerged arc welding are known as UOE pipes, while those employing high-frequency resistance welding are referred to as ERW pipes.
ERW steel pipes, which rely on high-frequency resistance welding, differ from submerged arc welding in their approach. During the ERW process, no additional welding materials are added, and the weld formation does not involve thermal melting. Instead, the weld metal undergoes a recrystallization process, ensuring that the chemical composition of the weld remains consistent with that of the base metal. Post-welding, the steel pipe undergoes an annealing process, which helps relieve both cold working stresses and welding-induced stresses. Consequently, ERW steel pipes exhibit superior comprehensive mechanical properties.
In contrast, straight seam submerged arc welding (UOE steel pipes) involves a post-weld cold expansion process to increase the pipe’s diameter. This results in UOE steel pipes having more precise geometric dimensions. When UOE pipes are joined together, their joints display excellent quality, and the welding process ensures reliable quality control. Additionally, UOE pipes often use multi-wire welding techniques, such as three-wire or four-wire setups. These methods generate minimal heat input and reduce the impact on the heat-affected zones of the base metal. Multi-wire welding also allows the initial weld pass to mitigate stresses produced during welding, thereby enhancing the mechanical properties of the pipe. Compared to spiral welded pipes, UOE pipes feature shorter weld lengths, leading to fewer welding defects and reduced overall impact. Furthermore, the base material of high-pressure pipelines made from straight-seam pipes can undergo 100% ultrasonic testing, ensuring compliance with stringent standards. However, the higher cost of UOE pipes can deter budget-conscious users.
Spiral steel pipes, on the other hand, have their welds arranged in a helical pattern. Typically, the weld area, including the heat-affected zone, represents the section of the pipe with inferior mechanical properties. Since the maximum internal stress in pressure pipes generally aligns axially, spiral welded pipes tend to be weaker at these stressed areas. By positioning the weld away from the direction of maximum internal stress, spiral welded pipes enhance their structural integrity. Nevertheless, the shape and height of spiral weld seams pose challenges for external corrosion protection. Gaps may form between adjacent welds, compromising durability. To address this issue, some manufacturers apply a horizontal winding technique to apply three layers of polyethylene (PE) or two layers of PE, effectively resolving the corrosion concerns associated with spiral steel pipes.
In summary, both types of welded pipes offer distinct advantages and disadvantages depending on specific application needs. While UOE pipes excel in dimensional accuracy and welding quality, their higher cost remains a limiting factor. Conversely, spiral welded pipes provide enhanced strength by avoiding critical stress points, albeit with potential corrosion-related drawbacks. Understanding these nuances is crucial when selecting the appropriate type of welded pipe for different industrial applications.