Welded pipes are divided into spiral welded pipes and longitudinally welded steel pipes according to the forming process, and high-frequency resistance welding and submerged arc welding according to the welding process. Spiral welded pipes all adopt submerged arc welding process. For longitudinally welded pipes, submerged arc welded pipes are referred to as UOE, and longitudinal high-frequency resistance welding is referred to as ERW.
High-frequency electric resistance welded steel pipe (ERW steel pipe) because its welding process is compared with submerged arc welding. The ERW process does not add any welding materials during the welding process. The weld formation does not undergo a thermal melting state, but the weld metal undergoes a recrystallization process. Therefore, the formed weld seam is completely consistent with the chemical composition of the base metal. After the steel pipe is welded, it is annealed to produce the cold working internal stress and welding internal stress. Therefore, the comprehensive mechanical properties of the ERW steel pipe are better.
Straight seam submerged arc welding (UOE steel pipe) adopts post-weld cold expansion process to expand the pipe, so the geometric dimensions of UOE steel pipe are more accurate. When UOE steel pipe is used for butt joints, the quality of the counterpart is good to ensure the welding quality. Eliminate part of the internal stress. In addition, multi-wire welding (three-wire, four-wire) is used when welding UOE steel pipes. This welding process generates small heat input and has a small impact on the heat-affected zone of the base material. After multi-wire welding, the welding wire of the first pass can eliminate the stress generated during welding, thereby improving the mechanical properties of the steel pipe. Compared with spiral welded pipes, LSAW pipes have shorter weld lengths, so that welding defects and effects are relatively small. The base material of the straight seam pipe in the high-pressure pipeline can achieve 100% ultrasonic flaw detection of steel plates one by one, which meets the requirements of the high-pressure pipeline for the base material. However, although UOE steel pipe has better overall performance than other steel pipes, its high price discourages users with tight funds.
Spiral steel pipe welds are distributed in a spiral shape. Generally speaking, the weld area of the steel pipe includes the weld heat-affected zone, which is the part with poor mechanical properties of the steel pipe. The maximum internal stress of the pressure pipe is distributed along the axial direction, and the spiral welded pipe will be weaker. The part avoids the direction of maximum internal stress, thereby improving the performance of the steel pipe. In addition, the welding seam forming of the spiral steel pipe and the height of the welding seam increase the difficulty in external anti-corrosion. There may be gaps between the two welds. At present, some manufacturers use lateral winding to coat three layers of PE or two layers of PE. The process can solve the anti-corrosion of spiral steel pipe.