Analysis of the process characteristics and quality performance of three common ERW, SSAW, and LSAW welded pipes

The long-distance pipelines we are more familiar with use three types of straight seam high-frequency welded pipes (ERW), spiral submerged arc welded pipes (SSAW), and straight seam submerged arc welded pipes (LSAW). The following is an analysis of the process characteristics and quality performance comparison of the three types of welded pipes:

Straight seam high-frequency welded pipes (ERW) are divided into two forms: induction welding and contact welding according to different welding methods. They use hot-rolled wide steel coils as raw materials. After pre-bending, continuous forming, welding, heat treatment, sizing, straightening, cutting and other processes, compared with spiral welded pipes, they have the advantages of short welds, high dimensional accuracy, uniform wall thickness, good surface quality, and high pressure resistance. However, the disadvantage is that only small and medium-caliber thin-walled pipes can be produced, and gray spots, unfused, and groove corrosion defects are easily generated at the welds. At present, the most widely used fields are urban gas, crude oil and refined oil transportation, etc.

Spiral submerged arc welded pipe (SSAW) is a strip steel coil with a forming angle (adjustable) between its forward direction and the center line of the forming pipe. It is welded while forming, and its weld is a spiral line. The advantage is that the same specification of strip steel can produce steel pipes of various diameters, the raw material has a wide range of adaptability, the weld can avoid the main stress, and the stress condition is better. The disadvantage is that the geometric dimensions are poor, the weld length is longer than the straight seam pipe, and it is easy to produce welding defects such as cracks, pores, slag inclusions, and welding deviations, and the welding stress is in a tensile stress state. The general oil and gas long-distance pipeline design specification stipulates that spiral submerged arc welded pipes can only be used in Class 3 and Class 4 areas. After improving this process abroad, the raw material was changed to steel plate, so that the forming and welding were separated. After pre-welding and refinement, cold expansion after welding, the welding quality is close to UOE pipe. At present, there is no such process in China, which is the direction of improvement of spiral pipe factories in my country. The spiral pipes used in the "West-East Gas Transmission" are still produced according to traditional processes, but the pipe ends are expanded. The United States, Japan and Germany generally deny SSAW and believe that SSAW should not be used in trunk lines; Canada and Italy partially use SSAW, and Russia uses SSAW in small quantities, and they have all formulated very strict supplementary conditions. Due to historical reasons, most domestic trunk lines still use SSAW.

Straight seam submerged arc welded pipe (LSAW) is produced by using a single medium and thick plate as raw material, pressing (rolling) the steel plate into a tube billet in a mold or a forming machine, and using double-sided submerged arc welding and expanding the diameter. The finished product specification range is wide, the weld has good toughness, plasticity, uniformity and density, and has the advantages of large pipe diameter, thick pipe wall, high pressure resistance, low temperature resistance and strong corrosion resistance. When building high-strength, high-toughness, high-quality long-distance oil and gas pipelines, most of the required steel pipes are large-diameter thick-walled straight seam submerged arc welded pipes. According to API standards, in large oil and gas transmission pipelines, when passing through Class 1 and Class 2 areas such as high-cold areas, seabeds, and densely populated urban areas, straight seam submerged arc welded pipes are the only designated applicable pipe type.