Seamless Characteristics of ERW Steel Pipes

The biggest difference between straight-seam high-frequency resistance welded pipes (Electric Resistance Welding, referred to as ERW) and seamless steel pipes is that ERW has a weld seam, which is also the key to the quality of ERW steel pipes. Modern ERW steel pipe production technology and equipment, thanks to years of unremitting efforts internationally, especially in the United States, have made the seamlessness of ERW steel pipes more satisfactory. Some people divide the seamlessness of ERW steel pipes into geometric seamlessness and physical seamlessness. Geometric seamlessness is to remove the internal and external burrs of ERW steel pipes. Due to the continuous improvement and perfection of the structure and cutting tools of the internal burr removal system, the removal of internal burrs in large and medium-diameter steel pipes has been better handled. Internal burrs can be controlled at around -0.2mm ~ +0.5mm. Physical seamlessness refers to the difference between the metallographic structure inside the weld and the base metal, which leads to a decrease in the mechanical properties of the weld area. Measures need to be taken to make it uniform and consistent. The high-frequency welding heat process of ERW steel pipes causes a temperature distribution gradient near the edge of the tube blank, and forms characteristic areas such as melting zone, semi-melting zone, overheated structure, normalizing zone, incomplete normalizing zone, and tempering zone. In the superheated zone structure, due to the welding temperature above 1000°C, the austenite grains grow rapidly, and a hard and brittle coarse grain phase will be formed under cooling conditions. In addition, the existence of a temperature gradient will generate welding stress. In this way, the mechanical properties of the weld area are lower than those of the base metal. The physical seamlessness is achieved through the local conventional heat treatment process of the weld, that is, using a medium frequency induction heating device to heat the weld area to AC3 (927"C), and then Carrying out an air cooling process with a length of 60m and a speed of 20m/min, and then water cooling when necessary. The use of this method achieves the purpose of eliminating stress, softening and refining the structure, and improving the comprehensive mechanical properties of the welding heat-affected zone. Currently, the world's most advanced ERW units have generally used this method to process welds, and have achieved good results. High-quality ERW steel pipes not only cannot identify the welds but also have a weld coefficient of 1, achieving a match between the weld area structure and the base material. At the same time, ERW steel pipes use hot-rolled coils as raw materials, and the wall thickness can be uniformly controlled at around +0.2mm. The two ends of the steel pipes are trimmed and beveled according to the American API standard or GB/T9711.1 standard, and the length is fixed. Goods and other advantages. In recent years, various natural gas pipeline network projects and gas companies have widely used ERW steel pipes as the main pipe materials in urban pipeline networks.