The biggest difference between straight seam high frequency electric resistance welded pipe (referred to as ERW) and seamless steel pipe is that ERW has a weld, which is also the key to the quality of ERW steel pipe. Modern ERW steel pipe production technology and equipment, due to years of unremitting efforts internationally, especially in the United States, has made the seamlessness of ERW steel pipes a relatively satisfactory solution. 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 the ERW steel pipe. Due to the continuous improvement and perfection of the structure of the internal burr removal system and the cutting tools, the removal of burrs in large and medium-diameter steel pipes has been better treated. The internal burr can be controlled within -0.2mm~+0.5mm. Physical seamlessness refers to the difference between the internal metallographic structure of the weld and the base metal, which leads to a decrease in the mechanical properties of the weld area, and measures need to be taken to make it uniform and consistent. The high-frequency welding thermal process of ERW steel pipe creates 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 fire zone. Among them, the superheated zone structure due to the welding temperature above 100, the austenite grains grow up sharply, which will form hard and brittle coarse crystal phases under cooling conditions. In addition, the existence of temperature gradients will cause welding stress. In this way, the mechanical properties of the welding dimension are lower than that of the mother village.
Physical seamlessization is through the local conventional heat treatment process of the weld, that is, the medium frequency induction heating device is used to heat the weld area to AC3 (927°C), and then perform 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. So far, the world's advanced ERW units have generally adopted this method to process the welds, and good results have been obtained. The high-quality ERW steel pipe not only cannot distinguish the weld, but also the weld coefficient reaches 1, which realizes the match between the weld area and the base metal.
At the same time, the ERW steel pipe has the advantages of using hot-rolled coil as the raw material, and the uniform wall thickness can be controlled at about plus or minus 0.2mm. The two ends of the steel pipe are in accordance with the American APl standard or GB/T9711.1 standard. Goods and other advantages. In recent years, various natural gas pipeline network projects and gas companies have widely adopted ERW steel pipe as the main pipe material for urban pipeline networks.
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