The biggest difference between high frequency resistance welded pipe (Electric Resistance Welding, 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 tool, the removal of burrs in large and medium-diameter steel pipes has been better handled. The internal burr can be controlled within -0.2mm～+0.5mm. Physical seamlessness refers to the difference between the metallographic structure and the base metal inside the weld, which causes the mechanical properties of the weld area to decrease, 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 tempering zone. Among them, the superheated zone structure due to the welding temperature above 1000 ℃, the austenite grains grow rapidly, and the hard and brittle coarse crystal phase will be formed under cooling conditions. In addition, the presence of temperature gradients will cause welding stress. In this way, the mechanical properties of the weld area are lower than that of the base metal. The physical seamlessness is to heat the weld area to AC3 (927°C) with an intermediate frequency induction heating device through the local conventional heat treatment process of the weld, and then perform 60m The length and speed of the air cooling process are 20m/min, and then water cooling is needed when needed. 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. At present, advanced ERW units in the world 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, realizing the match between the weld area and the base metal.
