The seamless characteristics of ERW steel pipe

　　ERW steel pipe and seamless steel pipe is that ERW has a weld seam, which is also the key to the quality of ERW steel pipe.

　　Modern ERW steel pipe production technology and equipment, due to the unremitting efforts of the world, especially the United States, have 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 ERW steel pipes. Due to the continuous improvement and improvement of the structure of the internal burr removal system and the cutting tools, the removal of the internal burrs of large and medium-diameter steel pipes has been better handled. The inner burr can be controlled at about -0.2mm～+0.5mm. Physical seamlessness means that there is a 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, and measures need to be taken to make it uniform and consistent.

　　The high-frequency welding heat process of ERW steel pipe causes the 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, due to the welding temperature above 1000°C in the superheated zone, the austenite grains grow rapidly, and a hard and brittle coarse-grained phase will be formed under cooling conditions. In addition, the existence of temperature gradients 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 to heat the weld area to AC3 (927°C) with a medium-frequency induction heating device through the local conventional heat treatment process of the weld, and then carry out a 60m Air cooling process with length and speed at 20m/min, and 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.