ERW welded pipe is formed by using the skin effect and proximity effect of high-frequency current to rapidly heat the edge of the formed pipe blank to the welding temperature and then extrude and weld it. According to the different feeding methods, ERW welded pipes can be divided into high frequency contact welding and high frequency induction welding. Both use the skin effect and proximity effect of high-frequency current to heat the edge metal of the coil to a suitable welding temperature, and at the same time carry out pressure welding to form a steel pipe under the action of the squeeze roller.
ERW contact welding is to use two electrodes or welding feet to contact the two edges of the formed tube respectively. The skin effect and proximity effect generated by high-frequency current are used to make the welding current concentrate on the V-shaped notch, and the metal at the V-shaped notch is instantly Heating to the welding temperature, at the same time pressurized by the extrusion roller to weld into the steel pipe; ERW induction welding is to first form the hot-rolled coil into the tube through the forming unit, and when the tube passes through the induction coil (the high-frequency welding machine The induction coil generates a high-frequency magnetic field due to the high-frequency current), and a high-frequency induced eddy current will be generated in the tube. The formed tube blank is quickly heated to the welding temperature due to its own resistance, and at the same time, it is pressure welded into a steel tube under the extrusion action of the extrusion roller.
ERW welded pipe has great development potential and market competitiveness due to its high dimensional accuracy, uniform wall thickness, high production efficiency, good compressive performance and comprehensive mechanical properties, and no perforation cracking. There have been a lot of applications in transportation and so on. With the increase in the number of domestic ERW welded pipe production lines and the accumulation of production experience, the pipe making and inspection process of ERW welded pipes are also becoming more and more perfect, thus effectively ensuring the market demand and the continuous improvement of welded pipe quality. A commonly used standard is ASTM A53 ERW.
1. Analysis of influencing factors on weld quality of ERW induction welded pipe
The quality of the weld joint of ERW induction welded pipe directly determines the overall quality of the welded pipe, so the influencing factors of the weld quality are the main factors affecting the overall quality of the welded pipe. From the process of making and inspecting ERW induction welded pipes, it can be seen that there are many factors that affect the quality of ERW induction welded pipes. These influencing factors can be roughly divided into two categories: one is the quality factor of welded pipe raw materials; the other is Pipe-making process factors for welded pipes.
a. The influence of raw materials
The chemical composition, physical and chemical properties and other quality indicators of welded pipe raw materials will play an important role in the quality of ERW induction welded pipe forming, welding, heat treatment and other pipe manufacturing processes.
The quality requirements of welded pipe raw materials mainly refer to whether the pipeline steel coils produced by the steel mill through the thermomechanical rolling technology meet the raw material control requirements of the steel grade specified by the API standard. For the control indicators of the quality of the steel coil, not only the manufacturer should do the corresponding physical and chemical performance test, and issue a quality assurance certificate of qualified products, but also after receiving the raw materials, the pipe manufacturer should also conduct a third-party supervision unit in accordance with the relevant API standards. Under the supervision of the staff, the physical and chemical properties of the raw materials are tested. After the inspection items all meet the relevant standards, the batch of raw materials can be used to start pipe making, so as to ensure the use of qualified steel coils to make pipes and further reduce the factors that cause pipe defects.
b. The influence of the pipe making process
The unique pipe-making process of ERW induction welded pipe determines that the quality of each process will affect the quality of welded joints. The pipe-making process factors that affect the weld quality of ERW induction welded pipes can be divided into the geometrical dimensional accuracy of the steel strip before forming the pipe blank, the forming quality of the pipe blank, and the quality of related processes such as welding and heat treatment.
2. Influence of ERW induction welding process parameters on weld quality
As one of the three core pipe-making processes of ERW induction welded pipes, the high-frequency welding process not only directly affects the effect of the next heat treatment process, but also directly determines the final welding quality of the weld joint. Generally, the influence of welding process on welding quality is manifested in welding parameters such as welding input heat, welding extrusion force, welding speed, opening angle, edge shape of tube blank, electrode or inductor, and placement position of impedance. Each welding process parameter is affected by a variety of factors, and the parameters are related to each other, which together affect the weld quality of the welded pipe.
(1) Reasonable optimization of pipe making and inspection processes is an important control method to ensure the quality of ERW induction welded pipe welds and improve production efficiency.
(2) The use of welded pipe raw materials that meet the requirements of relevant standards is a prerequisite for ensuring the quality of ERW induction welded pipe welds.
(3) The geometric dimensional accuracy of the steel strip before the tube blank forming and the quality of the key pipe-making processes such as welded pipe forming, welding, and heat treatment have an important impact on the quality of the ERW induction welded pipe weld.
(4) The main welding process control parameters such as input heat, extrusion force, opening angle, welding speed, and placement of induction coils and impedance resistors directly affect the welding quality of ERW induction welded pipe welds.