Welding Journal - October 2012

There are a wide range of important applications for metal tube materials in various fields. The production process of high-frequency longitudinally welded pipe includes first using a roller to coil the long steel strips of a certain size into a cylinder shape, and then join them by heating the steel materials at the welding edge with a high-frequency welding machine — Fig. 1. The processes include uncoiling, strip straightening, end cutting, butt joint welding, loop storage, pipe sizing, passing a hydrotest, printing, and coating. Highfrequency welded pipes are widely applied due to advantages such as no weld filler needed, small heat-affected zone (HAZ), high-precision pipe forming, and highefficiency continuous production. High-frequency welded pipe quality mainly depends on the quality of the weld, and the temperature distribution of the highfrequency induction heating is a key factor affecting the quality. The competition focus in the steel pipe industry has been gradually formed in the development of high-end welded tubes, equipment manufacturing, and process innovation; thereby, the primary thrust in future development is improving the quality of welded tubes. Pipe production should first turn steel plate to tubular, then use the coil with high-frequency electric current transfer energy to the tube edge to complete welding. A science and technology key is to solve the stability problem of weld quality. In practice, the welding process is often adjusted depending on the welder with years of experience who will observe the splash of weld sparks, weld glitch shape, and color of a thermally affected zone. However, this mode of production not only requires a welder with a high technical level, but it is also unable to guarantee continuous and stable weld quality. It is often required to take samples of the formed tubes so as to perform tests of metallurgical and mechanical parameters to investigate whether the quality of the welding area can meet the manufacturers’ requirements, not only increasing the product cost, but also extending the production cycle and reducing the production efficiency. With the increasing market demand for high weld quality, the traditional method of controlling the welded tube quality by relying on postproduction tests and metallurgical observation of the welding area not only requires a great deal of manpower, materials, and financial resources, but is also inefficient, and more importantly, it is unable to guarantee precise control of welded tube quality, a decrease in the rate of waste pipes, and stable production of high-quality welded tubes. The welding process has been optimized to reduce power consumption and improve the weld quality with computer simulation technology so as to efficiently allocate coils, magnetic bars, and rollers. It is difficult to establish a multifield coupling model during a heating process of longitudinal welded pipe due to the special form of magnetic flux distribution under a complex V-shaped geometric area of the pipes to be welded, and highly nonlinear changes of the physical properties of material parameters and other factors. In practice, either the edge is overheated due to high power, or there is a cold weld at the weld joint center due to low power even though the edge is properly welded. Wei et al. (Ref. 1) measured and analyzed the residual stress of joint welds in high-frequency welded pipe with the blind-hole method and an ANSYS finite element analysis program. It was discovered that the residual stress distribution of high-frequency welded pipe is consistent with that of general arc welding. However, a general moving thermal source that is similar to arc welding has been adopted to obtain the temperature field without actually considering the characteristics of high-frequency welding due to the restrictions of the computer hardware and software at that time. Important factors influencing temperature distribution such as the special V-shaped geometric pipe area were not considered, failing to obtain the accurate hourglass-shaped temperature distribution. Asperheim and Grande (Ref. 2) conducted two-dimensional finite element analysis on a high-frequency induction heating section of a longitudinal welded tube, focusing on the study of the horizonNumerical Analysis of a High-Frequency Induction Welded Pipe