To improve the weld seam quality of HF-ERW (high frequency-electric resistance welded) pipes, a monitoring system that combines optical and electrical measurement techniques is developed. In the welding process, it is known that there are type 1, 2, and 3 phenomena depending on the welding speed, the input power and the Vee convergence angle. We found that type 2 phenomena can be subdivided into three conditions, normal type 2, instable (type 3) and the two phased reduction of the Vee convergence angle (type 2′) regions in production processes, caused by the facing angle of the edge surfaces, which may not have the I-shape. Among them, only the condition under a narrow range of type 2′ allows to prove melting in the whole area of the edge surfaces and to excrete the oxide inclusions from the weld seam. Following the welding mechanism, the defect rate in the weld seam is kept at minimum. Accordingly, we developed a monitoring system to control the welding condition to the type 2′ range. An optical image processing and an electric impedance measurement method are combined into this system, which is based on remote photographing, high-precision V point detection, temperature measurement techniques with image processing, and a multi-frequency filtering method of the transient impedance waveform. Furthermore, operational indices are constructed through the analysis of the welding mechanism and the experimental data. We find that the weld defect area ratio lower than ever is steadily achieved in the trial products, as a result of continuously controlling the heat-input condition.

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