Recently, there is a tendency that 9Cr-1Mo steel has been more likely to be applied to thermal power generation thick-walled piping due to the rise of operating steam temperature. In order to reduce construction period of thermal power plant, high efficient welding method with satisfying required quality, especially automatic narrow gap GTAW of multi-handling, has been expected to be used in on-site welding of 9Cr-lMo steel thick-wall piping. However, there were several obstacles for applying this method. The machine has to be operated at outside and narrow working area. This means that not only the risk of blow hole induced by wind would become drastically high, but also narrow working area restricted the operating space of machine. Also, low fitting accuracy of on-site groove joint caused the bad penetration bead. Therefore, automatic narrow gap GTAW system for on-site welding of thermal power generation piping was developed, including jig for wind protection, suitable device configuration for on-site welding, and welding head design permitted to improve the robustness of automatic GTAW. Also, the automatic weld line tracking by image-processing was developed for multi-handling, which is the method that operator can control two weld machines at the same time. Developed system was applied in construction of combined thermal power plant, and contributed to reduce construction period with showing good weld quality.
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