IMPROVEMENT OF LOW-TEMPERATURE TOUGHNESS IN WELD METAL MADE OF 9CR-1MO-V STEEL BY GTAW METHOD

摘要

Energy demand will increase due to global population growth in the future. As one of solutions for the demand, it will be necessary to operate petroleum service plants more efficiently. To improve refining efficiency, operation at higher temperatures is required of reactors used in high-pressure hydrogen service at the plants. 9Cr-1Mo-V steel has excellent creep strength compared to 21/4Cr-1Mo steel and 21/4Cr-1Mo-V steel, which have been conventionally applied to reactors, and has been already put into commercial use for boilers of thermal power plants, etc. Further application of 9Cr-1Mo-V steel is expected for reactors at petroleum refining plants. As materials applied to reactors, low-temperature toughness should be considered for weld joints in addition to creep strength. However, 9Cr-1Mo-V steel has poor low-temperature toughness compared to 21/4Cr-1Mo steel and 21/4Cr-1Mo-V steel. As for the welding methods applied to reactors, Submerged Arc Welding (SAW), Shielded Metal Arc Welding (SMAW), and Gas Tungsten Arc Welding (GTAW) can be used. The 9Cr-1Mo-V steel weld metals formed by SAW and SMAW have a high oxygen content, and their low-temperature toughness is inferior to that of the weld metal formed by GTAW. On the other hand, the GTAW weld metal has a low oxygen content and excellent low-temperature toughness. Therefore, GTAW is an effective way to improve the toughness of the weld metal. However, GTAW has low productivity compared with others, so it is necessary to apply to a narrow groove and a hot wire method to improve the productivity. In this paper, the application of narrow gap GTAW using the hot wire method was considered for welding of 9Cr-1Mo-V steel. When using the hot wire method, productivity of GTAW increases in comparison to the conventional method, leading to increased weld pass thickness. With the increase in pass thickness, the area of coarse grains increases because of decreasing thermal effect by the subsequent pass, then the low-temperature toughness decreases. Therefore, in order to improve the low-temperature toughness by refining the grains of the GTAW weld metal, the melt-run method, arc re-melting without adding fillers, was conducted after the former weld pass metal solidified. The weld metal from the melt-run method had finer grains compared with those of the weld metal without the melt-run method, and the low-temperature toughness increased. On the other hand, the melt-run method requires two processes: welding and melt-run. Therefore, a tandem electrode GTAW machine was produced in which an electrode for welding and the other one for melt-run were placed continuously to make it possible to execute welding and a melt-run without a time lag. As a result, it is possible to manufacture reactors made of 9Cr-1Mo-V steel for petroleum refining plants with sufficient low-temperature toughness by applying a welding method with narrow gap GTAW and a melt-run method combined.