Effect of Underwater Operation and Surface Rust Layer on Friction Stir Weldability of Carbon Steel

To secure the safety during decommissioning of Fukushima nuclear power plant, temporary repair of rusted steel structures with cracks, possibly located in water, is needed. Friction stir welding (FSW) could be used to repair these structures in water without the elimination process of rust layers, but availability of underwater FSW to rusted steels has hardly been examined. To clarify effect of underwater operation and rust layer on weldability of 0.45%C steel, rusted steel was welded by conventional and underwater FSW at the various rotational speeds, and then the thermal histories, FSW machine outputs, and microstructure and mechanical properties of the stir zone were examined. Underwater FSW could produce the sound welds with no welding defects on 0.45%C steel, but the range of the rotational speed for the sound welds was slightly narrow during underwater FSW. Underwater FSW exhibited the lower maximum temperature and the much higher cooling rate than conventional FSW, resulting in the higher hardness and tensile strength of the stir zone through formation of the microstructure including bainite. Moreover, the rusted steel could be successfully welded by conventional and underwater FSW. The rust layer, about 50 mu m in thickness, hardly affected the weldability, and microstructure and hardness distributions of the weld. However, the fragmented rust was trapped and left near the top surface of the stir zone at the higher rotational speed, resulting in a decrease in strength of the stir zone.