Research On Effect Of Boric Acid On Welding Process And Quality Of Underwater Wet Welding

With the rapid development of nuclear industry, the demand for nuclear security and nuclear maintenance technology become pressing gradually. As a key technology for nuclear maintenance and emergency rescue, underwater wet welding is very significant. Considering the theory of underwater welding, the particularity of underwater welding which is applied to repair nuclear plant is mainly embodied in the boric acid underwater environment except for irradiation conditions. Especially, many issues of underwater welding in boric acid solution, such as the metal transfer process and the interaction mechanism of environmental media and the welding process, are not clear so far. Therefore, in this paper, the fundamental theoretical research of underwater wet welding in boric acid environment is carried out. The study focuses on investigating the effect of boric acid on metal transfer process, stability of welding process and weld quality of underwater wet welding. It is believed that this research can fill up the blank of related theory for underwater wet welding, and supply theoretical foundation for the development and popularization of the underwater repair technology in the nuclear plant.The monitoring method and system for underwater wet welding process are proposed and established based on X-ray high-speed imaging. Through this method and system combined with the mathematical modeling and analysis, the characteristics of metal transfer during underwater wet welding is researched. Besides, the force model of drops is built, and the mechanism of welding spatter is revealed. Furthermore, the influence of boric acid environment on metal transfer is illuminated detailedly. The research results show that the mixed repelled globular transfer and solid short circuit transfer mode and the mixed repelled globular transfer and surface tension transfer mode are two kinds of main metal transfer modes during underwater welding in boric acid solution. Respectively, the droplet repelled spatter and explosive spatter are observed when drops transfer by these two modes. If the concentration of boric acid solution is low, the main transfer mode is the former. However, the proportion of the latter mode increases with the concentration of boric acid solution, which is caused by the larger gas flow drag force and electromagnetic force because of higher rising speed of the bubbles. In addition, the molten pool shock spatter is formed in the whole underwater welding process.The arc signal of underwater wet welding in boric acid solution with different concentration is collected, and then be processed by computer. Four assessment methods, which refer to the oscillogram, probability distribution, U-I diagram and variation coefficient, are adopted to explore the influence of boric acid on the arc stability in underwater wet welding. The results suggest that the proportion of short circuit and arc interruption in the underwater wet welding increase with the concentration of boric acid. So the fluctuation of arc voltage and welding current increase, and the arc stability is deteriorated gradually.The effect of boric acid concentration on the macro forming, microstructure and composition as well as mechanical properties(strength, toughness and hardness) of welded joint is studied. The result shows that with the increasing of boric acid concentration, the weld width enlarge, but weld penetration lessens. Although the phase composition of weld is unchanged, the content of boric element in weld is increased. Besides, the equiaxial block ferrite will refine before coarse. The test results of mechanical properties indicate that the tensile strength, elongation of welded joint only have a little change even increasing the concentration of boric acid solution. Moreover, the average hardness of weld zone increases gradually, and the impact toughness of weld metal will rise first and then fall.