Research On Influence Of Calcium Fluoride On Welding Process And Quality Of Stainless Steel Underwater Wet Welding Using Flux Core Wire

With the development of marine industry, stainless steel has been widely used in underwater structures owing to its excellent corrosion resistance. As an essential technique for the fabrication, maintenance and emergency rescue of the stainless steel construction, the underwater wet welding is very significant. For the welding consumables of underwater wet welding, which plays a vital effect on the welding quality, a mass of calcium fluoride is added due to its well capacity for dehydrogenation and make the amount of pore in the weld decrease which makes the mechanical property improve. However, the F- produced by Ca F2 during welding process has strong corrosion of stainless steel materials, which easily forms the perfoliate corrosion hole and destruct the structure. Thus, it is necessary to develop the fluoride-free flux-cored wire suitable for underwater wet welding. In order to reduce the content of fluoride in welding consumables of underwater wet welding under the premise of ensuring the welding quality, it is imperative to clear the impact of Ca F2 on the underwater wet welding process. For this reason, in this paper, the fundamental theoretical research of underwater wet welding using flux-core wire with various Ca F2 content is carried out. On the basis of exploration of welding procedure of self-developed self-shielded flux-cored wire for underwater wet welding of stainless steel, the study focuses on investigating the effect of the content of Ca F2 in flux-cored wire on mass transfer process and metallurgical behavior is studied systematically.In this paper, the stainless steel flux cored wire special for underwater wet welding is successfully developed. And the welding appearance, microstructure and mechanical properties of the welded joint made via the flux cored wire is evaluated. The result reveals that there is complete coverage of welding slag and uniform welding appearance without obvious detects. The crystalline structure distribute reasonably, and there is a large area of equiaxed fine grain. The mechanical properties of joints are excellent, and the highest tensile strength reaches 545 MPa while the highest impact toughness reachs 132 J/cm2.The characteristics of metal transfer, welding spatter and electrical signal during underwater wet welding with flux-cored wire containing different calcium fluoride contents is researched via the synchronous system of X-ray high-speed imaging and electric signal acquisition. The force model of drops is built, and the mechanism of metal transfer and welding spatter is revealed. Furthermore, the influence of Ca F2 on mass transfer process is illuminated detailedly. The research result indicates that there are three kinds of main metal transfer modes, i.e. globular repelled transfer mode, surface tension transfer mode and “spatter-like transfer mode”. The “spatter-like transfer mode” is a special transfer mode in underwater wet welding firstly proposed in this paper, and its proportion reduce with the increase of calcium fluoride content. Five kinds of welding spatter modes in underwater stainless steel wet welding are observed. The gas evolution spatter and molten pool shock spatter are the most common spatter type for underwater wet welding. The droplet rotated spatter and droplet repelled spatter more likely to occur with low calcium fluoride content. And explosive spatter is low frequency in this study. When comes to the arc stability, the short circuit and arc extinction exist in all the underwater wet welding. Increasing the content of calcium fluoride content, the proportion of the short circuit and arc extinction both are climb up and then decline.The influence of calcium fluoride content in the flux-core wire on the microstructure, composition and mechanical properties(strength, toughness and hardness) of welded joint is researched. The results suggest that the proportion of cellular crystal zone increase while the proportion of fine grained zone decrease with the increasing calcium fluoride content in the flux-core wire. All the elements, except for nickel element, play the role of solid solution formers in nickel-based weld, and their contents decrease with the increase of calcium fluoride content. The addition of the calcium fluoride prominently improve the strength and toughness of weld joint. While the tensile strength and i MPact toughness decrease with further adding the calcium fluoride.