Study On The Root Bead Slag Detachability Of Large Thick Steel Plate During Submerged Arc Welding

Since the reform and opening up, large thick steel plate have been widely applied in a variety of industries with the rapid development of international trade and industry. Currently, shipping, pressure vessels, electric power, petroleum, chemical industry and so on require a lot of welding used in thick plate. Therefore, the thick plate welding process is particularly important, which submerged arc welding is generally carried out using multi-pass welding. Requirements that submerged arc flux should have a more favorable agglomerating process performance are proposed to meet the improvement of the efficiency in actual production. The slag detachability especially is an important factor which could improve welding efficiency in large thick plate with groove.Some ordinary alkaline slag system has been changed in this article. A high alkaline slag system with fluoride has been developed. That is MgO-Al₂O₃-CaF₂slag system. The effect of agglomerated components, slag microstructure, slag phases, slag thermophysical properties and other aspects of agglomerated flux on slag detachability in large thick plate with groove have been studied and the relationship between slag adherence in agglomerated fluxes with high-basicity and mechanical properties of welded joints have also been studied.In order to measure the rate of slag detachability of flux sample, uniform design method and the falling ball test have been used. The effect of slag system on slag detachability has been studied through experimental results analysis and to a mathematical model for slag detachability and slag system has been established using regression analysis. The reason for slag sticky and slag detachability mechanism with high alkalinity have been deep studied. The results show that it is benefit for the slag detachability when the microstructure of slag was coarse, the second phase of microstructure presented strip-shaped distribution, and the slag structure is compact. And it is bad for slag detachability when exists more spinel phase which could easily growth from weld metal. On the other hand, it is good for slag detachability when slag melting range is higher than weld meta about100℃, slag viscosity is0.3Pa·s and linear expansion coefficient is large different. By measuring the impact properties and tensile properties of welded joints and using optical microscopy, SEM and TEM, the shape and distribution of inclusions have been observed in welded joints, the relationship between slag adherence in agglomerated fluxes with high-basicity and mechanical properties of welded joints have also been studied. The results show that slag with high melting point, good stability stable, and spinel-like is easily to slag sticking in weldment. Thus inclusions distribution in weldment have been affected. Slag adherence determined by flux components has greater effect on impact properties but smaller effect on tensile properties through mechanical properties test.Finally, the paper has acquired the slag system composition for the agglomerated flux with high-basicity which has good slag detachability and mechanical properties of welded joints, i.e. MgO is25%-28%, CaF₂is20%-23%, Al₂O₃is18%-20%, SiO₂is5%-15%, CaO is20%-25%and TiO₂is5%-10%.