Welding fluxes, in agglomerated or fused forms, are indispensable components for satisfactory end-user performances. However, recyclability, and, to a larger extent, sustainability of welding fluxes has long been generally deemed as unattainable due to compositional and functional alterations compared with the original recipes. Current technical challenges are rooted from insufficient understanding with respect to thermodynamic factors, such as, but not limited to, due chemical reactions as well as element transfer behaviors occurring in the welding pool.

In this talk, we report a systematic approach towards deciphering key sustainability related issues for welding fluxes aimed for high heat input applications. Fused SiO₂-MnO, CaF₂-SiO₂ and CaF₂-SiO₂-MnO flux systems have been designed and applied to join EH36 shipbuilding steel, a low carbon low alloy grade, under submerged arc welding conditions. It has been demonstrated that SiO₂ and MnO exert synergistic effects on higher partial pressures of the gases (O₂ and SiO) in the plasma, improvement of the O level in the weld pool, and enhanced activities of the oxides (SiO₂, MnO, and FeO) at the slag–metal interface. Moreover, differences between welding slags and virgin fluxes are originated from the reactions during welding. For these three fluxes, the decomposition of SiO₂ and MnO in the arc cavity ushers in compositional changes. The accumulation of FeO in welding slags constitutes the most influential factor due to O chemical potential difference between the molten flux and the molten pool, which facilitates O transfer to the weld metal during subsequent welding applications. After welding, it shows that O content in the weld metal and FeO in the welding slag could be effectively controlled within a certain range, which provides a reliable approach achieving flux sustainability.