Probing Viscosity and Structural Variations in CaF2–SiO2–MnO Welding Fluxes

Recent demand on welding fluxes for high-strength low-alloy steel grades has necessitated the development of CaF2-free fluxes towards high heat input applications. To meet such requirements, a flux design strategy has been postulated by varying MnO/(CaF2 + MnO) mass ratio in the CaF2–SiO2–MnO flux system. The effect of MnO substitution for CaF2 on the flux viscosity and structure has been investigated, and a quantitative relationship between viscosity and structural units has been put forward. In addition, it has been suggested that the degree of polymerization (DOP) of silicate network is reduced with increasing MnO/(CaF2 + MnO) mass ratio, indicating MnO plays the role of network-breaker. The spectra of F1s and 19F prove that F− predominantly bonds with Ca2+ rather than Si4+, revealing CaF2 acts as a diluent and does not effectively depolymerize the silicate network. Linear correlations can be obtained between the logarithm viscosity and the average non-bridging oxygen per silicon atom (NBO/Si), which can potentially be used as a DOP index to quantify the impact of silicate structure on the viscosity of the flux.