High-Temperature Crystallization Characteristics of CaF2-SiO2 Fluxes Geared Towards High Heat Input Submerged Arc Welding

Crystallization behaviors of CaF2-SiO2 fluxes with varying SiO2 contents from 10 wt.% to 40 wt.% have been thoroughly investigated using single hot thermocouple technique (SHTT), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The resulting continuous cooling transformation (CCT) and time-temperature-transformation (TTT) diagrams showed that the onset crystallization temperature increased and the incubation time for crystallization decreased with increasing SiO2 content. The primary crystalline phase changed from CaO to Ca2SiO4 and then to Ca4Si2O7F2 with increasing SiO2 content. For two dominant initial crystalline phases, the growth of Ca2SiO4 crystals followed a one-dimensional pattern while that of Ca4Si2O7F2 a three-dimensional one. The effective activation energy for the whole crystalline process decreases with higher SiO2 content and the most negative value of effective activation energy for flux with 40 wt.% SiO2 indicated that Ca4Si2O7F2 phase that led to poor slag detachability should be avoided, while the CaSiO4 phase should be tuned and obtained accordingly due to the large planer disregistry towards enhanced slag detachability. [GRAPHICS]