SiO₂-Bearing Fluxes Induced Solidification Path Transition in Weld Metals of EH36 Shipbuilding Steel

Fluxes play significant roles in fine-tuning weld metal compositions, which may alter pertinent solidification behaviors and ultimately influence ensuing microstructures as well as end-user properties. However, identifying and quantifying phase transformations during weld solidification present an outstanding challenge. Here, we investigate the impact of SiO₂ content on the solidification path of EH36 shipbuilding steel welds using designed CaF₂-SiO₂ fluxes, through in situ observation enabled by confocal scanning laser microscope. With the addition of SiO₂ from 5 to 40 mass pct, it has been demonstrated that significant undercooling caused by the rapid cooling rate mimicking the welding process, drives the solid–liquid interface to shift from a two-stage peritectic solidification mode—where the growth rate surges from approximately 22–10600 μm/s at the onset of peritectic transition—to primary δ solidification, with the L/δ interface migrating at a rate of approximately 290 μm/s. Such a change is directly related to element transfer behaviors between the flux and the weld metal. Moreover, such transition may bode well for γ columnar grain (CG) size evolution, and the unique two-phase (L + γ) region in peritectic solidification carries significant potential in refining columnar grains.