Ferrite features in simulated transition zone of EH36 shipbuilding steel submerged arc welded by CaF₂–SiO₂–MnO fluxes

Ferrite features in the simulated transition zone welded with CaF₂–SiO₂–MnO fluxes containing various MnO contents have been investigated. Confocal laser scanning microscopy has been applied to simulate the thermal cycling of the transition zone and the phase transformations during cooling have been in-situ observed. It has been found that the appearance temperature for ferrite side plate decreases with increasing Mn content in the weld metals caused by MnO content increasing. Meanwhile, growth rates for both ferrite side plate and acicular ferrite are significantly enhanced with a higher Mn content of weld metal. Furthermore, from the statistical fractions of salient microstructures, for all samples, the acicular ferrite, grain boundary ferrite, and polygonal ferrite take over more than 90%. It has also been demonstrated that with the increase in Mn content, the ferrite side plate fraction increases slightly from 5% to 10% and the acicular ferrite fraction shows a tendency of first increasing and then decreasing, which experiences the maximum with the flux containing 30 wt.% MnO. This phenomenon is believed to be controlled by the O and Mn contents in weld metals synergistically.