Ferrite features in simulated transition zone of EH36 shipbuilding steel submerged arc welded by CaF_(2)-SiO_(2)-MnO fluxes

Ferrite features in the simulated transition zone welded with CaF_(2)-SiO_(2)-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.

Impact toughness variations of EH36 weld metal treated by CaF_(2)–SiO_(2)–MnO submerged arc welding flux

Fused ternary CaF_(2)–SiO_(2)–MnO fluxes have been manufactured and applied to join EH36 shipbuilding steel under high heat input submerged arc welding.Five fluxes have been designed to clarify the effect of MnO content in CaF_(2)–SiO_(2)–MnO flux on the impact toughness of the weld metal,with the added amount of MnO from 10 to 50 wt.%at the expense of CaF_(2).With the increase in MnO content,the Charpy impact energy increases first and then decreases,experiencing a maximum value at 30 wt.%MnO.Microstructure of the weld metals has also been studied to account for impact toughness variations.It has been demonstrated that the highest acicular ferrite volume fraction in the weld metal is achieved at 30 wt.%MnO,which is concurrent to the maximum value of Charpy impact energy.It is believed that the Mn and O content variations in the weld metal contribute synergistically to such an interesting phenomenon.