Impact toughness variations of EH36 weld metal treated by CaF₂–SiO₂–MnO submerged arc welding flux

Fused ternary CaF₂–SiO₂–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 CaF₂–SiO₂–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 CaF₂. 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.