Elucidating the Roles of SiO₂ and MnO upon Decarburization During Submerged Arc Welding: A Thermodynamic Study into EH36 Shipbuilding Steel

Abstract

By employing CaF₂-SiO₂ and CaF₂-SiO₂-MnO system fluxes, the roles of SiO₂ and MnO in decarburization behaviors during submerged arc welding of EH36 shipbuilding steel have been quantified and evaluated. All possible reactions associated with C transfer and interfaces at which these reactions occur are systematically discussed. It is concluded that the addition of SiO₂ and MnO exerts synergistic effects on the extent of decarburization due to increased partial pressures of O₂ and SiO gases in the plasma, improved O level in the weld pool, and higher activities of the oxides, such as SiO₂, MnO, and FeO, at the slag–metal interface. The investigation over the macrographic detached slag surfaces shows that the possibility of bubble nucleation is highly influenced by flux formula. The effect of heat input on decarburization is discussed, and the optimal flux compositions expected in the present study are analyzed.

Appendix A: ΔG Calculations for Possible Decarburization Reactions in Submerged Arc Welding

The ΔG values of possible decarburization reactions were calculated by FactSage 6.4.[33] The reaction module in FactSage was applied using FToxid, FSstel, and FactPS databases. The ΔG values for the reactions were adjusted to present dilute solute concentrations of C, Si, O, and Mn at one mass percent reference state in liquid Fe by using the free energy of solution values from the literature.[34] The calculations were performed from 1773 (melting point of iron) to 2573 K (temperature of the hot spot at the arc root).[24] The ΔG values were calculated with the following assumptions and summarized in Figure A1.

Fig. A1

Calculated ΔG values for possible decarburization reactions in SAW

• 1. The setting of partial pressures of gases (O₂, SiO, and CO) was based on the fact that their partial pressures in the plasma are much smaller than unity.[15]

• 2. The $P_{{{\text{O}}_{2} }}$ value of 2.57 × 10^–5 atm is the equilibrium $P_{{{\text{O}}_{2} }}$ for the FeO-Fe system at 2573 K with unit activities of FeO and Fe.[35]

• 3. The setting of maximum activities of SiO₂ and MnO expected in CaF₂-free flux (Flux F-12) was based on the assumption that the addition of CaF₂ to the flux has a dilution effect on MnO and may react with SiO₂ (via Reaction [A1]) to lower their activities in the slag.[11,16] The activities of SiO₂ and MnO were referenced from the work of Rao and Gaskell,[36] as it was reported that activity data of steelmaking could be used in SAW in terms of thermodynamic analysis.[18,23,25]

  $$ 2({\text{CaF}}_{2} ) + ({\text{SiO}}_{2} ) = 2({\text{CaO}}) + {\text{SiF}}_{4} ({\text{g}}) $$

  (A1)

• 4. The setting of FeO activity was based on the approximate FeO level analyzed in the post-weld slags in our previous studies, assuming the ideal behavior of FeO in the slags at low FeO concentrations.[16,17]