Role of SiO₂ upon Weld Metal Inclusion Characteristics in EH36 Shipbuilding Steels Treated by CaF₂–SiO₂ Fluxes

Abstract

EH36 shipbuilding steel has been submerged arc welded employing CaF₂–SiO₂ fluxes, and the weld metal inclusion characteristics contingent upon SiO₂ content has been examined. As a function of SiO₂ content, predominant inclusions change from Si–Mn–Al–(Ti) to Si–Mn oxides. Inclusion number density and volume fraction increase from 6 × 10⁶ to 8 × 10⁶ N/mm³ and 8.5 × 10^–2 to 3.0 × 10^–1 pct, respectively. Inclusion size is largely restricted by fast cooling rate despite significant O uptake from the flux.

Appendix: Calculation of Gibbs-Free Energy Change Δg 2 for Reaction [2]

It is suggested that the effective temperature of chemical reactions in weld pool is 2000 °C.^[33] The standard Gibbs-free energy change of Reaction [2] (from 1713 °C to 2000 °C) is referenced from the previous work as Eq. [A1].^[34] Gibbs-free energy change of Reaction [2], namely ΔG₂, is then given by Eq. [A2]. Since dilute solutions of Si and O in Fe liquid behave ideally according to Henry’s law and the slag forms ideal solution, both γ_Si and γ_O is determined to unity. [pct Si] and [pct O] are determined from nominal Si and O compositions from BM and wire as reported previously.^[18]

$$\Delta G_{2}^{\theta } = 542915.10 - 203.11T\,\left( {{\text{J}}/{\text{mol}}} \right) $$

(A1)

$$ \Delta G_{2} = \Delta G_{2}^{\theta } + RT\ln \frac{{\gamma_{{{\text{Si}}}} \left[ {\text{pct Si}} \right] \cdot \gamma_{O}^{2} \left[ {\text{pct O}} \right]}}{{\alpha_{{{\text{SiO}}_{{2}} }} }}\,\left( {{\text{J}}/{\text{mol}}} \right) $$

(A2)

where γ_Si and γ_O are the activity coefficients of Si and O; $\alpha_{{{\text{SiO}}_{{2}} }}$ is SiO₂ activity, which was calculated by FactSage 8.1 software (Equilib Module) using the FToxide and FactPS database, and the calculated results of $\alpha_{{{\text{SiO}}_{{2}} }}$ and ΔG₂ are presented in the Table AI above.

Table AI The calculated results of $\alpha_{{{\text{SiO}}_{{2}} }}$ and ΔG₂