Effect of MgO Content and CaO/Al₂O₃ Ratio of CaF₂–CaO–Al₂O₃–MgO Slag on the Oxidation Behavior of Mg in Mg-Containing GH3128 Alloy

Mg can refine and disperse carbides in GH3128 alloy, but it is easily oxidized by certain slag components during electroslag remelting, resulting in severe oxidation loss. To address this, CaF₂–CaO–Al₂O₃–MgO slags with varying MgO contents and CaO/Al₂O₃ (C/A) ratios were designed. Slag-metal reaction experiments were conducted using a crucible in a resistance furnace at 1923 K, and thermodynamic and kinetic analyses were used to understand how these slags affect the oxidation loss of Mg. The results indicate that increasing the MgO content from 8 to 14 wt.pct and the C/A ratio from 0.7 to 1.3, the activity of MgO increases while the activity of Al₂O₃ decreases, thereby reducing the oxidation loss ratio of Mg from 80.43 to 67.35 pct and from 83.52 to 63.54 pct, respectively. A higher MgO content inhibits the diffusion of MgO generated at the slag-metal interface into the slag. A higher C/A ratio enhances the bonding between CaO and Al₂O₃, thereby inhibiting the diffusion of Al₂O₃ from the slag to the slag-metal interface. A slag composition of 50 wt.pct CaF₂, 22 wt.pct CaO, 17 wt.pct Al₂O₃, and 11 wt.pct MgO is suitable for remelting Mg-containing GH3128 alloy.