Oxygen Supply Challenge of MgO Crucibles in Vacuum Induction Melting

In nickel-based superalloy production, the vacuum induction melting (VIM) process is critical. Yet one often under-appreciated challenge is that the MgO crucible itself can act as an inadvertent oxygen source.

The phenomenon of oxygen supply

Earlier studies (e.g., Wu Chaowan 1964) indicated that during VIM of pure iron, the crucible wall may release oxygen into the melt, causing the metal’s oxygen content to rise again after initial reduction. Recent research (Li et al., 2024) confirms that even high-purity MgO crucibles can undergo slight decomposition under high vacuum and high temperature conditions (MgO(s) ⇌ Mg(g) + ½ O₂(g)).

Impact on casting quality

• In pure nickel melts, oxygen levels decline initially but then rebound;
• In nickel-based alloys, increased O can lead to Al₂O₃ inclusions, reducing creep resistance and tensile properties;
• These inclusions degrade the alloy’s long-term performance and reliability.

Weiert Ceramics’ technical control strategy

• Use of ultra-pure MgO raw materials (≥99.7%) to minimise internal oxygen source;;
• Strict vacuum control the melt temperature (~1,550 °C).

Verification & results

In comparative trials, Weiert Ceramics’ modified crucibles reduced melt oxygen content from ~3.2 × 10⁻⁶ to ~1.4 × 10⁻⁶, with oxygen supply rate cut by approximately 60%.

Summary

Oxygen supply from crucibles is a hidden contamination source in VIM. By employing high-purity materials and protective coatings, Weiert Ceramics offers advanced crucible solutions that enable improved alloy purity and process repeatability.

References:

• Wu Chaowan, Studies on MgO Crucible in Vacuum Induction Furnace for Pure Iron Melting, Acta Metallurgica Sinica, 1964.
• Li Xu-Ze et al., Deoxidation of Nickel-Based Superalloy Using Carbon under High Vacuum Considering MgO-Crucible Decomposition, Metallurgical & Materials Transactions B, 2024.
• Yan Jing, Vacuum Induction Melting of BY-Series Deformation Nickel-Based Superalloy: Loss Mechanisms of MgO Crucibles, Journal of Large-Scale Forgings & Presses, 2024.