Deoxidation of Nickel-based Superalloy Using Carbon under High Vacuum Degree

The vacuum carbon deoxidation process via CO formation has the ability to achieve high cleanliness of nickel alloys in vacuum induction melting. In the present study, the effect of vacuum degree in melting chamber, melt temperature, and initial carbon content on deoxidation efficiency was studied. The reactions of vacuum carbon deoxidization and MgO decomposition were strongly affected by chamber pressure and melt temperature. Low chamber pressure and high melt temperature resulted in a severe MgO-crucible decomposition reaction and increased oxygen supply to molten nickel alloy, and hence, decreased the deoxidation efficiency. Therefore, moderate vacuum degree in the chamber and lower melt temperature would improve the vacuum carbon deoxidation efficiency. The reaction rates of vacuum carbon deoxidization and MgO decomposition were controlled by the mass transfer of oxygen in liquid boundary layers near the reaction interfaces. The nitrogen in molten nickel alloy could be well removed together with carbon deoxidation under the vacuum conditions in the present study. A prediction model of deoxidation and carbon loss in vacuum melting process was established to determine the optimum temperature and vacuum conditions in vacuum carbon deoxidation process.