Evolution of inclusions in vacuum induction melting of superalloys containing 70% return material

The variation law of inclusions type and size in the vacuum induction melting process and ingot of Ni-based superalloy containing 70% return material was studied by industrial test sampling, and the mechanism of inclusions formation was analyzed with thermodynamic calculations. The results show that there are mainly two types of composite inclusions in the vacuum induction melting of Ni-based superalloys, which are nitride- and oxide-based composite inclusions, like Al₂O₃–SiO₂–Cr₂O₃, TiN–(Mo, Nb)C, etc. The type and proportion of inclusions from the center to the edge of the vacuum induction ingot did not change significantly. The number density of inclusions from the center to the edge of the ingot varied less, and the size of inclusions became smaller from the center to the edge. In addition, thermodynamic calculations show that oxides (M₂O₃) are present in the liquid phase and mainly contain Al, Ti, Cr, Fe and O elements. The nitride consists mainly of Ti and N and contains small amounts of Cr, C, Nb, and Mo elements. This is consistent with the results of industrial tests. As the temperature decreases, the precipitation phases such as M₂O₃, MN, γ, MC, δ, \(\upgamma^{\prime }\) and μ phases are gradually precipitated, where oxides and nitrides are present in the liquid phase. The contents of O and N elements are the main influencing factors for the inclusions content and precipitation temperature; when the nitrogen content is reduced to below 0.0015%, it can make MN precipitate below the liquid-phase line.