Formation and Precipitation Mechanism of TiN Inclusion in Mg-Treated GCr15 Bearing Steel

TiN inclusions in both original and Mg-treated GCr15 bearing steels have been observed. The results of the observation show that most of the TiN inclusions observed in original the GCr15 bearing steel are single particles, while many of the TiN inclusions in Mg-treated GCr15 bearing steel are TiN–MgO, TiN–MgS, or MgO–MgS–TiN. In Mg-treated GCr15 bearing steel, the TiN inclusions were refined by MgO, MgS, or MgO–MgS inclusions, and the number of TiN inclusions with sizes below 3 μm obviously increased. Thermodynamic calculations indicated that the TiN inclusions precipitated in the mushy zone in both the original and Mg-treated GCr15 bearing steel. The order of precipitation of the inclusions in the Mg-treated GCr15 bearing steel is MgO > MgS > TiN. To understand the formation mechanism of TiN inclusions at the atomic scale, the DFT method was used to investigate the structures and thermodynamic properties of the pre-nucleation metastable phase (TiN)n clusters and the interface between TiN and MgO (or MgS). It can be concluded from the calculation results that the formation of TiN inclusion crystal is controlled by the formation of (TiN)n clusters, and TiN inclusions are more easily formed in a heterogeneous way than in a homogeneous way. TiN inclusions were formed by the homogeneous transformation of the (TiN)n clusters (n > 20), while TiN inclusions on the surface of MgO were formed by the transformation of the (TiN)n clusters (n > 3), and TiN inclusions on the surface of MgS were formed by the transformation of the (TiN)n clusters (n > 4).