Deeply revealing the coupled formation mechanism of inclusions and precipitates in 654NMo ingot

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This study systematically investigates the formation, evolution, and distribution of inclusions in a 654NMo ingot during solidification by combining lattice mismatch degree with multi-scale characterization, and elucidates the coupled precipitation mechanisms of inclusions and precipitates. The dominant inclusions in the ingot are Al 2 O 3 -MnS-CrN composite inclusions. Initially, MnS nucleates on Al 2 O 3 inclusions, followed by the heterogeneous nucleation of CrN on the Al 2 O 3 -MnS interface, ultimately forming Al 2 O 3 -MnS-CrN composite inclusions. The inclusion number density is significantly higher at the ingot edge compared to the center, while the bottom zone exhibits a greater density than the top and middle. This distribution pattern is attributed to the higher solidification rates at the edge and bottom. Conversely, the center zone contains a larger proportion of large-sized inclusions, resulting from slower solidification, which promotes inclusion collision and coalescence. Furthermore, the Mo 3 Si phase precipitates around the Al 2 O 3 -MnS-CrN composite inclusions. This phenomenon occurs because CrN nucleation on the Al 2 O 3 -MnS composite inclusions decreases the Cr and N of the surrounding matrix, creating a solute-depleted zone that facilitates Mo 3 Si precipitation. These findings provide a scientific basis for alloy design and solidification process optimization of high-nitrogen stainless steels.