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

Abstract

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 Al2O3–MnS–CrN composite inclusions. Initially, MnS nucleates on Al2O3 inclusions, followed by the heterogeneous nucleation of CrN on the Al2O3–MnS interface, ultimately forming Al2O3–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 Mo3Si phase precipitates around the Al2O3–MnS–CrN composite inclusions. This phenomenon occurs because CrN nucleation on the Al2O3–MnS composite inclusions decreases the Cr and N of the surrounding matrix, creating a solute-depleted zone that facilitates Mo3Si precipitation. These findings provide a scientific basis for alloy design and solidification process optimization of high-nitrogen stainless steels.

Keywords

• 654NMo ingot
• Solidification
• Inclusion
• Precipitation phase