Cerium refinement of grains and primary carbides during solidification of Cr4Mo4V bearing steel

The effect of cerium (Ce) on the solidification microstructure of Cr4Mo4V bearing steel was investigated via a combined experimental and theoretical method. With a trace amount (0.056 wt%) of Ce addition, the coarse columnar grains in as-cast microstructure transform into equiaxed ones, and the average diameter is reduced from 56 to 27 μm. The network-like and bulky primary MC and M₂C carbides at the interdendritic regions become disconnected and refined, and their volume percentage decreases from 4.15 vol% to 2.1 vol%. Ce-inclusions acting as heterogeneous nucleation agents of prior-austenite grains and Ce atoms segregating at grain boundaries, both contribute to the refinement of grains. Thermodynamic calculations reveal that primary carbides are precipitated after γ-austenite forms near the end of the solidification process. The modification of primary carbides in size and amount is mainly attributed to the isolated remaining melt separated by refined γ-austenite grains in which the nucleation of carbides is promoted, while the growth is restrained owing to the less segregation of alloying elements.