Evolutions of Micro- and Macrostructure by Cerium Treatment in As-Cast AISI M42 High-Speed Steel

The as-cast M42 high-speed steels containing different contents of cerium (Ce) were manufactured to investigate the effects of Ce on the solidification structures at the micro- and macroscale. The results indicate that the addition of Ce could modify MgO·Al₂O₃ and MnS into the Ce-containing inclusions. The addition of Ce refined the dendrite structure and eutectic carbides, which can be ascribed to the heterogeneous nucleation of primary austenite on Ce₂O₂S or Ce₂O₃ and the increase in both the compositional supercooling at the dendrite forefront and the restriction on the dendrites coarsening. Both the secondary dendrite arm spacing (SADS) and total eutectic carbides content decrease gradually with increasing Ce content. M₂C and M₆C carbides are the predominant precipitates. The increase of Ce content made the morphology of M₂C carbides change from long lamellar or straight-rod morphology into shorter curved-rod or honeycomb morphology because of the overgrowth of eutectic austenite, but it had no significant effect on the morphology of M₆C carbides. Ce₂O₂S and Ce₂O₃ can serve as the very effective heterogeneous nucleation sites for M₆C carbides because of the low lattice disregistry between them, hence Ce addition improved markedly the macroscopic distribution of M₆C carbides in the cast ingot and promoted the formation of M₆C carbides at the expense of M₂C carbides.