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

Abstract

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.