Significant Improvement of Cleanliness and Macro/Microstructure of As-Cast AISI M42 High-Speed Steel by Mg Treatment

The modification effects of surface-active element Mg on the inclusions and macro/microstructure of as-cast M42 high-speed steel were investigated by microscopy analyses, thermodynamic calculations, and first-principles DFT calculations. The results indicated that Mg treatment changed the inclusion type from MgO·Al 2 O 3 and MnS to MgO and MgS. The contents of O and S as well as the average size and number density of inclusions gradually decreased with the increase of Mg addition amount due to the accelerated floating up of inclusions. Increasing Mg content reduced the secondary dendrite arm spacing and the average thickness of eutectic ledeburite. The refinement of as-cast microstructure by Mg treatment could be attributed to the combined effects of heterogeneous nucleation of primary austenite on MgS inclusions and the solute segregation effect of Mg. The enrichment of solute Mg in the interdendritic regions was identified by ToF-SIMS. Increasing Mg content significantly reduced the precipitation of eutectic carbides by decreasing Mo enrichment and C activity in the residual liquid. Moreover, increasing Mg content inhibited the formation of M 6 C eutectic carbides and promoted the precipitation of M 2 C eutectic carbides by increasing C segregation and decreasing Mo segregation, and thus, significantly improved the homogeneity of as-cast macrostructure.