Abstract: Rare earth element cerium (Ce) has been widely used as a modifier in high-speed steel to refine its microstructure, thereby enhancing mechanical properties. The beneficial effect of Ce addition is closely correlated with the oxygen content. In this study, using a combination of experimental and computational methods, an attempt to improve the understanding of the role of Ce in carbide refining under very low oxygen conditions is made. Trace amounts of Ce were added to M2 high-speed steel, in which the oxygen content was controlled to be 3 ppm through deep deoxidation. The morphological and compositional evolution of primary and secondary carbides resulting from Ce addition was investigated using electron microscopy, micro-computed tomography, 3D atom probe tomography, and electron energy-loss spectroscopy. The results demonstrate that the addition of Ce to steel can prevent the continuous development of carbides and significantly refine carbide size, which is primarily ascribed to the compositional change induced by Ce addition. The added Ce mainly segregates to the steel matrix, providing additional solute trapping sites for carbide-forming elements such as W and Mo. This leads to the development of small-sized M₂C carbides, which readily decompose into finer M₆C and MC carbides upon subsequent hot processing.

Key words: M2 high-speed steel, Rare earth Ce, Carbide refining mechanism, Element segregation, Thermoplasticity