Effect of Cerium on the Behavior of Primary Carbides in Cast H13 Steels

Different amounts of elemental cerium were added into H13 ingots to investigate the effect of cerium content on primary carbides' behavior. The enrichment of the elemental Cr, Mo, V, and C in the last-to-solidify region was identified by EPMA. The enrichment of alloy elements is the main reason for the precipitation of primary carbides during solidification. As the increase of cerium content, the size of the last-to-solidify region decreases first and then increases. We found a huge difference between the 2D and the 3D observations of primary carbides. The number density of the primary carbides appeared higher, while their size was smaller in the 2D observation. In the 3D observation, as the cerium content increases, the primary carbides' size also decreases first and then increases, while the morphology of the primary carbides changes from dendritic structure to sheet structure and then to dendritic structure again. The cerium content has little influence on the thermodynamic precipitation process of primary carbides. The enrichment of the elemental cerium in the last-to-solidify region observed by TOF-SIMS accelerates the nucleation of the austenite, further leading to smaller last-to-solidify region size. The smaller last-to-solidify region size provides less space for the further growth of the primary carbide, resulting in the smaller primary carbide size.