Cerium Alloying in Nanobainitic Steel: A Double-Edged Effect on Austenite and Toughness

The intricate interplay among cerium (Ce) alloying, phase transformation kinetics, and mechanical behavior in a high-carbon nanobainitic bearing steel was systematically investigated. Steels with Ce contents of 0, 0.032, and 0.054 wt.% were subjected to isothermal bainitic treatments, revealing that Ce acts as a critical phase transformation regulator and microstructure refiner. Contrary to expectations, low-level Ce addition significantly suppressed the bainitic transformation completion, leading to incomplete reaction and a two- to fourfold increase in the volume fraction of retained austenite (RA). This abundant, stable RA is identified as the primary source for a substantial and continuous improvement in impact toughness, even as the fracture mode remained quasi-cleavage. Furthermore, the dual effect of Ce was revealed: addition of 0.032 wt.% Ce refined prior austenite grains from 57 to 40 µm and modified inclusions into fine oxysulfides, whereas a higher content of 0.054 wt.% resulted in grain coarsening and the formation of large, detrimental Ce-phosphide inclusions. These negative microstructural changes partially offset the toughness gains, explaining the diminished rate of improvement at higher Ce levels. These findings provide a new strategy for designing high-toughness nanobainitic steels by controlling the RA content through Ce alloying.