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

ABSTRACT

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.