Effect of Al on the Microstructure and Mechanical Properties of High Carbon Lightweight Bearing Steel

The evolution of microstructures and mechanical properties in high carbon lightweight bearing steels with different Al contents was systematically studied. The results demonstrated that with increasing Al content, the ferrite fraction in the matrix gradually rose. In 10-wt pct Al steel, ferrite could not be fully eliminated even after austenitizing at 1150 °C, primarily due to a significant elevation in the austenitizing temperature. Regarding precipitate evolution, aluminum strongly facilitated the nucleation and growth of κ-carbides, which competitively suppressed the formation of conventional M3C cementite. In addition, due to the variation of solubility of Al in austenite, the decreased Al solubility in austenite promotes ferrite precipitation along prior austenite grain boundaries, leading to a feather-like morphology. The local Al depletion simultaneously lowers the Ms temperature in the affected zones, thereby suppressing the martensite transition in these regions. Notably, lenticular martensite was initially observed in Fe–Cr–Al–C steel, primarily attributed to grain coarsening and elevated martensitic transformation stress induced by substantial Al addition. In terms of mechanical properties, the hardness of 6 and 8 wt pct Al steels was primarily governed by ferrite content, both reaching peak values of approximately 60 HRC at 900 °C and 1050 °C, respectively. The 10-wt pct Al steel failed to achieve a fully martensitic structure, resulting in lower hardness. In terms of toughness, the attainable peak impact energy of 6-wt pct Al steel significantly surpassed that of 8- and 10-wt pct Al steels, mainly attributed to its high grain boundary density, optimal ferrite fraction, and lath martensite structure. This study highlights a novel strategy of lightweight bearing steel by optimizing the added Al content and its consequent microstructure.