Effects of Al and Ca Addition on Nonmetallic Inclusions and Rolling Contact Fatigue Life in GCr15 Bearing Steel

Four heats of bearing steel with Al contents of 0.0167, 0.0994, 0.4666, and 0.9988 wt.% were produced. Below 0.4666 wt.% Al, inclusions were MnS, Al 2 O 3 and their composites. At 0.4666 wt.% Al, AlN precipitated independently or as fine secondary particles on Al 2 O 3 , while at 0.9988 wt.% Al, AlN accounted for 49.57% of inclusions, coarsening up to 11.74 μm. Thermodynamic analysis of the Al–N equilibrium showed that AlN nucleation and growth are governed by solute segregation and solid fraction. Two additional heats with Ca–Al alloy revealed that Ca markedly reduced S content and average inclusion size, converting MnS into (Mn, Ca)S and CaS with globular morphologies. Rolling contact fatigue (RCF) testing demonstrated that increasing Al significantly decreased both L 10 and L 50 lives, whereas Ca‐treated steels exhibited similar life distributions, albeit with shorter lives than steels free of Al and Ca. Extreme value statistics revealed that large exogenous inclusions, particularly oxides, remain the principal life‐limiting defects, while Ca primarily modifies sulfides without removing large particles. These findings provide guidance for optimizing inclusion control and improving the RCF reliability of bearing steels.