Effect of rare earth elements on microstructure and mechanical properties of bainite/martensite bearing steel

This study aims to investigate the effect of rare earth (RE) elements on the microstructure and mechanical properties of bainite/martensite (B/M) bearing steel. For this purpose, GCr15 bearing steels with different RE contents were prepared and analyzed after experiencing B/M heat treatment. The results show that RE addition significantly delays the bainite transformation by reducing grain boundary energy and carbon diffusion rate, leading to a reduction of bainite volume fraction from 27.3% for bearing steel without RE addition to less than 14.5% for RE-treated steels, and the corresponding increase of martensite volume fraction from 39.1% to more than 50.2%. The volume fraction of retained austenite (RA) remains nearly unchanged. Meanwhile, the width of the bainite needles gets refined in RE-treated steels while martensite and RA blocks become bigger. Compared with bearing steel without RE addition, larger RA blocks in RE-treated steels unexpectedly have higher stability due to higher dislocation density and surrounding phases strength. Accompanied by the change of microstructure, the hardness in RE-Treated steels improves slightly, while the fracture toughness dramatically deteriorates which can be directly explained by two mechanical phenomena during toughness test. The one is that the RA in RE-treated steels is harder to transform into martensite and thus consumes less energy; The other one gives that cracks tend to propagate along grain boundaries or martensite interfaces with less energy consumption easily in RE-treated steels instead of breaking off bainite.