Characterization of bainitic and martensitic microstructures in bearing steels

Yurtseven, Ece Naz
Bainitic transformations in steels is a well-known phenomenon for more than 60 years. However, in recent years, the lower bainite in high carbon steels gained importance due to their superior mechanical properties since bainitic transformation takes place at very low temperatures, i.e. 160°C -200 °C. In this study, the morphology and mechanical properties of lower bainite are studied after applying different heat treatment procedures. The study covers SAE 52100 grade bearing steel. The specimens are austenitized at either 825C, 875C or 1000C. As austenitization temperature increases the time required for obtaining 100% bainite increases also. All the bainitic specimens yielded hardness values in the range 595HV645HV whereas the hardness of the as-quenched specimens yielded hardness values in the range 670HV-780HV. As far as the retained austenite contents are concerned, the amount of retained austenite in bainitic samples are always lower than 10%. However, the retained austenite content of austenitized and quenched specimens are in the range 10%-55%, such that an increase in austenitization temperature yields higher amount of austenite. For conditioning this retained austenite a tempering operation at 235C is needed so that the retained austenite can transform to martensite upon cooling to room temperature. vi The bainitic specimens which are transformed to bainite after austenitizing at 1000C showed brittle behavior and their tensile testing could not be performed. The bainitic specimens yielded UTS values higher than 2000MPa.
Citation Formats
E. N. Yurtseven, “Characterization of bainitic and martensitic microstructures in bearing steels,” Thesis (M.S.) -- Graduate School of Natural and Applied Sciences. Metallurgical and Materials Engineering., Middle East Technical University, 2019.