Comparative micromechanics assessment of high-carbon martensite/bainite bearing steel microstructures using in-situ synchrotron X-ray diffraction

The mechanical behavior of different microstructural constituents in SAE 52100 bearing steel has been studied at room temperature in relation to quenching and partitioning (QP) and bainitization (B) process parameters, and compared to the standard quenched and tempered (QT) microstructure using high-energy synchrotron X-ray diffraction in situ during tensile loading. Owing to a larger degree of carbon entrapment in its body-centered cubic lattice and associated lattice distortion, martensite in the QT microstructure showed a larger lattice parameter and broadened diffraction peaks as compared to lower bainitic ferrite or partitioned martensite. A reduction in diffraction peak broadness in tempered martensite occurs at a true stress value of ~1800 MPa, and preserves its peak broadness even after subsequent unloading. In contrast, an equivalent effect in peak broadness is detected at ~1500 MPa in the lower bainitic ferrite or mixed bainitic/martensitic matrix characteristic of the B and QP microstructures. In all studied microstructures, the metastable austenite phase transforms when a critical stress is reached, the value of which increases with the bainitic ferrite/martensite fraction and with the carbon content in austenite, but remains lower in the QP and B microstructures compared to the standard QT steel. These results suggest that the carbon solid solution strengthening and associated lattice distortion in bainitic ferrite or martensite are key in determining the mechanical performance of the constituent phases in the steel, with the phase fraction and local carbon content playing an additional role on the austenite mechanical stability.