Formation of lower bainite in a high carbon steel – an in-situ synchrotron XRD study

The microstructural evolution of and simultaneous dimensional changes in high-carbon SAE 52100 bearing steel were monitored continuously during austempering for 120 min at selected temperatures the range 210 °C-270 °C, also its subsequent tempering to 340 °C an additional min, via high-energy X-ray diffraction real time in-situ dilatometry. austenite-to-bainitic ferrite transformation induces lattice defects internal stresses that increase with lower temperatures. These are evidenced by full-width half-maximum relevant reflections diffraction. parameter bainitic takes highest value early stages austempering, then gradually decreases as progresses. This observation points initial state carbon supersaturation ferritic is likely reducing due segregation close dislocations, fine carbide precipitation within ferrite, partitioning into surrounding austenite. austenite particular higher 240 270 which there a noticeable remaining longer times. dimensions specimens governed volume change formation relaxation distortion absence further phase transformation.