Effect of prior martensite on bainite transformation in nanobainite steel

Nanobainite transformation behavior was comparably studied using in situ neutron diffraction measurements, scanning electron microscopy and electron backscatter diffraction observations for two heat treatments: with and without partial quenching before isothermal holding at 523 or 573 K. Prior martensite transformation was found to accelerate the subsequent nanobainite transformation. Bainitic lathes formed adjacent to a pre-existing martensite plate exhibited an almost identical orientation. Dislocations introduced in austenite due to stress relaxation of transformation strains are believed to assist bainite transformation accompanying variant selection. Diffraction profiles of austenite were found to show symmetric broadening with martensite transformation whereas nonsymmetric broadening occurred with nanobainite transformation, indicating the generation of two populations of austenite. Diffraction line broadening analysis using the convolutional multiple whole profile method provided a dislocation density of 1.51 × 1015 m−2 in austenite after partial martensite transformation.