Nature of upper and lower bainitic transformations revealed by High Energy X-Ray Diffraction

This study provides new insight into the bainitic transformation in steel through a high-resolution, time-resolved analysis using in situ high-energy X-ray diffraction (HEXRD), coupled with post-mortem electron microscopy and electron back-scattering diffraction (EBSD). By systematically investigating isothermal transformations between 350 °C and 500 °C, we reveal the distinct thermokinetic pathways that define lower and upper bainite, not as sharply separated microstructures, but as part of a continuous transition governed by carbon redistribution and precipitation dynamics. At low temperatures (350–400 °C), the transformation is complete and uninterrupted, with carbon enrichment in austenite occurring only after bainite formation. Carbides form within ferrite laths, and no signs of interfacial carbon accumulation are detected, while microstructural observations support the presence of transition carbides at low temperatures, suggesting a tempering-like mechanism. In contrast, upper bainite (450–500 °C) exhibits incomplete transformation (stasis), early austenite enrichment, and delayed carbide precipitation, including inter-block cementite. Peak asymmetry analysis confirms significant carbon heterogeneity in austenite at such high temperatures. Across all conditions, bainitic ferrite forms in a carbon-supersaturated state, with supersaturation decreasing with increasing temperature. The findings challenge classical thermodynamic models of transformation stasis and support a gradual, rather than abrupt, transition between upper and lower bainite, governed by a continuous evolution of kinetics, precipitation, enrichment behavior, and microstructure.