Microstructure of low C steel isothermally transformed in the M _S to M _f temperature range

An isothermal transformation was observed when a fully austenitized lean-alloyed, low C steel was quenched to a temperature in the M _S to M _f temperature range and held at the quenching temperature. The dilatometric analysis revealed that the isothermal transformation was distinct from the bainitic transformation. Internal friction (IF) measurements and X-ray diffraction (XRD) analysis showed that the dislocation density in the isothermal transformation product was larger than in lower bainite, and lower than in athermal martensite. Microstructural analysis by transmission electron microscopy (TEM) revealed that the isothermal transformation product had a specific microstructure consisting of large lath-type constituent units with wavy boundaries, with a Nishiyama-Wassermann orientation relationship (NW OR) with respect to the parent austenite. The isothermal transformation below M _S proceeds by the thickening of athermally formed laths.