Transformation kinetics of carbide-free bainitic steels during isothermal holding above and below MS

The transformation kinetics of a low carbon bainitic steel was investigated using in situ confocal microscopy and dilatometry for isothermal treatments above and below the martensite start (M S ) temperature. Both martensite and bainite are formed during isothermal holding below the MS, but bainite was the dominant transformation product. The prior martensite formed during cooling to the holding temperature strongly influenced the bainite precipitation kinetics. A critical volume fraction of prior martensite, f c P M , was identified. In order to accelerate the bainite transformation kinetics by austempering below the MS, the prior martensite fraction should be less than f c P M . The best combinations of strength and ductility were obtained in steels by austempering below the MS temperature with an optimum martensite fraction (∼5%). Finally, the isothermal bainite transformation kinetics is analyzed by Johnson-Mehl-Avrami-Kolgomorov (JMAK) kinetics model. The analysis suggests that the nucleation of bainite approaches site saturation when the isothermal holding temperature is below the MS.