Isothermal transformation kinetics, microstructure and mechanical properties of a carbide free bainitic steel

The isothermal transformation kinetics, microstructure, and mechanical properties of a 0.23C high silicon steel, which is isothermally treated both below and above the martensite start temperature (Ms) are investigated. Results show that the small amount of athermal martensite (AM) in below Ms treated sample gives the additional nucleation sites for bainitic transformation, which accelerates the kinetics of bainitic transformation. The nucleation rate accelerates by two orders of magnitude at below Ms compared to above Ms. The microstructural analysis identifies the isothermal decomposition product formed in below and above Ms treated sample as bainitic ferrite. Furthermore, as compared to above Ms, below Ms isothermal treatment shows higher stability of austenite, giving rise to much improved ductility. Moreover, the below Ms isothermal treatment refines the bainitic lath more effectively.