On low temperature bainite transformation characteristics using in-situ neutron diffraction and atom probe tomography

In-situ neutron diffraction was employed to monitor the evolution of nano-bainitic ferrite during low temperature isothermal heat treatment of austenite. The first 10 peaks (austenite, γ and ferrite, α) were monitored during austenization, homogenisation, rapid cooling and isothermal holding at 573 K. Changes in the α-110 and γ-111 peaks were analysed to determine the volume fraction changes and hence the kinetics of the phase transformation. Asymmetry and broadening in the α-200 and γ-200 peaks were quantified to lattice parameter changes due to carbon redistribution as well as the effects of size and dislocation density. Atom Probe Tomography was then used to confirm that, despite the presence of 1.5 mass% Si, carbide formation was evident. This carbide formation is the cause of poor ductility, which is lower than expected in such steels.