Diffusion controlled bainitic phase transformation
The bainitic structure is of considerable importance in the design of high strength steels because of its combination of excellent strength and toughness. This project aims at the description of bainitic phase transformation which is mainly diffusion controlled and the simulation of bainitic microstructure formation by means of phase-field software MICRESS®. In order to deeper understand the mechanisms of bainite reaction, High Resolution TEM analysis, Local field Electrode Atom Probe (LEAP) investigations and 2D phase-field simulations were performed in high carbon steel 100Cr6.
The complete kinetics model according to Azuma et al. was implemented to simulate bainitic transformation kinetics. The modelled kinetics results and the experiments have a good agreement. To model the bainitic microstructure evolution, 2D phase-field simulations were performed, which shows the bainitic sheaf development and also predicts its growth kinetics. Carbon diffusion as well as the interfacial mobility during bainitic phase transformation was considered in simulation. The phase-field predicted phase fraction and microstructure evolution results were compared against the dilatometry and HRTEM experimental results. High resolution TEM analysis show fine bainitic microstructures in the steel and Atom Probe investigations reveal the distribution of elements, especially Fe, C and Cr, in both matrix and carbides in the steel. The detection of ε-carbide in lower bainite implies a large excess of carbon trapped in bainitic ferrite when it first forms. There is no significant segregation of substitutional elements in the interface between bainitic ferrite and cementite. Atom Probe concentration results were compared with the para-equilibrium diagram which was calculated using Calphad method and supported as an input for phase-field simulations.3 Dimensional Atom Probe (3DAP) analyses also reveal the existence of ε-carbide in lower bainite. The detection of ε-carbide in lower bainite implies a large excess of carbon trapped in bainitic ferrite when it first forms.