Theoretical Modeling of the Bainitic Transformation in TRIP steels

摘要

A 2D FEM based model of the bainitic transformation in TRIP steels is currently under development at the LISM at Ghent University. The model conceives the bainitic transformation as a sequence of nucleation and growth of ferritic subunits. The growth of a subunit is assumed to take place through a displacive mechanism. The accompanying transformation strain is calculated using the phenomenological theory of martensite formation. An elastoplastic finite element solver computes the stress state that is induced in the system by the accommodation of the transformation strain. This allows a straightforward calculation of the dissipated energy in the austenite surrounding the subunit. The growth step is immediately followed by carbon rejection from the supersaturated bainitic ferrite into the surrounding austenite. This uphill carbon diffusion phenomenon is described using a nonlinear finite element solver. Both stress state and carbon concentration profiles in the austenite will strongly determine the renucleation kinetics of a new subunit. The transformation stops when formation of a new subunit becomes impossible. The results of the finite element calculations show that it is possible to compute some geometrical features of the bainitic microstructure combining mechanical and thermodynamic considerations. It is also shown that the formation of a subunit introduces an autocatalytic effect on the transformation.