Modelling of Microstructural Banding during Transformations in Steel

Microstructural banding is defined as alternating layers of two different microstructures in steel, often ferrite and pearlite. It is caused by fluctuations in the concentration of alloying elements, primarily manganese, due to microsegregation introduced during solidification. In this thesis, a model is presented to simulate banding using phase transformation kinetics theory. An existing program that simulates the decomposition of austenite to allotriomorphic ferrite, Widmanstätten ferrite and pearlite upon cooling was significantly modified to treat steels with an inhomogeneous distribution of solute, with the focus on manganese. The concentration profile was divided into discrete concentration steps (“slices”) and paraequilibrium conditions were assumed. The slices interact by the partitioning of carbon between them. After each time step, the concentration of carbon in untransformed austenite is calculated by taking into account the amount of ferrite formed in all slices, effectively assuming infinitely fast carbon partitioning. Simulations were carried out using three sets of input parameters, one of them being a typical steel with parameters chosen to clearly show banding and two of them taken from the literature for comparison of the model with experimental data. Input parameters were systematically varied to test the behaviour of the program. Trends for varied cooling rate, austenite grain size and concentration fluctuation amplitude are in accordance with the expected results. The model was capable of reproducing the suppression of banding above a critical cooling rate, although this rate did not quantitatively agree with experimental findings for all the test cases implemented. Results differ from experiments mainly for high cooling rates, probably due to the unrealistic assumption of infinitely fast carbon partitioning between the slices. A method is suggested on how the current model could be improved to include a finite carbon partitioning velocity. The work nevertheless represents the most comprehensive treatment of the phenomenon of banding to date.