Time-Resolved X-ray diffraction Investigation of Austenite and Transformation to Bainite

In-situ time-resolved X-ray diffraction technique with synchrotron radiation was used to track austenite decomposition in steels. Measurements from a Fe-C-Si-Mn steel indicate two lattice parameters in the austenite, possibly due to the development of carbon-rich and carbon-poor austenite prior to transformation. The lattice parameter became uniform with the progress of transformation and the fraction of carbon-poor austenite decreased. The bainitic ferrite itself exhibited a range of lattice parameters during the early stages of transformation, due to the trapping of carbon. The results are in agreement with atom probe data. In order to evaluate the generality of this phenomenon, the experiments were repeated in Fe-Ni and Fe-Ni-C steels. Preliminary evidence shows the tendency for the development of austenite peak splitting even in Fe-Ni-C alloys.