In-Situ High Temperature X-ray Studies of Austempering Transformation in High Silicon Cast Steel

The in-situ X-ray diffraction observations of the bainitic transformation of high silicon cast steel were performed using the high temperature X-ray diffraction technique. The volume fraction and carbon content of austenite depend on the transformation temperature. The experimental result has shown that the volume fraction of austenite ceases to a constant value which indicate that the transformation is almost finished after holding for about 1000 s. Asymmetry diffraction peaks are obtained for samples at the early stage of transformation due to a heterogeneous distribution of carbon in different regions of austenite and thus exists two types of austenite: low-carbon austenite (gamma(LC)) and the high-carbon austenite (gamma(HC)). The volume fraction of bulk austenite with low carbon decreases greatly at the early stage of transformation and then tends towards zero. The lattice parameter of both low-carbon and high-carbon austenite increases with the holding time due to the carbon partition from the supersaturated ferrite to the austenite. The experimental results supports that the bainite growth is by a diffusionless mechanism when austempering temperature is in the lower bainite transformation temperature range.