The role of austenite twins on variant selection during bainitic decomposition of a low carbon (0.06 wt%) steel

Abstract

Thermomechanical Controlled Processing (TMCP) is widely used to control the microstructure and properties of linepipe or high strength low alloy steels (HSLA). These steels are often joined by welding and used in demanding environments such as the Arctic. In these materials, the thermal path the steel experiences is critical for understanding microstructural evolution during processing. A key step is the solid-state phase transformation during cooling from the high-temperature austenite to the room-temperature microstructure which significantly influences the final mechanical properties. We used 3D electron backscatter diffraction (EBSD) to explore the relationship between the austenite phase and the room temperature microstructure. A significant result for the present work is the collection, and analysis, of data from a large volume (150 x 150 x 100 um3, with a (200 nm)3 voxel size) which enables analysis of a complete prior austenite grain. This grain is twinned, allowing us to analyse the variants at the twin boundary in this grain, which offers new insights into the mechanisms of the transformation to the low temperature phase by highlighting the significant of the twin boundaries on the variants present. This suggests opportunities to engineer novel microstructures by controlling the high-temperature grain boundary character.