Crystallography of the nugget zone of bainitic steel by friction stir welding in various cooling mediums

Abstract In this study, the crystallography of the nugget zone of bainitic steel was examined by applying friction stir welding (FSW) and submerging in various cooling mediums. In air, the variants mainly occurred in a single Bain group because of the plastic accommodation of transformation strain. However, in water and a mixture of water and CO2, the variants generated in two and four close-packed (CP) groups, respectively, and this was attributed to the self-accommodation of transformation strain. In this case, the variant selection was inhibited by both the severe deformation during FSW and low temperature of cooling medium. Furthermore, the reduced cooling-medium temperature resulted in refined bainitic and martensitic blocks with an increased fraction of high-misorientation variant pairs owing to the increased transformation driving force.