Microstructural Evolution in Friction Stir Processed API X80 Steel

The evolution of microstructure associate with friction stir welding is very complex.  Although is has been studies extensively in friction stir welded aluminum, there has been limited work in steels.  The complexity of such studies increases when allotropic phase transformations are present such as in steel and titanium. In this paper, Electron Backscattered Diffraction (EBSD) is used as a means to reconstruct prior austenite (PA) grain morphology, orientation, and texture.  Both the room temperature and PA microstructures were used to investigate the nature of deformation and recrystallization in the austenite, and the effects of the elevated temperature deformation on the resulting transformation and room temperature microstructure. Analyses show evidence of shear deformation textures such as A1* (111)[-1-12], B (1-12)[110], and -B (-11-2)[-1-10], as well as rotated-cube (R) recrystallization texture in the reconstructed PA. Existence of rotated-cube texture as well as polygonal grain structure of PA implies that recrystallization is partially occurring in prior austenite. Room temperature ferrite exhibits well-defined shear deformation texture components. The observed shear deformation texture in the room temperature microstructure implies that FSW imposes deformation below the A₃ temperature. The evolution of both elevated and room temperature microstructures in FSW’ed API X80 steel will be presented.