Challenges Concerning the Characterization of Cementite in Low Carbon Steel Using Electron Backscatter Diffraction

Abstract

Understanding the location and morphology of cementite precipitation in steel has long been addressed almost exclusively with transmission electron microscopy, particularly in low carbon steels. However, scanning electron beam techniques such as electron backscatter diffraction (EBSD) could leverage automated diffraction pattern analysis and high scan rates to improve statistics in cementite analysis. This paper discusses challenges specific to EBSD through the lens of a case study concerning low carbon microalloyed pipeline steel. Correlations between secondary electron (SE) and EBSD micrographs of a low carbon, microalloyed steel etched with 2 pct nital could not effectively verify the presence of cementite identified using EBSD. Pixels identified as cementite by the EBSD software often held a 43.6° || <100> axis/angle relationship reflected in grain boundary texture misorientation distributions. Further investigations of the ferrite/cementite interfaces displaying this 43.6° || <100> relationship were undertaken by utilizing pole figures of three well-known orientation relationships (OR) between ferrite and cementite, which resulted in the best match with the Bagaryatskii OR. Some questions concerning interaction volumes and indexing/phase identification algorithms are presented with respect to cementite characterization, as well as a proposal for future work to consider possible pseudosymmetric phenomena in specific ferrite orientations.