Correlating Microstructure and Magnetic Domain Behaviour in Non-Oriented Electrical Steels via EBSD and Kerr Microscopy : A Study of Hidden Loss-Factors

Sammanfattning

The global transition towards sustainable energy relies heavily on the efficiency of electrical machines, making the continuous minimisation of core energy losses in non-oriented electrical steels (NOES) a critical engineering challenge. The magnetic behaviour of these materials is strictly governed by the dynamics of magnetic domains, which are heavily influenced by local microstructural features. This master's thesis investigates the direct correlation between these structural features and magnetic domain behaviour to uncover hidden loss factors within industrial NOES. Specifically, the study examines two as-produced NOES samples exhibiting significantly different bulk power losses, alongside their stress-relief annealed (SRA) counterparts, to provide industrial insights into unexplained performance deviations. To accurately characterise and observe the magnetic microstructure, the research employed a correlative approach combining electron backscatter diffraction (EBSD) and in-situ wide-field Kerr microscopy. Large-area EBSD mapping revealed that both the standard-loss sample and the high-loss sample possessed nearly identical grain size distributions and crystallographic textures. Furthermore, high-magnification Kerr imaging demonstrated matching optical hysteresis curves and demagnetised domain states in optimally oriented grains across both samples. Consequently, the elevated hysteresis losses in the underperforming sample were attributed to secondary microstructural factors not captured by standard surface analysis, such as localised internal stresses. Additionally, the SRA treatment yielded a highly anomalous result, actively increasing the core losses of the standard-performance sample. This unexpected deterioration is correlated to an oxidation-induced vacancy agglomeration mechanism, which actively introduces new, isolated dislocation loops that pin domain walls and counteracts the intended healing effects of the annealing process. From a methodological perspective, this study demonstrates that correlative EBSD-Kerr analysis provides a more robust diagnostic tool for industrial quality control than conventional power-loss measurements alone. Because Kerr microscopy is limited to an optical penetration depth of approximately 20 nm, it primarily visualises complex surface closure domains generated by grains intersecting the surface at high deviation angles, rather than representative bulk magnetic behaviour. Therefore, EBSD provides the essential structural information required to accurately put these localised, potentially misleading surface responses in context. Finally, a numerical method for vectorised in-plane magnetic domain imaging was successfully developed. By using alternating longitudinal and transverse Kerr imaging, this proof-of-concept allowed for the clear differentiation between 90-degree and 180-degree domain walls, establishing a robust framework for future advanced in-situ magnetic evaluations of NOES.