Degradation of Magnetic Properties in Transformer Steel: Role of Prior Elastic Deformation

Transformer steel, fully processed Hi-B (high permeability) grade of cold rolled grain oriented (CRGO), was subjected to relatively minor, 0 to -55 MPa, stresses. Detailed magnetic and microstructural characterizations were then carried out. Shift of hysteresis loop and clear degradation in magnetic properties, namely increase in core loss and drop in permeability, were noted. Though absence of mesoscopic plastic strains was confirmed, possibilities of minor local plastic deformation cannot be ruled out. Prior deformations: 1) modified residual strain/stress signatures in both CRGO laminates and Forsterite (Mg₂SiO₄) coating and 2) caused corresponding degradation in magnetic performance. Redistribution of residual stresses through local plastic deformation appears to be the plausible justification for these observations. This paper brought out clear correlations between degradation in magnetic properties with introduced residual stress/strain.