Effect of pre strain on magnetic ageing in Fe-Si steels

The durability and performance of electrical steels, essential for electric motors, are compromised by magnetic ageing, which mainly arises from microstructural changes such as carbide precipitation. This study examines how plastic pre-strain influences the magnetic ageing behaviour and microstructural evolution of an ultra-low carbon Fe-Si electrical steel during isothermal ageing at 200°C. Strain quantification was performed using controlled digital image correlation; samples were pre-strained to 2% and 5% plastic deformation prior to ageing. Magnetic properties were measured with single strip testers, and microstructural changes were quantitatively analysed through SEM imaging and image analysis at different ageing intervals. The results show that increasing pre-strain significantly slows down carbide nucleation and growth kinetics, contrary to the usual expectation that dislocations accelerate precipitation. This slowdown is explained by carbon atoms segregating to dislocations, forming Cottrell atmospheres that reduce the carbon available for carbide formation. Although magnetic losses increase with pre-strain due to enhanced domain wall pinning by dislocations, overall magnetic ageing is reduced because precipitate formation is suppressed. A modified precipitation model that includes dislocation density and carbon segregation effects successfully predicts the experimental data. These findings provide new insights into the complex relationship between plastic deformation, precipitation kinetics, and magnetic ageing in electrical steels, guiding strategies to improve motor efficiency and lifespan.