Iron losses are an important consideration when assessing the energy efficiency of an electrical motor. When Fe-Si steels are exposed to moderate temperatures, their in-service magnetic properties degrade. This occurrence is caused by the precipitation of carbides or nitrides at low temperatures, which correspond to the normal operating temperatures of electrical equipment. The carbon concentration influences magnetic ageing, as well as Si-content: high-Si content helps to preserve from magnetic ageing.

A crucial factor that may change precipitation kinetics and hence magnetic ageing is the strain state of the materials, which has never been studied in the context of magnetic ageing. Electrical steels are subjected to deformation methods during the development and manufacturing of electrical devices, building a strain and stress state in the materials that may have a negative impact on their magnetic performance in operating conditions. In particular, interaction between dislocations, interstitial atoms and precipitates may modify the level of magnetic ageing and/or its kinetics.

The primary objective of this study is to determine the effect of pre-straining on the magnetic ageing of Fe-Si alloys. In this study, Fe-Si alloys investigated contain less than 3% silicon and 60ppm to 90ppm carbon. Two types of specimens were tested, one non-prestrained and the other with a (< 5%) plastic strain. To verify the homogeneity of the straining conditions, strain was measured by Digital Image Correlations (DIC) analysis. The axial plastic strain data using DIC demonstrate that homogeneous plastic strain may be achieved across the sample for plastic strain higher than 2%. Specimens were cut based on the dimensions required for Scanning Electron Microscopy (SEM) analysis and magnetic measurements using Single Sheet Tester (SST). Specimens are aged at temperatures between 160°C and 200°C, typical operating temperature range. The SEM analysis are preformed to capture images at different ageing time to study the evolution of mean radius and volume fraction of carbide precipitates formed during ageing. The SST measurements were performed on samples with identical ageing times to the SEM analysis. This method allows us to study how microstructural changes influence the magnetic behaviour of the Fe-Si alloy.

 When precipitation kinetics and magnetic losses are compared, the pre-strained sample exhibits a different behaviour in terms of magnetic property degradation than the non-prestrained sample. Pre-straining is expected to alter the carbon distribution that segregates on dislocations, modifying distribution and kinetics of carbide precipitation.