Investigation of magnetic properties and Barkhausen noise of electrical steel

Magnetic characteristics of grain oriented electrical steel (GOES) are usually measured at high flux densities suitable for its applications in power transformers. There are limited magnetic data at low flux densities which are relevant for the characterisation of GOES for applications in metering instrument transformers and low frequency magnetic shielding in MRI (magnetic resonance imaging) medical scanners. Magnetic properties of convention grain oriented (CGO) and high permeability grain oriented (HGO) electrical steels were measured and compared at high and low flux densities at power magnetising frequency. HGO was found to have better magnetic properties at both high and low magnetisation regimes. This is because of the higher grain size of HGO and higher grain-grain misorientation of CGO. As well as its traditional use in non-destructive evaluation, Barkhausen Noise (BN) study is a useful tool for analysing physical and microstructural properties of electrical steel which control their bulk magnetic properties. Previous works deal with measurements carried out at high flux densities (0.2 T and above) but this work demonstrates that BN has different characteristics at low flux densities. The results show that the amplitude sum and the rms BN signals are higher for HGO than CGO steels at high flux densities. Below 0.2 T, the BN signal becomes higher for CGO steel. This is because of grain size/misorientation effects. Mechanically scribing of HGO samples on one surface transverse to the rolling direction was found to reduce the BN amplitude at high flux densities due to the decrease of domain width by scribing. The trend reverses again at low flux density. Removal of the coating from the surface of CGO and HGO electrical steels was found to increase the BN due to the widening of the 180° domains as a result of the release of the tensile stress imparted to the materials during coating. The BN characteristics of decoated samples with a 3 MPa tension applied were found to be similar to those observed before decoating demonstrating the close similarity between the effects of coating stress and externally applied stress on BN due to their similar roles in domain refinement. A strong correlation between average velocity of domain wall movement and changes in BN in conventional and high permeability steels was found which demonstrates that the dominant factor responsible for BN v emission is the mean free path of domain wall movement and hence the width of the predominant 180° domains in these materials. BN of commercially produced non-oriented electrical steel was found to be influenced by silicon contents and sample thickness. BN was found to increase with decreasing strip thickness and increase with increasing silicon contents owing to eddy current shielding effects. The rms values of the BN and the total sum of amplitudes were found to increase with the rate of change of flux density at all the peak flux densities measured. The findings show that the influence of sample thickness and silicon content is significant and must be taken into consideration when measuring and interpreting BN in non-oriented electrical steel