Role of phosphate coating in determining the magnetic properties of Goss-oriented silicon iron

During the production of Goss-oriented silicon iron, a hard glass-like coating is produced that is augmented by a hard phosphate coating applied to the strip to provide the interlaminal insulation when the steel is built into a transformer core. The insulating coating has a beneficial effect on the stress sensitivity of the steel (the sensitivity of magnetostriction and power loss to stresses in the plane of the sheet). One likely reason for this could be that the steel is held in a state of tension owing to the differential contraction of the steel and coating during cooling. Laboratory experiments have been carried out to determine the role of the coating by studying the effects both of removing the coating and of heat treatment under tension on the stress sensitivity of magnetostriction and power loss of silicon iron. These investigations have shown that the coating plays two main roles in determining the stress sensitivity of the steel; not only does it hold the steel in a state of isotropic tension, caused by the different contractions, but it can also hold the steel in a state of uniaxial tension along the rolling direction. The existence of the two mechanisms found in the samples investigated is verified by comparison of the experimental results with the expected theoretical changes in stress sensitivity.