Magnetic measurement to evaluate material properties of ferromagnetic structural steels with planar coils

Nondestructive measurement experiments were carried out with a planar coil to evaluate changes of material properties of ferromagnetic structural steels. Examined steel were of A533B that is a low-alloy steel and of SUS410 that is a martensitic stainless steel. The planar coil has two windings; one is of primary for excitation and the other secondary for induction of output voltage. The coil was placed on a test plate with a magnetic yoke for application of a bias dc magnetic field, and excited with a constant current of 25 Hz. Then the output voltages were measured while slowly changing the bias field by excitation of the magnetic yoke with a triangular-wave form current of 0.005 Hz. Changes of output voltages with different test plates were correlated with their mechanical and magnetic properties. The correlation is so good that this measuring method could be applied to nondestructive evaluation of material degradation in ferromagnetic structural steels.