Eddy current profiling of magnetic coercivity in layered ferromagnetic materials

Abstract

Abstract The eddy current response of a ferromagnetic material having a surface modified layer is analyzed for the case where an external magnetic field is swept from negative to positive saturation. Using a plane wave model, it is shown that the impedance of the layered material can display either one or two peaks as a function of field depending on the shape of the hysteresis loops and hence of the reversible permeabilities of core and surface materials. By varying the eddy current frequency different thicknesses of material are probed, resulting in changes in the relative contribution of core and surface material. This causes changes in both the amplitude and position of the peaks with frequency, a feature that can be used for characterizing surface layers. The case of single peaked impedance curves is analyzed in detail as it corresponds to the often encountered practical case of a hardened surface layer. In particular it is shown that as the frequency is varied from a low value corresponding to penetration of the bulk material to a high value corresponding to penetration of the surface layer only, the peak in impedance shifts gradually from the coercivity, H c , in the core to H c of the surface. However, at intermediate frequencies, effective values of H c which are greater than either that of the core or the surface can occur. This is explained in terms of reflections at the core/surface layer interface. Experimental results are then presented for carburized materials of various geometries and shown to be in agreement with the calculations. The frequency f max at which the coercivity reaches its maximum value, and hence where reflection effects are a maximum is also shown to be a useful feature for nondestructively determining the thickness of carburized layers and nugget penetration in resistance welds.