MAGNETOINDUCTIVE DIFFUSION LAYER THICKNESS GAUGING

Abstract Measuring the properties of thermo-chemically produced surface layers, in particular gauging the thickness of such layers, presents a nondestructive testing problem hitherto not satisfactorily solved. To date, magnetoinductive or eddy current methods have failed to eliminate the influence of the smelting charge and the method of treatment on the resultant signal. Smelting charges differ in conductivity and magnetic permeability and, hence, the calibration curve obtained for a particular charge cannot be translated to any other charge. Thus, application of eddy current methods to this problem area on a broad scale has not been possible. The idea of eliminating the influence of charge is to avoid measuring the properties of the layer in absolute terms and, instead, quantify the differences between the properties of the bulk material and those of the layer. To this end, information for various depths of the material can be obtained by both discrete multifrequency and broad-band test signals. This paper presents thickness gauging criteria for carburized layers which, by means of a vector rotation method and spectral analysis of eddy current response signals, are applicable irrespective of a particular charge.