An investigation concerned with the nondestructive determination of case depth by harmonic voltage analysis measurements was continued. A prototype test instrument was designed and constructed which permits rapid, accurate measurement of the amplitude, phase, and in-phase and quadrature components of harmonic frequencies generated by the test piece under the influence of sinusoidal applied field. The design of the instrument is such that a wide range of frequency and strength of applied field can be investigated. It has been established by com¬parative measurements on homogeneous plain carbon steels that the results obtained by harmonic analysis are primarily produced by the additional surface carbon content achieved by the carburizing process rather than such secondary factors as residual stress or retained austenite. The ability to measure several voltage parameters has also been shown to be of value in effectively separating two simultaneously changing variables>i.e., case depth and tempering temperature.
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