Case depth measurements of surface hardened steel parts are important for quality control. The magnetic properties, including initial permeability, differential permeability and saturation magnetization of a series of induction-hardened and carburized steel rods were studied through measurements, with the aim of developing new methods to evaluate the case depth nondestructively. Four-point alternating current potential drop (ACPD) method also has the potential to evaluate different case depths. The potential drop was measured on a series of 1" diameter induction-hardened and carburized steel rod samples using a co-linear probe with pins aligned parallel to the axis of the rod.

For comparison with the above-mentioned electromagnetic measurements, hardness profiles were obtained by microhardness measurements.

It seems that measurements of initial permeability on surface hardened rods do not give a clear indication of case depth. Differential permeability was plotted versus applied magnetic field H. It is interesting to see that, when H is around 18 Oe, the differential permeability increases more-or-less in the sequence of case depth d in induction hardened samples. Differential permeability measurements give a good indication of case depth in induction hardened rods, but not carburized rods. The saturation magnetization decreases as the case depth increases. It was shown that four-point ACPD is a promising technique for distinguishing between different depths of case hardening. Especially in the low-frequency range of the real part of the normalized impedance, there is very good distinction between the case depths of the samples.