Assessment of depth of case-hardening in steel rods by electromagnetic methods

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.