Abstract: A method for determining a case depth of a hardened layer in a surface of a metal object includes: (a) placing an eddy current probe at a location adjacent the surface; (b) using the eddy current probe, generating a time-varying eddy current in the object; (c) using the eddy current probe, outputting a measured eddy current and providing a signal representative of the measured eddy current to a computer; (d) using the computer, comparing the time-varying measured eddy current to a correlation of measured eddy currents to known case depths; and (e) determining the case depth at the location of the probe based on the correlation.