Alternating current potential-drop measurement of the depth of case hardening in steel rods

The case depth of induction-hardened steel rods has been determined using multi-frequency alternating current potential-drop measurements. Experimental results are analyzed using a model which approximates the variation in the material properties of a hardened rod by assuming that a homogeneous core is surrounded by a homogeneous case-hardened layer of uniform thickness. Experimental measurements on an untreated rod are used to estimate the core conductivity and permeability of similar hardened rods. The implicit assumption is that the material parameters in the core region are unchanged in the hardening process. The case depth is found by parameter fitting to minimize a penalty function representing the overall difference between multi-frequency potential-drop measurements and theoretical predictions. Case-depth values found nondestructively show reasonable agreement with those found using Rockwell hardness measurements on sectioned rods.