Application of Four-Point Probe Method to NDE of Case Depth on the Surface of Steel Hardened by the Induction Hardening Process

摘要

A way of evaluating case depth on the surface of steel hardened by the induction hardening process was proposed. Change in electrical resistivity due to hardening was measured using a four-point probe method. Assuming the hardened material to be a two-layer structure with different resistivity, an analytical solution obtained for a flat two-layer structure was used. The procedure to evaluate case depth using the potential drop measured with two sensors that differ in the probe spacing was shown. This method was applied to a shaft-like specimen that is often treated by the induction hardening process. When the diameter of a shaft is small in comparison with the probe spacing, the shape effect cannot be ignored. A simple way of considering the shape effect was proposed. This method was applied to specimens with different diameters and its validity proved. Considering the shape effect is thus important in evaluating case depth of shaft-like materials.