Evaluation of case depth in induction-hardened steels: Magnetic hysteresis measurements and hardness-depth profiling by differential permeability analysis

We have studied case depth of induction-hardened steel rods using magnetic hysteresis technique. A step-like behavior of magnetization curve, a decrease of maximum flux density, and an increase of coercivity with case depth, typical for surface-hardened steels, were observed, reflecting a mixture of magnetically hard hardened layer and soft core. Differential permeability exhibited a double peak structure with high- and low-field peaks due to hardened layer and core, respectively. Analysis for the area and position of the permeability peaks, based on a simple model, demonstrated that the volume fraction of hardened layer can be quantitatively and directly inferred from the permeability data alone. Although this method is generally restricted to the case in which a double peak structure in differential permeability is observed, this analysis method can be a possible technique to non-destructively construct depth profile of Vickers hardness from magnetic measurements.