Abstract: Surface heat treatment can induce micro-structural changes in the components surface to form hardened and transition layer above the core zone, which can be treated as a three-layer structure. The case depth of the hardened layer is an important criteria for evaluating the mechanical properties of the surface material. Magnetic hysteresis model for multilayer structure is established based on point-like magnetic dipole for quantitative evaluation of case depth. Double peaks occur in the predicted differential permeability curve of three-layer structure. The heights of the two peaks and the height ratio almost linearly change as the increasing case depth to produce a new case depth evaluation method. The measured differential permeability curves, which are obtained from high-frequency induction hardened 40Cr steel rods, show that the peak heights ratio is more suitable for case depth evaluation as compared to coercive force. The variation coefficients among the repeated testing results of the two peak heights are lower than 1%, and the linear regression coefficients are higher than 0.95. The proposed hysteresis model can act as theoretical tool for case depth evaluation using magnetic testing method, and the proposed method can be applied for plat or curved components.

Key words: case depth, differential permeability curve, magnetic hysteresis model, quantitative characterization