Applications of Impedance Plane and Magnetic Differential Permeability in Microstructural Characterization of AISI D2 Tool Steel

Two nondestructive electromagnetic techniques, hysteresis loop and eddy current methodologies, have been used to characterize the microstructural changes of D2 tool steel in the course of quench and tempering treatments. To measure the retained austenite fraction in the quenched microstructure, six specimens were austenitized in the range of 1000-1130 °C. Samples austenitized at 1080 °C were also tempered in the range of 200-650 °C for characterization by eddy current and magnetic hysteresis loop outputs.. Impedance point movement and maximum differential permeability were measured as a function of austenitizing/tempering temperature to characterize the microstructural features. The study showed that good correlations exist between microstructural variations detected by destructive methods (hardness, XRD and microscopic observation) and outputs of the nondestructive techniques.