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

Two nondestructive electromagnetic/magnetic techniques including hysteresis loop and eddy current methodologies have been used to characterize microstructural changes of AISI D2 cold work tool steel as a result of quench and tempering treatments. To measure the fraction retained austenite in 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, nondestructively. Impedance point movement and maximum differential permeability were measured as a function of austenitizing/tempering temperature to characterize the microstructural features. The study shows that there are good correlations between microstructural variations detected by destructive methods (hardness, XRD and microscopic observation) and outputs of the nondestructive techniques.