Multi-frequency electromagnetic sensor measurement of ferrite/austenite phase fraction—Experiment and theory

A multi-frequency electromagnetic sensor was used to measure the impedance and inductance of samples containing varying fractions of ferromagnetic phase over a range of frequencies (100 Hz to 1 MHz). The impedance values are approximately linearly related to ferrite fraction for random microstructures up to about 40% ferrite, at which point the ferrite increasingly forms connected pathways and the impedance signal does not increase significantly. The zero cross-point frequency was found to distinguish between samples with higher (>40%) ferrite percentages and can be related to the effective relative permeability of the sample.