This paper presents the measurement of ferrite/austenite phase fraction using amulti-frequency electromagnetic sensor. A simple analytical model was established that can describe the response of thesensor for samples containing varying fractions of ferromagnetic phase over a wide range of frequencies(100Hz-1MHz).In particular, a new feature, the peak frequency of the imaginary part of the inductance, is found to be able to distinguish between samples across the whole range of the ferrite percentages. FEM models were used to simulate representative real microstructures from the samples and to relate the relative permeability to the ferrite fraction. Experimental results suggest that the accuracy of ferrite/austenite percentage measurement is within8%.

by heat treating Fe–0.8 wt.% C steel rods for various times in air at 1000 C. The inductance value of the sensor at different frequencies varied as a function of decarburization depth due to the difference in magnetic permeability between ferrite and pearlite. The relationship between sensor output and decarburized layer thickness was modelled using finite element software.

Abstract The link between the electromagnetic properties of steel and its microstructure is a complex one, depending on both phase fractions and morphology. n this paper, both analytical and three-dimensional finite element (3DFEM) modelling techniques were applied to the prediction of permeability for steel with a given ferrite fraction for random ferrite/austenite distributions. Experimental measurements from a multi-frequency electromagnetic sensor on samples generated by hot isostatic pressing (HIPping) of powder mixtures were used to evaluate the analytical and FEM predictions. Theoretical treatment of the relationship between the sensor output and the effective permeability is also given; in particular, it was found that the zero crossing frequency of the real part of the inductance is approximately linearly related to the permeability for high (>40%) ferrite percentages. The EM sensor can therefore be used to identify the samples across the full range (0-100%) of ferrite percentages using both the zero crossing frequency (>40%) and trans-impedance (0–40%). The effect of banded (non-random) microstructures on sensor output and the prediction of the upper and lower bounds of permeability are also discussed.

A robust feature in multi-frequency eddy current (MEC) testing has been found that can be directly linked to the thickness of the plate under test. It is shown mathematically that the peak frequency of the imaginary part of the inductance change when an air-cored coil is placed next to a non-magnetic metallic plate is inversely proportional to the thickness of the plate for a given material. Experimental results indicate that this relationship also holds for a ferrite-cored U-shaped coil. In addition, this peak frequency has been shown to be relatively independent of lift-off variations. Use of this new feature provides a fast and accurate method to gauge the thickness of plates. Measurements made for a sample air-cored and ferrite U-cored coil next to copper and aluminium plates of various thicknesses verified the proposed method.

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.