document: EM sensors exploit the difference in magnetic properties, such as relative permeability, and electrical conductivity between samples with different microstructural phase balances. In ferromagnetic steels, the change in relative permeability has a significant effect. Previously, multi-frequency EM sensors have been shown to be able to measure austenite/ferrite fraction from 0% to 100% in model (HIPped austenitic/ferritc stainless steel powder) alloys [7,8]. The large difference in magnetic properties of ferrite (ferromagnetic) and austenite (paramagnetic) phases makes the change in signal large and hence relatively easy to measure. EM sensors have also measured the levels of decarburisation (variation in ferrite content with depth) in steel rod [9,10]. The approach adopted to relate the overall steel EM sensor signal to its microstructure has been to construct a finite element (FE) model for the microstructure (phase, region size and distribution). The EM properties of the individual phases are assigned to those regions to give the overall EM properties of the steel. Within the model the particular sensor geometry is included (e.g. two-dimensional axisymmetric for a cylindrical sample and tubular sensor [10]) and the interaction with the steel and any external circuits predicted. In this way different microstructures and sensor designs can be compared.
extractive_keyphrases: austenite cylindrical sample decarburisation difference in magnetic properties electrical conductivity em properties of the individual phases em sensor em sensors external circuits fe ferrite ferromagnetic finite element microstructural phase balances microstructures overall em properties paramagnetic particular sensor geometry relative permeability sensor stainless steel powder steel
abstractive_keyphrases: em sensor signal ferromagnetic steels multi-frequency em sensors steel rod tubular sensor variation in ferrite content with depth