Quantification of the effect of changes in steel microstructural parameters on EM sensor signals

In an accompanying paper being presented in NDESAI, the utility of Multifrequency electromagnetic sensors has been shown to be able to detect steel microstructure changes, for example the austenite to ferrite transformation and
the presence of decarburisation in high carbon steel rods. In the present study, steels with various carbon contents have been used to study the effect of phase balance changes (ferrite, pearlite, un-tempered martensite and tempered martensite) on the EM readings. With an increase in pearlite content in ferrite/ pearlite microstructures, the relative permeability and hence inductance value dec-reases. Changing the microstructural state from pearlite to martensite, in a high carbon steel, decreases the rel-ative permeability and hence inductance value, whilst tempering increases these values. In addition, steel wires, with a fully pearlitic microstructure have been used to determine whether the EM sensor can be used to quantify interlamellar spacing changes. The low frequency inductance value was found to increase approximately linearly with an increase in the interlamellar spacing for the range of values investigated.