Measurement and modelling of the electromagnetic response to phase transformations in steels

An electromagnetic (EM) sensor, capable of detecting the formation of ferromagnetic ferrite from paramagnetic austenite below the Curie temperature, has been developed and assessed. In this article, results obtained using an a.c. EM sensor for a medium (0.45 wt pct)-carbon steel slow cooled through its transformation-temperature range are presented. It was found that the EM sensor can successfully detect the formation of ferrite below the Curie temperature, but that the transimpedance values can be significantly affected by the formation of a decarburized ferrite ring around the samples. It was also found that the transimpedance value is monotonically (nonlinearly) related to the ferrite volume fraction and depends on the morphology/distribution of the ferromagnetic phase and, hence, is influenced by the prior-austenite grain size. Results from finite-element (FE) simulations designed to enable prediction of the transimpedance from the microstructure are also presented, showing that two-dimensional (2-D) FE simulations can be successfully used to model the experimental trends observed. The combination of modeling and measurement has shown that EM sensors can be used to indirectly monitor the ferrite transformation (below the Curie temperature), thus providing a measure of ferrite volume fraction and also a means of identifying the ferrite distribution in the microstructure.