Overview of non-destructive evaluation of steel microstructures using multifrequency electromagnetic sensors

The characterisation of steel microstructures is an important tool for metallurgists as mechanical properties are controlled by microstructural parameters such as grain size, phase balance and precipitates. The majority of microstructural characterisation tools are indirect (inference from measurement of temperature), destructive (optical metallography, SEM and X-ray) or require small samples and laboratory equipment (dilatometry and DSC). This paper describes a multifrequency electromagnetic sensor that has been designed to detect changes in the relative permeability and resistivity of steel, which can be related directly to changes in microstructure. COMSOL multiphysics modelling software, considering the sensor design, sample geometry and sample microstructure, has been used to relate the measured sensor signal to changes in the steel microstructure. Examples presented in this paper are where the sensor has been used to monitor phase transformation (austenite to ferrite) below the Curie temperature (∼770°C) during online processing of steel; detection of decarburisation, both online and offline; and evaluation of the effect of long term thermal exposure on alloyed steels (changes in martensitic/ bainitic microstructures). © 2011 Institute of Materials, Minerals and Mining.