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Abstract
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).
The authors would like to thank the UK Engineering and Physical Sciences Research Council and Tata Steel Europe for their support of this research. Contributions from Dr Wuliang Yin from the University of Manchester, Dr Xinjiang Hao and Dr Jun Liu from the University of Birmingham and Peter Morris from Tata Steel Europe are gratefully acknowledged.