Measurement and modeling of the electromagnetic response to phase transformation in steels
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2/20/04
10:47 AM
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Measurement and Modeling of the Electromagnetic Response to Phase Transformation in Steels M.P. PAPAELIAS, M. STRANGWOOD, A.J. PEYTON, and C.L. DAVIS 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.
I. INTRODUCTION
AFTER hot rolling, strip steels are cooled to obtain the microstructure and, hence, mechanical properties required by the customer. To increase microstructural homogeneity and, hence, improved consistency in the mechanical properties, variations in the temperature distribution and cooling rate must be avoided. High-strength multiphase strip steel is hot rolled as austenite, water cooled on the run-out table to give the ferrite transformation, and then coiled at a prescribed temperature to allow any second phase to develop.[1] Accurate control of the cooling process is required to form the desired fraction of ferrite and second phase in the steel. Optical pyrometers can be used to monitor the strip temperature, however, their performance is significantly affected by interference from water droplets, steam, and dust in the cooling zone, resulting in reduced accuracy.[2] A number of alternative on-line nonmagnetic methods have been proposed to directly measure microstructures, including X-ray diffraction,[3] X-ray attenuation to monitor density changes,[4] laser ultrasonics,[5] and, for off-line measurement, differential thermal analysis (DTA)[6] and dilatometry. Electromagnetic (EM) techniques also provide the opportunity for monitoring phase transformations by detecting changes in electrical conductivity and magnetic permeability within the steel. This is particularly useful, because conductivity and permeability are known to be directly influenced by t
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