Magnetic detection of chromium depleted regions in metastable Fe-Cr-C alloy

Abstract

In this work, vibrating sample magnetometry was used to analyse and correlate the magnetisation versus temperature behaviour in the metastable, hypoeutectic Fe-Cr-C alloy with the evolution of the intradendritic segregation, especially chromium depleted regions, as a consequence of applied heat treatment. The Fe-24.5-0.79C alloy used in this work was synthesised in a suction casting device under a high purity argon atmosphere. The investigated alloy contains chromium-rich carbides (paramagnetic phase) and a Fe-Cr solid solution (ferromagnetic phase) in the form of primary and secondary dendrites. The microstructure of the alloy was investigated in the as-cast state and after heat treatment for 4h at 650, 800 and 1000°C using scanning electron microscopy and atomic force microscopy, while the evolution of the domain structure was determined by magnetic force microscopy. The occurrence of chromium depleted regions in the rapidly solidified alloy was confirmed using energy-dispersive X-ray spectroscopy, as well as magnetisation measurements by means of vibrating sample magnetometry. It was found that the chromium depleted regions with a width below 1.5μm, found in the suction cast rod, disappear after heat treatment for 4h at temperatures greater than 800°C. Magnetic measurements and behaviour analysis of the Curie temperature, herein reported, exhibit volumetric character and were successfully applied to detect the chromium depleted regions, as well as to qualitatively evaluate the effect of heat treatment on homogenisation in the investigated alloy.