Investigation of Microstructure and Magnetic Properties of HP40-Nb Reformer Tubes

In this paper, the results of nondestructive evaluation, structure characteristics, and magnetic properties of HP-Nb steam reformer tubes of a direct reduction unit are presented. An eddy current testing technique based on a special normal probe was performed to estimate the service life of the outer surface of the reformer tubes at a frequency of 60 kHz. The magnetic properties were investigated by vibrating sample magnetometer testing. Microstructural transformations of reference samples were performed by an optical microscope equipped with image analysis software and scanning electron microscopes. With exposure to severe operational conditions in oxidizing atmospheres and high temperature, microstructural changes such as oxidation and transformation of M23C6 and M7C3 carbide in the carburization zone, and chromium depletion in the austenite matrix, can be intensified. The tubes were inspected on a point basis and the eddy current data were presented as an impedance map. The results of the eddy current testing and vibrating sample magnetometer showed that microstructural changes due to operational conditions affect the electromagnetic response of the tubes.