Magnetic and Microstructural Characterization of Carburized 25Cr35NiNb Alloy

During the service of an HP tube, carbon diffuses into the tube wall, forming a carburization layer and reducing the mechanical strength. To quantitatively measure the carburization layer thickness, its magnetic properties should be accurately characterized. Magnetic properties, including saturation magnetization and magnetic permeability, of chromium-depleted HP alloy have been characterized by a vibration sample magnetometer and the eddy current method. The microstructural observations were made using optical microscopy and scanning electron microscopy. The change of the magnetic properties with chromium content has been quantitatively obtained. The results show that the saturated mass magnetization and magnetic permeability have abrupt increases when chromium content is less than 14.6% and 11.8%, respectively. The obtained properties were further used to quantitatively evaluate the carburization layer thickness in a slice of ex-serviced tube. The estimation error was less than 0.33 mm, indicating the characterized magnetic properties are effective for the carburization layer measurement.