Detection of Carburization in Ethylene Pyrolysis Furnace Tubes by a C Core Probe with Magnetization

A technique was developed for detection of the magnetic carburization layer generated inside a ethylene pyrolysis furnace tube with magnetic oxide and nitride layers at the external surface. A C core probe was used for detection of the carburized layer through a magnetic layer at the external surface of the tube. Both preliminary experiments and finite element analysis for the measurement properties of the probe were conducted using test pieces that simulated a tube with magnetic oxide and nitride layers at the external surface. The results indicated successful detection of the carburized layer through the magnetic layer, due to the magnetic circuit between the probe and the test piece. Furthermore, eddy current measurements using the C core probe with magnetization to cancel the external magnetism were conducted, because the cancellation effect of the magnetic oxide and nitride layers at the external surface on the magnetic flux generated from the C core probe was still large. Impedance plots were measured for used tubes with the C core probe and with magnetization, and the carbon and nitrogen concentrations of the tubes were analyzed using infrared absorption spectroscopy. The impedance plots corresponded with the analytical results for the carbon concentrations.