Studies on eddy current inspection of steel tubes with external surface ferromagnetic layer

HP steel tubes used in reforming furnaces operate under extreme temperature and pressure, leading to the formation of creep voids in the inner wall. Nucleation of these voids leads to the formation of multiple cracks, when left undetected, may prove catastrophic. In this work, we have investigated eddy current inspections to detect creep induced axial cracks occurring in clusters, emulated by machined notches, located in the sub-surface of retired reformer tube samples. As those tubes were initially exposed to oxidising environments, there is a formation of a thin ferromagnetic layer on the surface of the tube. The probe used for this study, in addition to a rectangular geometry coil for excitation and a Hall sensor for detection, includes permanent magnets to saturate this layer. A study by Finite Element Method (FEM) was conducted to determine the required magnetic remanence to partially saturate the ferromagnetic layer. Boundary Element Method (BEM) was used to rapidly estimate the perturbed magnetic field components due to multiple flaws, in a simplified two-layer stacked geometry. Experiments have been performed over the tube samples containing axial cracks. The results indicate that the proposed technique can detect sub-surface cracks that are nearly 30% of tube thickness.