In the steel tube of a large-sized heating furnace in an oil-refining plant, both the front side and rear side are carburized because of being used for many years in high-temperature environments. If these carburization depths increase the steel tube will explode suddenly and a big accident may occur. Therefore, the measurement of both the front side and rear side carburization depth are important in order to prevent accidents. In this paper, an electromagnetic inspection method for the carburized depth of both the front side and rear side of the steel tube is proposed. In this method, the alternating magnetic field of the two kinds of exciting frequency using one electromagnetic sensor is applied to the examined steel tube. Both carburization depths are obtained by evaluating the flux density in layers with and without carburization using the 3-D nonlinear FEM. It is shown that the inspection for the carburization depth of both the front side and rear side of the steel tube is possible by using the deference in permeability and conductivity of carburized layer.

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When testing ferromagnetic materials with an eddy current test (ECT), it may be necessary to saturate the material magnetically with either a DC electromagnet or permanent magnet. The authors already reported that pulsed eddy current tests with inner probe coils might have ability to detect flaws of ferromagnetic heat exchanger tubes by suppressing the magnetic noise that would be caused by variation of the magnetic property of ferromagnetic material. However, it was necessary to use a permanent magnet as a supplement because it wasn’t possible to suppress the magnetic noise sufficiently. In this study, a dynamic magnetizing process was taken into account to suppress the magnetic noise. After a pulsed magnetization has been supplied to the tubes, a short time is needed for falling of the permeability of tubes. Thus a burst wave is applied to execute ECT while the tubes behave as nonferrous material according to pulsed excitation. Adjusting the delay time of the burst wave appropriately enhanced the SN ratio of ECT signals caused by the proposed method in this paper.

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