The ferromagnetic steel tubes are used for heat transfer tubes of heat exchangers in power plants or oil plants. The outside defect in the ferromagnetic steel tube is generated by friction with the supporting steel plate (baffle). In this research, in order to detect the outer side defect in the ferromagnetic steel tubes, the electromagnetic inspection sensor-probe of the type inserted in the ferromagnetic steel tubes using square wave alternating magnetic field with DC bias is proposed. The flux density and eddy current in the ferromagnetic steel tubes is estimated by the 3-D nonlinear FEM taking account of minor loop curve using the play hysterons model method. The usefulness of this proposal inspection method is shown also from comparison with an experimental verification.

We had proposed a combined 3D solid core and 1D steel plate modeling method for accurate magnetic field and iron loss calculation of laminated iron cores with low computation cost. The effective permeability including the laminated effect can be obtained and fed back to the 3D model. However, only the initial BH curve and classic eddy currents are considered in the 1D model, thus it is improved to consider the DC hysteresis property and anomalous eddy current loss in this paper. The improved 1D model is applied to the dynamic hysteresis loop calculation of non-oriented and grain-oriented silicon steels and confirmed to be effective by comparing with the measurement results.

Pipes made of ferromagnetic alloys are widely installed in petrochemical plants and the steel plants. To implement non-destructive corrosion inspections on these pipes, remote field eddy current testing (ECT) which installs a search coil inside the pipe is a practical inspection method. Remote field ECT has the advantage of being able to detect external surface defects while the distance between the detection coil and the excitation coil is twice of the outer diameter of the testing pipe. Nevertheless, the inspection principle such as the frequency characteristics etc. of ECT signals have not been elucidated, inspection devices based on remote field ECT had not been employed widely. In this paper, we measured the frequency characteristics of ferromagnetic pipes with artificial outer surface defects by remote field ECT, and then detailed finite element analysis are carried out by using the nonlinear eddy current analysis to clarify the inspection principle of the remote field ECT. In both measurement and analysis, peak signal occurs directly under the defect and the optimal frequency range are the same. The phase angle of flux density due to the excitation current and flux density due to eddy current are found to greatly affect the frequency characteristics.

In this paper, we present a novel magnetic flux leakage method to estimate the circumferential position of a crack in cylindrical ferromagnetic rods by modelling the crack as a magnetic dipole. The first-order Fourier coefficients of the radial component of the magnetic flux leakage are measured using two coils with widths proportional to | cos ϕ| and | sin ϕ|, where ϕ is the circumferential location of the rod surface. The circumferential and axial location of a crack is estimated using only two sensors, as demonstrated for an S45C cylindrical rod.

The inspection of the hardened depth of surface-hardening steel is important in the maintenance of various mechanical parts, and for other purposes as well. There is a difference in the electromagnetic properties in the layers with and without hardening. Therefore, the evaluation of the hardened depth is possible by detecting the difference in the electromagnetic characteristics. In this paper, the electromagnetic method for measuring the depth of a hardened layer using an alternating magnetic field of low frequency is proposed. The flux density in the surface-hardening steel is estimated by a 3-D nonlinear finite-element method taking account of the hysteresis characteristics of the layers with and without hardening. In addition, the experimental verification is also carried out.