Electromagnetic Inspection Method of Outer Side Defect on Small and Thick Steel Tube Using Both AC and DC Magnetic Fields

As thick steel tubes of small diameter are used in heat exchangers in various electric power plants and petrochemical plants, the inspection of these steel tubes is very important. The outer side defects are often observed on these small steel tubes. Moreover, since these steel tubes in heat exchangers are stored as a bundle, it is necessary to inspect the outer side defects of these steel tubes from the inside of the tube. In this paper, the electromagnetic inspection method using a direct current (dc) magnetic field and an alternating current (ac) magnetic field is proposed. The behavior of flux in steel is examined using 3-D finite-element method (FEM) taking account of hysteresis (minor loop) and eddy current. It is shown that the detection of outer side defect is possible by the differential permeability of minor loop of which the position on the hysteresis loop is affected by the existence of the defect.