Analysis of Electromagnetic Inspection Method of Opposite Side Carburization Depth in Steel Plate Taking Account of Minor Loop

There is a need to inspect the opposite side of carburized layer in a heating furnace steel tube in an oil-refining plant. The conductivity of the layer with carburization is larger than the layer without carburization in the steel, and its permeability is smaller than the layer without carburization. Then, the estimation of the opposite side carburized depth in the steel is possible by using the differences of these electromagnetic properties. In this paper, the electromagnetic inspection method using a dc magnetic field and a minute ac magnetic field is proposed. The behavior of flux in steel is examined using 3-D nonlinear FEM taking account of minor loops and eddy current. It is shown that the inspection of opposite side of carburized depth in the steel is possible using the differential permeability of minor loop of which the position on the hysteresis loop is affected by the depth of carburized layer.