Experimental Evaluation of Defects in Ferromagnetic Steel Tube by Electromagnetic Sensor Using Square Wave AC Magnetic Field with DC Bias

Regular inspections are required for the heat transfer tubes of multitube heat exchangers in power plants and oil plants.Since the number of these steel tubes is large, high-speed inspection is desired.The use of an AC magnetic field is known to be a high-speed method of inspecting nonmagnetic tubes.On the other hand, since large electromagnetic noise is generated in a ferromagnetic steel tube with nonuniform initial magnetic permeability, it is difficult to inspect a steel tube by an electromagnetic inspection method using a minute AC magnetic field.However, the nonuniformity of the initial permeability in the steel tube is reduced by a minute AC magnetic field with a large static magnetic field.In this research, an electromagnetic inspection method for detecting defects in a steel tube by applying a magnetic field that is a combination of a large static magnetic field and a minute AC magnetic field to the steel tube is investigated.We propose an electromagnetic sensor probe using a square wave AC magnetic field with DC bias that is inserted into the steel tubes for inspecting steel tubes.In addition, it is shown that the detection sensitivity of defects in the steel tube when using the square wave AC magnetic field with DC bias is higher than that when using a sinusoidal AC magnetic field with DC bias.Phenomena that occur during the inspection by this method are investigated in detail by the 3D nonlinear finite element method, taking account of the magnetization properties of the minor loop.