The eddy current flaw detection of steel pipe is a flaw detection method that uses the principle of electromagnetic induction to detect surface defects of precision steel pipes, components, and metal materials in steel pipes. The detection method is the structure of the detection coil and its classification and detection coil.

The eddy current flaw detector is a non-destructive flaw detection technology that induces eddy currents on the surface of the workpiece. It is suitable for defect detection of conductive materials, including ferromagnetic and non-ferromagnetic metal material components. Due to the eddy current flaw detection, the coil and the component are not required to be in close contact during the detection, and there is no need to fill the coupling agent between the coil and the component, so it is easy to realize the inspection automation. However, eddy current testing is only suitable for conductive materials, and can only detect defects on the surface or near the surface layer, which is inconvenient for components with complex shapes. In thermal power plants, it is mainly used to detect condenser tubes, steam turbine blades, steam turbine rotor center holes, welds, etc. 

When alternating current is passed into the coil, if the voltage and frequency used remain unchanged, the current passing through the coil will also remain unchanged. If a metal tube is placed in the coil, a circumferential current is induced on the surface of the tube, that is, an eddy current. The direction of the eddy current magnetic field is opposite to the magnetization direction of the applied current, so it will cancel a part of the applied current so that the impedance of the coil and the magnitude and phase of the passing current will change. Variations in tube diameter, thickness, electrical and magnetic permeability, and the presence of defects all affect the impedance of the coil. If other factors remain unchanged, only the signal of the impedance caused by the defect is taken out, amplified by the instrument, and detected, and the purpose of flaw detection can be achieved. The eddy current signal can not only give the size of the defect but at the same time, because the eddy current under the surface can lag behind the surface eddy current by a certain phase during eddy current flaw detection, the position t (depth) of the defect can be judged by using phase analysis. In the eddy current inspection, the detection coil is for To adapt to different flaw detection purposes, according to the relationship between the detection coil and the component to be inspected, it is divided into three categories: through-type coil, inner-through coil, and inner-type coil. If it is necessary to insert the workpiece and pass the coil detection, the through coil is used. When testing pipe fittings, it is sometimes necessary to put the coil inside the pipe for inspection, so an internal through-type coil is used. When using a t-type (point-type) coil, place the coil on the surface of the workpiece to be inspected for detection. This kind of coil is small in size and generally has a magnetic core inside the coil. It has high sensitivity and is easy to carry. It is suitable for surface crack inspection of large components and plates and strips. According to the use of the detection coil, it can be divided into three types: absolute coil type, standard comparison coil type, and self-comparison type. Only one detection coil is called the absolute coil type. Two detection coils are connected in a differential form, which is called the standard comparison coil type. Two coils are placed on different parts of the same inspected component as a comparison standard coil, which is called the self-comparison type, which is a special case of the standard comparison coil type. The basic circuit consists of an oscillator, a detection coil signal output circuit, an amplifier, a signal processor, a display, and a power supply.

The eddy current test is to bring the conductor close to the coil with alternating current, and the coil establishes an alternating magnetic field. The alternating magnetic field passes through the conductor and has electromagnetic induction with it to establish an eddy current in the conductor. The eddy current in the conductor will also generate its own magnetic field. The action of the eddy current magnetic field changes the strength of the original magnetic field, which in turn leads to changes in the coil voltage and impedance. When a defect appears on or near the surface of the conductor, it will affect the intensity and distribution of the eddy current, and the change of the eddy current will cause a change in the voltage and impedance of the detection coil. According to this change, the existence of the defect in the conductor can be known indirectly.

Due to the different shapes of test pieces and different detection parts, the shape of the inspection coil and the way of approaching the test piece are not the same. In order to meet various detection needs, various detection coils and eddy current detection instruments have been designed.