Detection principle is based on the excitation coil that generates eddy currents within the conductive member, the amount of change measured by the detection coil eddy current, to obtain information about the defect member. According to the shape of the detection coil can be divided into through-type (for the detection of wire, bar and tube), probe type (local test for the member surface) and plug-in (for the internal pore detection) three species. Eddy current inspection to AC electromagnetic coil flaw in the metal structure of the surface nondestructive testing technology induction. It is suitable for the conductive material, including defect ferromagnetic and non-ferromagnetic metal material member is detected. It is not required in the detection coil in close contact with the member, the coupling agent is not filled between the coil and the member, easy test automation. But only for the eddy current testing of conductive material, can detect defects or near the surface layer, inconvenient to use in complex shape components. In the power plant is mainly used to detect the condenser tubes, turbine blades, turbine rotor center hole and weld and so on. When the alternating current passed through the coil principle, if the voltage and frequency are used unchanged, the current through the coil will change. If placed in a metal tube, the circumferential surface of the tube current is induced in the coil, i.e. the vortex.
Eddy current is high frequency welded pipe production is one very important process, high-frequency welded steel pipe in fluid transport, building components and container manufacturing and other fields have a wide range of uses. According to the technical requirements of pipe, the weld shall not have cracks, cracks, lack of fusion defects, the surface does not allow excessive scratches, crushing and other defects. Since the pipe production line with continuous, rapid production characteristics, welding speed of 15 to 60 m / min, and therefore, the quality of welded manually post hoc test alone is difficult to ensure; while the eddy current inspection method has a detection speed, no Coupled with the workpiece surface requires high detection sensitivity, detection can be carried out simultaneously with the production of the advantages of steel, which can be used instead of steel pipe hydrostatic testing, greatly reducing the steel production process brought by the root cumbersome hydraulic test test Therefore, it is suitable for quality inspection and quality control pipe production process.
Eddy current testing is a high-frequency welded pipe close to the conductor through the coil with alternating current, an alternating magnetic field created by the coil, the alternating magnetic field through the conductor, and with the occurrence of electromagnetic induction effect. Eddy currents in the conductor will generate its own magnetic field, the magnetic field changes the role of the original vortex strength of the magnetic field, leading to changes in voltage and coil impedance. When the conductor surface or near-surface defects will affect the intensity and distribution of the swirl, caused by the change of the eddy current and the coil voltage and the impedance change detection, according to this change, the guide can be indirectly known defects present in the body.
