The basic principle of magnetic flux leakage flaw detection of seamless pipes is based on the characteristic of high magnetic permeability of ferromagnetic materials. After the ferromagnetic seamless steel pipe is magnetized, its surface and near-surface defects form a leakage magnetic field on the surface of the seamless steel pipe, and the existence of the defect can be detected by detecting the change signal of the leakage magnetic field.
The steps of magnetic flux leakage testing
The magnetic permeability of the defect in the seamless steel pipe is much smaller than that of the steel pipe. When the seamless steel pipe enters the steel pipe detection line in an assembly line and passes through the detection area quickly, it will be magnetized by the transverse and longitudinal magnetic fields generated by the DC coil. If the seamless pipe has no defects, most of the magnetic force lines pass through the seamless steel pipe, and the magnetic force lines are evenly distributed at this time. If the seamless steel pipe is defective, the lines of magnetic force are bent, and some of the lines of magnetic force leak out of the surface of the seamless steel pipe. Transverse and longitudinal probe coils (sensors) are used to detect the leakage magnetic field escaping from the surface of the seamless steel pipe. Then according to Faraday's law of electromagnetic induction, the leakage magnetic field is converted into a defect signal, and the defect signal is further processed and analyzed to determine whether the defect exists and the size parameters related to the defect.
Features of magnetic flux leakage testing
Magnetic flux leakage testing can not only detect internal, external surface and subcutaneous defects, but also can know the characteristic dimensions such as defect depth and length and whether it reaches the set rejection level from the relationship between the established electrical signal amplitude and defect parameters without measurement. Strong detection ability and fast detection speed. But it is only suitable for ferromagnetic seamless steel pipes, the investment in testing equipment is large, and the sensitivity of detecting defects is low.