The New MFL Principle And Its Applications For Steel Pipes

As a highly efficient NDT (nondestructive testing) method, MFL (Magnetic flux leakage) has been widely used for testing and evaluation of various kinds of ferromagnetic materials, especially having obvious advantages in automatic inspection of steel pipe. Up to now, the studies of MFL have a long history of nearly one hundred years. With the increasing requirements and detection index, the further research of MFL testing method and its relevant new principle is of great importance to the theory system and of benefit to find a new MFL testing method for steel pipe.Firstly, starting with the basic magnetic interaction and its relevant physical feature in electromagnetism, and combining with analysis of detection approaches and their working characteristics, the forming course of MFL of defects is described into three steps:magnetic refraction, magnetic diffusion and magnetic compression, leading to a new leaking mechanism of the MFL of defects. The proposed new theory has been confirmed and analyzed by mathematical derivation, finite element method (FEM) and experiments. It is concluded that the background magnetic fields reduce the magnetic refraction but they strengthen the magnetic compression, resulting in a reduction of the MFL of defects, providing a theoretical basis for detection sensitivity of MFL.Further, a new MFL principle and its testing method based on the establishment of a near-zero background magnetic field are proposed, i.e. by reducing/or eliminating the background magnetic density and therefore the flux in the testing body leaking into the created near-zero background magnetic region with the largest refraction angle and without magnetic compression, the strongest MFL signals can be captured by magnetic sensors, reducing the signal noises caused by the background magnetic fields. During the course of simulation and experiments, the distortions and mutations of MFL in present MFL testing method are pointed out, helping to find the root of explanation for MFL inspection signals.On the basis of the work mentioned above, taking the high-speed testing of steel pipes as an example, a new MFL technology based on orthogonal magnetization is presented, being successfully applied in the in-line inspection for coiled tubing. Additionally, a new method of MFL inspection for omni-direction defects under a unidirectional magnetization is also put forward, designing an in-line MFL testing system also successfully applied in the detection for coiled tubing and steel strip. The two inspection apparatuses have worked for over a year in the production line of coiled tubing which is the first one in China and the third one in the whole world.Meantime, a new NDT principle by using PMFP (permanent magnetic field perturbation) is proposed at the time of searching an approach of the discrimination of internal and external defects, accomplishing the detection of external defects with simple structure and lowest cost. Moreover, the discrimination method of internal and external defects combining PMFP with MFL method is carried out, addressing the issue of discrimination of internal and external defects of steel pipe with a hundred percent and then accomplishing the uniform evaluation of the internal and external defects.Finally, aiming at the detailed distribution of MFL of defects, the detection signals characteristics and its formation mechanism is addressed by using finite element method of creating of multiple air region, laying a good foundation for the quantitative MFL inspection.