Abstract: Two electromagnetic induction phenomena essentially occur when steel pipe moves through the axial magnetizing coil at high speed in magnetic flux leakage(MFL) nondestructive testing. On one hand, the eddy current is generated in steel pipe which is cutting the magnetic lines. On the other hand, due to the change of the magnetic flux, the induced current is formed in the magnetizing coil. The magnetic field generated by the eddy current and induced current magnetizes the pipe with the magnetizing filed together, affecting the magnetization of steel pipe, eventually resulting in difference of the MFL signals from defects in different locations. To find out the influence mechanisms of the induced-magnetic field on the magnetization of steel pipe, based on the Lenz’s Law, the descriptions for the eddy current and induced current are established, and moreover the induced-magnetic field distributions were obtained. It was found that the induced-magnetic field in pipe tail and the magnetizing field have the same direction, resulting in the increase of the magnetization intensity, the direction of the induced-magnetic field in the pipe head is opposite to the one of the magnetizing field, which brings about the reduction of magnetization intensity, and the induced-magnetic field current in the pipe body nearly has no affection on the magnetization. The relevant experiments in different speeds are conducted by using the hi-speed MFL testing system for steel pipe to verified the influence of the induce-magnetic field, and the test results agree with the theoretical analysis well.

Key words: magnetization, eddy current, induced current, induced-magnetic field, magnetic flux leakage testing at high speed, steel pipe