Abstract: The existing magnetic flux leakage (MFL) method and apparatus by combined magnetization is presented and its limitations are also analyzed. The helical scanning approach limits the testing speed and does not suit the inspection of pipes unsuitable to rotate or with straight line weld. Afterwards, for the deficiencies, the feasibility of omni-directional defects MFL inspection under a unidirectional axial magnetization for steel pipe is found and verified by simulation of finite element model and experiments, particularly the feasibility of longitudinal cracks parallel to the axial magnetization, thus it is possible to realize testing by linear scanning. A method for ensuring the consistency of evaluation and testing for the defects in a same volume but different directions is also put forward. A MFL method and apparatus without needing a rotation of detection probes or steel pipe are developed and designed, thus resolving the problems of high-speed or ultra-high-speed inspection of seamless steel pipe and adapting to the inspection of pipes unsuitable to move helically or with straight line weld. The proposed method simplifies the conventional MFL method and reduces the cost of the testing apparatus.

Key words: Axial magnetization, High speed, Magnetic flux leakage (MFL), Nondestructive testing (NDT), Steel pipe