Contents I. Introduction
Steel pipes are widely applied in gas pipelines and nuclear power, which often bear high internal pressure. Once cracks initiate, they are very easy to expand, resulting in major safety accidents [1]. Therefore, it is essential to ensure the quality of the steel pipes, and nondestructive testing must be carried out after production [2]. The commonly employed inspection methods for steel pipes include ultrasonic testing (UT) [3], eddy current testing (ECT) [4], and magnetic flux leakage (MFL) testing [5], [6]. Magnetic induction tomography (MIT) can also be applied to detect structural defects [7], which is less sensitive to narrow cracks and more suitable for the detection of volume defects in ferromagnetic pipes [8]. UT can detect both internal and external defects, but the couplant is required [9]. The ECT does not need the couplant, and the surface defect detection sensitivity is high. However, the internal defects cannot be detected via ECT. The MFL method can detect internal and external defects simultaneously, and it is widely used in steel pipe testing because of its fast detection speed [10].
