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In oil field drilling operations, oil drill pipes are the key components that connect the surface rotation system, lifting system, and circulation system to the drill collars and drill bits. They enable the rotary table to rotate the drill bit, the hook to lift and lower the drill bit, and mud to be sent to the bottom of the well to facilitate circulation, thereby allowing the drill bit to break up rock formations and continue drilling. During normal drilling, oil drill pipes are subjected to complex alternating stresses such as tension, compression, torsion, and mud acidization underground; their operating conditions are extremely harsh. As the frequency of operation increases and the pipes are used for extended periods, fatigue cracks and severe corrosion pits can form. Therefore, whether oil drill pipes can function properly and safely is one of the key factors determining the smooth progress of drilling operations in oil fields, and it holds a crucial role in drilling activities. Applying magnetic flux leakage testing technology to inspect oil drill pipes, and exploring the use of this method, is of great significance for improving the efficiency of using oil drill pipes, understanding their quality status, reducing drilling accidents, and thereby enhancing overall economic benefits. Basic principle: The principle of magnetic flux leakage testing for oil drill pipes is based on the high magnetic permeability of ferromagnetic materials. By measuring the changes in magnetic permeability caused by defects in these materials, it is possible to assess the condition of oil drill pipes in use. Oil drill pipes (ferromagnetic materials) become magnetized under the influence of an external magnetic field. When there are no defects, most of the magnetic field lines pass through the ferromagnetic material, and at this time the distribution of magnetic field lines within the material is uniform ； When defects are present, the magnetic permeability of these defects is lower than that of the ferromagnetic material itself, which causes the magnetic field lines to bend; some of these field lines leak out from the surface of the material. By detecting this leaked magnetic field, it is possible to effectively identify the presence of defects, thereby enabling the detection and analysis of fatigue damage in oil drill pipes. Magnetization methods 1. Selection of magnetization method: Common magnetization methods include alternating current magnetization, direct current magnetization, and mixed AC/DC magnetization. Alternating current magnetization: Due to the effect of the \"skin effect,\" the detection depth decreases exponentially as the frequency of the magnetizing current increases. Moreover, high-power high-frequency alternating current sources are difficult to obtain. Considering that most oil drill pipes are thick-walled and require a certain level of detection depth, this method of magnetization is rarely used in most applications. DC magnetization: This method allows for detection at greater depths, simplifies signal processing, and high-power DC power supplies are readily available. AC/DC hybrid magnetization: In principle, this method combines the advantages of the first two approaches – it takes into account both the detection depth and the sensitivity for surface detection, making it an ideal magnetization method. However, in practical applications, it is less used because the mixed magnetization method results in bulky excitation detection equipment and complex signal processing. In summary, when performing magnetic flux leakage testing on oil drill pipes, which are mostly thick-walled tubes, a certain testing depth is required. Moreover, oxidation and corrosion occur on both the inner and outer surfaces of these tubes; therefore, direct current magnetization is a suitable method. The magnetic flux leakage testing method for oil drill pipes described in this article employs direct current magnetization. 2. Selection of magnetization method: Magnetization methods are generally divided into two types, namely axial magnetization and radial magnetization. They are applicable to through-type and rotational detection respectively (currently, some people abroad have used combined magnetization and rotational magnetization, but both are relatively complex). Here, an axial local magnetization method is introduced. To clearly explain the principle of the local magnetization method, we first analyze the conventional magnetization method. Due to the distance between the two magnetization coils, the magnetic flux density in the center is low, resulting in insufficient magnetization and reduced reliability of the detection results. To achieve a certain level of reliability, it is necessary to increase the magnetization current and determine an appropriate coil distance, so that the drill pipe between the two magnetization coils is fully magnetized and reaches magnetic saturation. By guiding the magnetic field to draw the poles toward the center, the distance between the two detection excitation coils is reduced, resulting in a significant increase in the magnetic flux density at the center. Therefore, by using only a smaller detection excitation current, it is possible to locally magnetize and saturate the inspected oil drill pipe, thereby effectively improving the reliability of the detection results. By reducing the detection magnetization current, the excitation power supply also becomes simpler. By using the local magnetization method, under the same flux conditions, the magnetic flux density at the center increases by 23 times, effectively enhancing the degree of magnetization and improving the reliability and accuracy of the detection results. Signal processing methods: 1. DC magnetization power supply: Supplies a direct current to the excitation coil of the detection probe in order to generate a direct current magnetic field. The petroleum drill pipe under inspection is magnetized to achieve magnetization. The output of this power supply must be adjustable, so as to select the optimal magnetization point according to different specifications.