Research on the magnetic leakage field of hardened zone based on the theory of magnetic charges

Abstract Accurate assessment of hardened zones in operational oil and gas pipelines contributes to the identification of structural weaknesses and enables proactive prevention of potential failure mechanisms such as hydrogen-induced cracking and sulfide stress corrosion cracking. To facilitate the detection and dimension evaluation of hardened zones, this study establishes a magnetic charge distribution model based on magnetic flux leakage detection. A computational method for determining the magnetic leakage field associated with hardened zones is proposed, and the effects of applied magnetic field strength and residual stress on the leakage field intensity are investigated. The accuracy of the proposed model is further validated through experimental studies. Research indicates that the intensity of magnetic leakage field induced by hardened zones is influenced by multiple factors, including the magnetic properties of both the surrounding metal and the hardened zones, the applied magnetic field, and the dimensions of the hardened zones. Holding other variables constant, an increase in hardened zone size corresponds to a higher intensity of the generated magnetic leakage field. The optimal applied magnetic field strength for detecting hardened zones in X70 pipeline steel ranges from 4 to 8 kA m −1 . Within this range, strong magnetic leakage field intensities can be consistently observed in all three spatial directions, and a positive correlation exists between the magnetic leakage field intensity and the sizes of the hardened zones.