Modeling study on the influence of plastic strain on the coercivity value of X80 pipeline steel

Stress can significantly alter the magnetic properties of ferromagnetic materials, making magnetic characterization techniques an effective method for the non-destructive testing of pipeline stress. However, changes in magnetic parameters caused by initial plastic strain during pipeline forming pose challenges for accurate stress assessment. In this study, different plastic strains were introduced into X80 pipeline steel through uniaxial tensile testing. The hardening behavior under the Bauschinger effect was clarified, the evolution of corresponding magnetic parameters was analyzed, and the material's soft/hard phase ratio was obtained by combining the results with metallographic analysis. An equivalent stress model was established based on the hardening behavior, with its parameters determined according to the two-phase ratio. The results showed that the proportion of the soft phase in the circumferential weld area and the base metal area was 72.8% and 65.2%, respectively. The Armstrong-Frederick model was used to describe the evolution of back stress, exhibiting extremely high fitting accuracy (R² = 0.9866 for the base metal and 0.9996 for the weld). This indicates that the model can accurately characterize the internal stress caused by plastic deformation. In most deformation stages, the model parameter k remained stable around 0.7, showing that the internal stress is mainly equivalent to the back stress that hinders magnetization. The best prediction was achieved for the axial coercivity during the small plastic deformation stage of the base material, with an R² of 0.9484. • Strain hardening law of X80 pipeline steel base material and welded joints • The influence of material plastic deformation on stress free magnetic properties • The ratio of soft phase to hard phase at the base metal and welding joint • The constitutive relationship between its stress, strain, and magnetic parameters.