Quantitative research on magnetic-based axial stress inspection technology and application of girth welds of high-grade steel pipe under alternating excitation

The pipe girth weld is subjected to significant axial stress due to the discontinuity of the structure and material, which is the weak point of pipeline failure. The alternating electromagnetic field test method is suited for inspecting weld stress. However, the stress magnetic quantitative relationship of the weld zone is unclear. It is challenging to quantify the axial stress of girth welds. The magneto-mechanical constitutive relationship of X80 steel weld material is established in this study. Based on this, a numerical simulation model of magnetic response under alternating excitation is established. Then, the overall hysteresis loop generated by the synergy between the weld and the base metal is calculated, and the quantitative relationship between magnetic characteristic parameters and axial stress is analyzed. Finally, an axial stress inspection method for pipe girth welds is proposed and applied. The results show that the R2 between simulation and experimental results is 0.97. Coercive force exhibits the highest sensitivity to stress. The coercive force changes by 1.1 A/m per MPa in 0–300 MPa and 2.7 A/m per MPa in 300–600 MPa. The X-ray inspection revealed 4 weld porosity at 7:35 position, which differed only 2.5° from the results obtained by the proposed method.