Thus,the tangential magnetic field strength of the eddy current did not change in the vertical direction and only changed in the horizontal direction.The tangential magnetic field strength of the eddy current of the double-layer sample was stronger than that of the single-layer sample.To clarify the tangential magnetic field images of the single tube and the double tube,cross-section diagrams are shown in Fig. 9. The lines of the cross-section diagram are shown as A–A 0 and B–B 0 in Fig. 8. The A–A 0 line was located at the position 60 mm in height,which was across the weld area,and the B–B 0 line was located at the position 4 mm in width direction,which was a long the weld area.In the cross-section diagram of the 1 kHz measurement,the tangential
magnetic field strength of the eddy current of the double-layer sample was more than double the strength of the single layer,which was the reason why the eddy current flows were generated
not only in the inner layer,but also in the outer layer.In the cross-section diagram of the 50 Hz type,the profile of the tangential magnetic field strength across the weld area was almost the same between the single-layer sample and the double-layer sample.The peak amplitude of the tangential magnetic field strength of the double-layer sample was slightly weaker than that of the single-layer sample,which was affected by the outer layer.