Origin of a magnetic easy axis in pipeline steel

Oil and gas pipelines are generally magnetically anisotropic, with a magnet ic easy axis in the pipe axial direction. This is of interest because magne tic flux leakage tools are commonly used for the detection and sizing of de fects. In the present study we investigate the origin of this magnetic easy axis, using an angular magnetic Barkhausen noise technique to characterize the magnetic anisotropy. The texture, microstructure, and residual stress are examined as possible causes of the easy axis, using x-ray pole figure a nalysis and microstructural examination along with high and low temperature annealing treatments. Our results indicate that plastic deformation and re sidual stress are responsible for the magnetic easy axis, since an eliminat ion of the residual stresses through low temperature "stress relief'' heat treatment produces a magnetically isotropic structure without altering the texture or microstructure. X-ray pole figure analysis supports the conclusi on that magnetic anisotropy is not related to texture in these materials. W e conclude that the axial magnetic easy axis is due to a compressive residu al hoop stress resulting from the cold bending and cold expansion of the pi pe during processing. (C) 1999 American Institute of Physics. [S0021-8979(9 9)06615-3].