Magnetic and microstructural investigations of pipeline steels

Pipelines exhibit a magnetic anisotropy with the easy axis in the axial direction of the pipe. Three different types of steel API 5L (X52, X56, X60) obtained from out-of-service pipeline sections were investigated with respect to their microstructure, magnetic properties, and magnetostriction. In order to measure the magnetic properties samples were cut parallel as well as perpendicular to the pipe axis. The investigated steels contain 98% iron and 2% of additive elements like Mn, Si, C, and traces of others. The microstructure of steel types X52 exhibited banding along the rolling direction, whereas that of the X56 and X60 steels was negligible. The grain size was between 10 and 25 μm. A similar grain size was observed by raster electron microscopy. The size of the magnetic domains was several micrometers which is distinctly smaller than the observed grain size. The saturation polarization was 2.07 ± 0.01 T which is 2% below that of pure Fe. Hysteresis loops were measured in a magnetically closed circuit. Saturation in the transversal direction was significantly more difficult to achieve than in the parallel direction. The loops showed clearly the effect of texture induced anisotropy. The stress sensitivity of the loop is caused by the magnetoelastic energy. The ratio of J_r/J_s was below the theoretical value of 0.75 for a cubic isotropic material indicating also the presence of a texture. These results are in agreement with the texture as observed in the microstructure and with the magnetostriction measurements (longitudinal and transversal) which also showed higher values in the transversal direction.