Use of Magnetic Barkhausen noise and magnetic flux leakage signals for analysis of defects in pipeline steel

We report on studies of the effects of hoop and axial tensile stresses on m agnetic flux leakage (MFL) signals from 50% penetration electrochemically m illed pits eroded in line pipe steel. It is observed that stress can change the MFL signal by more than 50%, depending on the magnetization of the pip e wall. The studies were performed on pits created in both the absence and presence of 330-MPa hoop or axial tensile stress, The MFL results obtained in the two cases show detectable differences if the applied stress is high enough to create plastic deformation in the regions of stress concentration near the pits, The effect of stress applied during pit erosion is less tha n that when the same stress is applied during the subsequent measurements. Magnetic Barkhausen noise (MBN) measurements have been used to study the st ress concentrations around electrochemically milled and mechanically drille d defects and have shown that significant additional stress and plastic def ormation can be introduced during mechanical drilling. The MBN results are used to assist interpretation of the stress-dependent MFL results.