Non-Destructive Investigation of Microstructural Transformations and Mechanical Hardening by Electromagnetic Techniques in Cold-Rolled Ferritic-Pearlitic Steel Wire of Flexible Pipelines

Offshore oil and gas exploration in deep areas raises the need to develop equipment monitoring techniques to detect possible defects and prevent catastrophic failures. This work sought to analyze the feasibility of using non-destructive electromagnetic techniques - Magnetic Barkhausen Noise (MBN), Magnetic Hysteresis and Electrical Resistivity - in the identification of predominant stresses, microstructural transformations and mechanical behavior of a steel used in the manufacture of flexible Risers. The material was analyzed as received and cold rolled with 5, 10, 20, and 35% reduction rates. The signals were related to data obtained by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and hardness tests. The results show that with increasing strain, the MBN peak amplitude decreases (95.0%), the energy required to reach magnetic saturation increases (23.2%), and the electrical resistivity increases (41.6%). The variation of these signs indicates an increase in structural heterogeneities, such as elongation of cementite lamellae, increase in dislocation density, and non-homogeneous stress field distribution, which increase the area of sites fixation of magnetic domains and decrease the flow of free electrons. The sensitivity in detecting changes of the techniques in question demonstrates their efficiency in inspecting flexible ducts installed in areas of difficult access.