Influência do tratamento térmico de esferoidização e da deformação a frio na microestrutura e propriedades magnéticas de arames de risers flexíveis

Abstract

Flexible pipelines used in the oil and gas industries are an advantageous technological option in the face of the challenges encountered in exploring deeper areas. Flexible risers are made up of different layers, including tensile reinforcement, built by a helical arrangement of steel wires. This is responsible for resisting traction, compression, and radial stresses from internal pressure and torsion. Due to the complex stress modes to which they are subjected, their failure modes become unpredictable, requiring monitoring methods, integrity verification and life estimation of these components. Electromagnetic techniques work as a good method for non-destructive monitoring of microstructural, morphological and mechanical properties transformations in deformed or heat-treated steels. In this work, the electromagnetic behavior of a wire used in the manufacture of tensile reinforcement, subjected to different combinations of temperature and time of heat treatments and plastic deformations by cold rolling, was investigated. These signals were related to microstructural transformations and changes in mechanical properties caused by the processes involved. To achieve this objective, Magnetic Barkhausen Noise (MBN), Magnetic Hysteresis, Inductance and Electrical Resistivity tests were carried out. The microstructural characterization was performed by Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC). Hardness tests were performed to evaluate the mechanical properties. The results indicate that the increase in deformation generates an growth in dislocation density and lower dislocation mobility, which causes an increase in hardness and in the area of the hysteresis curve, with a consequent decrease in the MBN signal due to the greater difficulty in domain wall mobility. Low strain rates (5 and 10%) cause an increase in the degree of spheroidization due to cementite breakage unfavorable to the rolling direction. The treatments carried out at 600 and 700°C indicate the recovery of microstructure and the spheroidization of cementite, facilitating the deformation of the material, however the behavior of the MBN envelope and the DSC curves suggest that recrystallization only occurred in the treatments at 700 and 800°C. There is dependence of the resistivity on the stress field, being greater for tensile stresses. The multiplication of structural heterogeneities hinders the magnetic flux, reducing magnetic permeability and inductance.