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Residual stress, hardness, and grain size development during processing in annealed and cold-rolled flat steel wire
Gallardo, Clarissa
Gallardo, Clarissa
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2020
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2021-07-03
Abstract
Flat high-carbon steel armoring wires are used to protect exploration cables used in aggressive environments in the oil and gas industry. Because of the environment that they operate in, they are manufactured and further cold-rolled to achieve high levels of strength and toughness, but they remain susceptible to a wide variety of failure modes, including stress corrosion cracking and hydrogen embrittlement, which have been studied extensively for steel specimens. However, the differences in residual stresses that are introduced during manufacturing processes and their effects have not been extensively documented for flat wires. Residual stresses can greatly affect fatigue life, fracture resistance, and general stability of the material. It is crucial to better understand how residual stresses are introduced and developed during processing and to define residual stress measurement methods that can provide accurate, fast, and repeatable residual stress calculations for wires and bars designed for oil and gas applications. Four wire conditions were studied. Two of the wires, with the same composition but different cross-sectional areas and shape profiles, were cold-rolled and annealed as a final step; the other two wires were further cold-rolled from each of the annealed wires. The objective of this study was to observe the development of residual stresses and corresponding hardness and grain size through processing stages. Residual stresses were measured with two techniques: the splitting method and the hole drilling strain-gage method. Complete stress profiles through thickness were estimated for all conditions. Microhardness maps were determined across each cross-section in order to observe the uniformity of the plastic deformation imparted during the last cold rolling step. Microstructural characterization and grain size analysis were completed in order to observe grain size refinement during cold-rolling. A stress-relieving annealing procedure was determined for each cold-rolled condition, and two stress-relieved conditions were introduced. The annealing procedure was designed to relieve tensile residual stresses near the surface of the cold-rolled material while maintaining the increased hardness achieved during cold-rolling. Residual stresses were measured via hole-drilling in the stress-relieved conditions. The splitting method was used to measure residual stresses parallel to a cut made in the longitudinal direction of the bar, which is parallel to the rolling direction. This method indicates the presence of compressive residual stresses at the surface of conditions annealed as a last step and tensile residual stresses at the surface of conditions cold-rolled as a last step. The hole drilling method was used to measure residual stresses up to a depth of 1 mm along the length and width of the bar. The hole-drilling method indicates very similar results, except for the presence of compressive residual stresses, followed by significant tensile stresses deeper into the specimen, in the surface of one cold-rolled condition. Both methods were in good agreement. The hole-drilling method provides more detailed information near the surface (up to a depth of 1 mm into the specimen), but it is sensitive to measurement errors, while the splitting method is a simple and accurate way of checking the residual stress state near the wire surface. Microhardness results indicate that annealed samples are harder around the edges but softer at the core, while cold rolled samples are harder at the core but softer around the edges, indicating non-uniform plastic deformation. The grain size analysis and microstructural characterization that compared cold-rolled to annealed conditions indicate grain size refinement for one cold-rolled condition and a change in morphology for the other cold-rolled condition. The residual stress profiles determined for the stress-relieved conditions indicate that the annealing procedure was successful in relieving tensile residual stresses. Stress-relieving annealing can be used to relieve harmful tensile residual stresses from the wire surface while maintaining the level of hardness necessary for the application.
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