Microstructure, Mechanical Properties, and Fatigue Resistance of High-Carbon Steel for Tensile Wires Used in Flexible Pipes for Offshore Oil and Gas Transportation

Tensile armor carbon steel wires are structural elements assembled in the complex construction of flexible pipes used in the oil and gas industry. Recently, failures of flexible pipes were associated with tensile armor fractures. Despite the relevant key of these elements in the structure of flexible pipes, manufacturers and steelmakers do not provide sufficient information about the materials and their production flow. The objective of the present study was to characterize a tensile wire made of cold worked high carbon steel regarding the microstructure and mechanical properties. The microstructure was investigated by high resolution scanning electron microscopy. The steel has nearly eutectoid chemical composition (0.69 % carbon) and presents a microstructure of very fine and deformed pearlite. Some cementite carbides appeared fragmented and spheroidized in the ferrite matrix. These microstructural features are typical of patented steel, which was subsequently cold deformed by rolling and drawing. The tensile curve of the material was obtained and modeled by Hollomon's, Voce's and Ludwik's constitutive equations. The S- N fatigue curve of the wire was constructed with four-point bending tests with ratio R = 0.1. The endurance limit for R = 0.1 was estimated, and the correspondent value for R = -1 was calculated based on Goodman's relation.