Mechanical behavior of pipeline steel after strain aging – Part I: Experiment

Low plastic straining combined with aging leads to critical changes in the mechanical properties of steels used in submarine pipelines constructed using the pipe reeling method. This includes alterations in the mechanical behavior of steel at the initial stages of plastic strain, which can be critical for the long-term operation of pipelines due to localized plastic deformation in the form of buckling. In this study, the effects of plastic preliminary strain of 3% through monotonic tension and aging at 250 °C for 1 h on the mechanical behavior of low-carbon steel in various microstructural states were investigated. These states differed in terms of the volumetric fraction of structurally free ferrite after quenching or normalizing. It was shown that the overall yield strength increase, as well as the contributions of strain hardening and bake hardening, depend on the volumetric fraction of ferrite and the heat treatment method. A direct proportional relationship between uniform elongation and the strain hardening exponent, both before and after strain aging, was confirmed. The results have been used for numerical modeling of the plastic strain of steel, aiming to predict and prevent the buckling of deep-water pipelines, as presented in Part II of the article.