Room temperature creep and strain-rate-dependent stress-strain behavior of pipeline steels

Creep deformation and the effect of strain-rate on stress-strain behavior of X-52, X-70 and X-80 pipeline steels at room temperature were studied using round tension test specimens. Depending on its chemical composition and the processing condition (as-received or fully annealed), a pipeline steel may exhibit a stress-strain curve with or without a yield point. The as-received and the annealed steels with both type of yielding behavior were creep tested at a constant stress either below or past the yield point / 0.2% offset yield strength. Independent of yielding behavior, significant post-yield creep deformation was observed in all the steels. The pre-yield creep, however, is strongly dependent on the yielding behavior. In the presence of a yield point, only a minor deformation was detected in the steels subject to the pre-yield creep. In the absence of a yield point, pre-yield creep deformation occurred to a relatively large extent. For the latter case, an annealing treatment further enhanced creep deformation. A strain-rate-dependent stress-strain behavior was also observed in all thesteels that show significant creep deformation. Dislocation mechanisms responsible for the creep behavior observed in the study are also provided in the paper.