Abstract
Static strain aging has been demonstrated to have a significant effect on the yield strength and yield point behavior of modern line pipe steels. Static strain aging can occur at many points during pipeline fabrication: during coating application, girth welding, and field joint coating. Each of these discrete thermal events is described by a unique thermal cycle (time-temperature profile). This makes it difficult to define a simulation heat treatment which would reflect the cumulative strain aging effect of all thermal events. The first section of this work describes the development of an analytical expression based upon classical strain aging theory that allows strain aging heat treatments to be normalized to an equivalent time at an isothermal temperature. The second section of this work compares thermal profiles from various strain aging heat treatments. Lastly, the mechanical response of modern line pipe steel to a variety of carefully controlled time-temperature combinations designed to simulate the causes of strain aging was examined. The analytical approach to compare strain aging heat treatments was found to be overly conservative when comparing longer times at lower temperature to shorter times at higher temperature which had been calculated to have equivalent heat inputs. Additionally, the yield strength was found to concomitantly increase with strain aging temperature, even when the effects of time were normalized. An alternate approach to the development of normalized strain aging heat treatments using a phase field model are briefly introduced.
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