Reversible cyclic softening in HSLA steels at low homologous temperatures

Critical mechanical components employed in energy applications require certification of their mechanical response under monotonic or cyclic loading at various constant temperatures. However, components often experience combined load and temperature changes, which are more complex and time-consuming to evaluate. This work presents previously unreported reversible cyclic softening in low-alloy steel undergoing loading sequences between 293 K and 453 K (0.15 to 0.25 of the homologous temperature). Mechanical cycling at 80 pct of the yield stress results in a similar cyclic response for all temperatures, while cycling at 90 pct of the yield stress doubles the cyclic strain when preceded by cycling at 453 K. The softening can be reversed by maintaining the material at elevated temperatures and cannot be explained by dislocation recovery mechanisms or changes in the microstructure. Instead, serrated deformation at specific deformation levels suggests that Dynamic Strain Aging (DSA) contributes to softening, which is further supported by a theoretical analysis of interstitials and dislocations mean free paths at various temperatures.