Effect of dynamic strain aging on cyclic deformation in piping material SA333 Gr-6 steel

Abstract

In piping structures, the low cycle fatigue loading arises from thermally induced strain cycles associated with start-up and shut-downs and fluctuation in loading conditions. SA333 Gr-6 steel is prone to dynamic strain aging (DSA) and the parameters that govern the DSA are temperature and strain rate. There are limited studies on DSA in the investigated steel. In the present investigation, fatigue experiments were conducted at a fixed strain amplitude of +/- 0.5 % with the variation of temperatures (RT to 400 degrees C) and strain rates (1 x 10(4) s(-1) to 1 x 10(-2) s(-1)). The results reveal that the cyclic deformation behavior depends on temperatures and strain rates. The steel shows cyclic hardening characteristics at elevated temperatures which indicates the occurrence of the DSA phenomenon. The negative strain rate sensitivity and temperature dependency of stress amplitude are the manifestations of DSA. The present study shows the deleterious effect of DSA on fatigue life. The DSA temperature regime found in the temperature 200-250 degrees C for 1 x 10(4) s(-1) strain rate, 250-300 degrees C for 1 x 10(-3) s(-1) strain rate, and 250-350 degrees C for 1 x 10(-2) s(-1) strain rate. The DSA regime gets shifted to a higher temperature with the increase in strain rate. Transmission electron microscopy investigations reveal that the fatigue failed specimen at the DSA regime shows severe dislocation activities and dislocation tangles, whereas minor dislocation activities and cell structures were observed at the non-DSA regime. The dislocation density calculated from the X-ray diffraction analysis was higher at DSA compared to the non-DSA regime.