Significant increase in the hardness of the ferritic phase due to structural changes at low temperatures in duplex stainless steels

Abstract

Duplex stainless steel samples were aged at low temperatures of the 300°C and 400°C for 3000, 5000 and 7000 hours. The changes at the micro-structure were followed during the annealing time using an optical micros-copy and measurements of phase percentages. Nano-hardness was used in order to identify the phase responsible for the increasing in the global hardness. The G phase precipitation and α'phase due to spinodal decomposition was identified by transmission electron microscopy. It was detected that the micro-structural changes affect the global properties, remarkably the global hardness. TEM results showed that the chromium rich precipitation occurs mainly in the ferrite phases. The results also showed a clear difference between the kinetics of precipitation of the lower chromium content sample and the higher chromium sample. The phenomena of precipitation and coalescence of chromium rich phases must be related to the increasing and decreasing tendencies of hardness, respectively.