For operation of existing nuclear power plants (NPPs) beyond their design lifetime (up to 80 years), one of th7e main issues is the assessment of the performance of its structures, systems and components (SSC) during the period of extended operation. The reactor pressure vessel (RPV) is one of the components that could determine the lifetime of a NPP. Neutron irradiation at the temperature of operation of the nuclear reactors facilitates the chemical equilibrium of the alloying elements of the RPV steels, especially copper, while producing nanoscale precipitates (Cu, Mn-Ni-Si, and ∼M6C carbides) that leads to embrittlement [1–4]; by the other side, neutron irradiation also produces dislocation loops. Both, nanoscale precipitates and dislocation loops, produce loss of toughness and hardness increase. One of the proposals to manage the RPV time life is the thermal annealing, however there are still some concerns about its application, one of them is a possible detrimental effect by the development of new microstructural features that could lead to thermal embrittlement. Thus it is important to have data about the microstructural and hardness evolution of the RPV steels submitted to thermal annealing treatments.
The results of thermal treatments (450°C, 500°C and 550°C) performed in a A533 C1.1 (JRQ) steel during 0.5 to 1000 hours are presented in this work. JRQ steel has a bainitic microstructure, but well separated ferrite and bainite islands are observed at high magnification. The hardness evolution of such islands as a function of the thermal treatment is correlated with the number of precipitates present after thermal treatment as well as with the chemical evolution in these microconstituents. Thermal treatments of JRQ steel at 450, 500 and 550°C promoted the increase of hardness in both, bainite and ferrite. For the thermal treatment at 550°C, it is observed a maximum of the number of precipitates per μm2 in the treatments during 500 hours, which coincides with the depletion of the alloying elements in the bainite matrix and a decrease of Vickers microhardness (HV) in bainite. Cooper rich nanoprecipitates have been observed in the samples treated at 550°C during 500 and 1000 hours. The Cu content in the nanoprecipitates increase according the ageing time.