RPV steels are industrial alloys with very complicated microstructures. The characterization of the evolution of microstructural features that can affect the hardness of this type of steel can provide information on parameters that may be considered for modeling the mechanical behavior of these materials. Thermal aging at temperatures higher than the operating temperatures of nuclear reactors may provide some information that could be related to enhanced diffusion irradiation effects. Thermal aging may also provide data about the evolution of RPV steels submitted to heat treatment within the temperature domain proposed for RPV thermal annealing. We present the results of heat treatments (450°C, 500°C, and 550°C) carried out on samples of an A533B Cl.1 (JRQ) steel up to 1000 h. JRQ steel has a heterogeneous microstructure, with well-separated ferrite and bainite islands observed at high magnification. The thermal aging of JRQ steel at 450°C, 500°C, and 550°C promoted an increase in hardness in both bainite and ferrite, with the increase more significant in bainite than in ferrite. For the thermal treatment at 550°C, a maximum of the density of precipitates (per μm2) in the treatments was observed over a period of 500 h. This coincides with the depletion of the alloying elements in the bainite matrix and a decrease of HV in bainite. Copper-rich nanoprecipitates (< 6 nm in size) were observed in the samples treated at 550°C for 500 and 1000 h. The Cu content in the nanoprecipitates increases with aging time. Finally, the HV of the samples treated at 550°C for up to 500 h is a function of the Geometrically Necessary Dislocation (GND) density, which was obtained from EBSD cartographies. GND density is at the same time a function of the density of the precipitates (40–300 nm in size).

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