Cu precipitation behaviour in long-term thermally aged, high-copper model reactor pressure vessel steels

Abstract

Nanoscale Cu-rich precipitates that were formed in Cu-containing reactor pressure vessel (RPV) steels during service have a deleterious effect on mechanical properties, which can result in RPV embrittlement and limit reactor operation life. To understand the nanoscale precipitation mechanism, thermal aging at 370°C for up to 13200 h of high-copper RPV model steels was performed to produce nanosized Cu-rich precipitates. Then the Cu-rich precipitates were systematically investigated by atom probe tomography (APT). The changes in the mechanical properties of the steels were characterized by Vickers hardness test. The results show that the Cu-rich precipitates as determined by APT construction analysis have a spatial core-shell structure. The core is enriched with Cu atoms, and the region near the precipitate/matrix interface is enriched with Mn, Ni, Mo, and Si and C atoms. Cu-rich precipitates lead to precipitation strengthening and hardening/embrittlement effects. Using the Russell-Brown model, we estimated the hardening due to Cu-rich precipitates in the matrix and observed that the measured hardness and the estimated changes in hardness were in good agreement.