There is long-standing research interest focused on revealing thermal ageing effect of model alloys and commercial pressure vessel steels in order to understand the thermodynamics and kinetics of the microstructural evolution [1, 2, 3, 4, 5, 6]. High quality thermal ageing data will be helpful to determine proper post irradiation annealing procedures to reduce or eliminate irradiation embrittlement effect [1, 4], and to estimate potential thermal embrittlement within the vessel lifetime (～300,000 h) [1]. The thermal data from the commercial pressure vessel steels are valuable for validation of theoretical models for prediction of microstructure and mechanical properties of the materials. This paper reports our recent results on hardness and microstructure evolution of two pressure vessel steel welds with different Ni content (0.29 at% Ni and 1.66 at%) but similar high Cu content (0.4 at%) during long-term thermal ageing at three temperatures (330°C, 365°C and 405°C respectively). The high Ni content steel was observed to have a much stronger hardening effect during thermal ageing, as shown in Figure 1. Precipitation of Cu-rich clusters is responsible for the hardening effect observed in the two alloys. 3-dimensional atom probe has been employed to characterize quantitatively the evolution of precipitation microstructure, cluster chemistry and cluster/matrix interface segregation. The strong influence of ageing temperature and of alloy Ni content has been confirmed in this study. The high Ni content steel shows a higher the number density of Cu-rich precipitates for a given temperature [1], as shown in Figure 2. The higher the thermal ageing temperature, the higher the Cu concentration in the core of Cu-rich clusters. Ni, Mn and Si show segregation to the cluster/matrix interface, and this is stronger at lower ageing temperatures. The effect of temperature on cluster/matrix interface chemistry indicates that the interface segregation is a thermodynamic effect.
展开▼
