Thermal Ageing Effect of Pressure Vessel Steels

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 (330degC, 365degC and 405degC respectively) were reported. The high Ni content steel was observed to have a much stronger hardening effect during thermal ageing. 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 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