Influence of ageing heat treatment on A-286 microstructure and stress corrosion cracking behaviour in PWR primary water

Abstract : The influence of ageing heat treatment on alloy A-286 microstructure and stress corrosion cracking behaviour in simulated Pressurized Water Reactor (PWR) primary water has been investigated. A-286 microstructure was characterized by transmission electron microscopy for ageing heat treatments at 670 °C and 720 °C for durations ranging from 5 h to 100 h. Spherical $\gamma$′ phase with mean diameters ranging from 4.6 to 9.6 nm and densities ranging from 8.5 × 10$^{22}$ m$^{−3}$ to 2 × 10$^{23}$ m$^{−3}$ were measured. Results suggest that both the $\gamma$′ phase mean diameter and density quickly saturate with time for ageing heat treatment at 720 °C while the $\gamma$′ mean diameter increases significantly up to 100 h for ageing heat treatment at 670 °C. Grain boundary $\eta$ phase precipitates were systematically observed for ageing heat treatment at 720 °C even for short ageing periods. In contrast, no grain boundary η phase precipitates were observed for ageing heat treatments at 670 °C except after 100 h. Hardening by $\gamma$′ precipitation was well described by the dispersed barrier hardening model with a γ′ barrier strength of 0.23. Stress corrosion cracking behaviour of A-286 was investigated by means of constant elongation rate tensile tests at 1.5 × 10$^{−7}$ s$^{−1}$ in simulated PWR primary water at 320 °C and 360 °C. In all cases, initiation was transgranular while propagation was intergranular. Grain boundary $\eta$ phase precipitates were found to have no significant effect on stress corrosion cracking. In contrast, yield strength and to a lesser extent temperature were found to have significant influences on A-286 susceptibility to stress corrosion cracking.