AbstractAtom probe field ion microscopy, analytical electron microscopy, and optical microscopy have been used to investigate the changes that occur in the microstructure of cast CF8 and CF8M primary coolant pipe steels after long term thermal aging. These cast steels have a duplex microstructure consisting of austenite with approximately 15 vol. -% ferrite. In material aged at 300 or 400°C for up to 70 000 h, the ferrite had spinodally decomposed into a modulated fine scale interconnected network consisting of an iron rich a phase and a chromium enrichedα′phase with a periodicity of between 2 and 9 nm. Roughly spherical G phase precipitates 2 to 10 nm in diameter were also observed at concentrations of more than 1021m−3. The degradation in mechanical properties of these materials is a consequence of the spinodal decomposition and G phase precipitation in the ferrite.MST/1184