AbstractCast duplex stainless steels of CF3, CF8, and CF8M grades, containing a wide range of chemical composition and ferrite content, were aged in the laboratory at 270–450°C to evaluate the kinetics and mechanisms of thermal aging embrittlement. Also, CF8 and CF8A steels were obtained from components of boiling and pressurised water reactors after 12–22 years of service. After mechanical property (namely, Charpy impact, tensile, and J integral–crack growth resistance R curve) and microhardness tests, the aged specimens were examined using transmission electron microscopy and field ion atom probe techniques to identify the aging mechanisms and the critical factors that control the embrittlement kinetics. From the results of microstructural characterisation, the primary and secondary embrittlement mechanisms were identified. A mechanistic correlation was obtained between the microstructural characteristics and the activation energy of aging (namely, the temperature dependence of the aging kinetics) for steels with a range of chemical compositions and subjected to various fabrication and thermal treatment conditions. The unified correlation, based on spinodal decomposition influenced by G phase nucleation and growth in ferrite and M23C6precipitation on ferrite/ austenite boundaries, provides a good explanation of the complex behaviour of the activation energy (ranging from 75 to 230 kJ mol−1) regardless of grade, chemical composition, or fabrication process.MST/1204