AbstractSamples of a 9Cr–1Mo and a 2·25Cr–1Mo steel (both wt-%) have been heat treated to produce microstructures that are likely to form in the heat affected zone of a weld. Two treatments were employed for this purpose, involving quenching (i) from an artificially high austenitising temperature and (ii) from an abnormally low temperature, within the intercritical region. Some of the samples in each condition were given aging treatments at 550°C, which were designed to simulate exposure to typical in-service temperatures. The microstructures produced by each heat treatment schedule were characterised using transmission electron microscopy (TEM). The effects of each treatment on the mechanical properties of the steels were assessed using ambient temperature tensile testing and Charpy tests at various temperatures. The results were compared with those obtained from samples for which standard austenitising temperatures were employed. The higher austenitising temperature was found to have very little effect on the mechanical properties of either steel. The lower temperature treatment affected the properties of both steels–its most significant effect being to increase the ductile–brittle transition temperature (DBTT) of the 2·25Cr–1Mo steel and reduce that of the 9Cr–1Mo steel, such that the DBTT of each steel converged to a similar value. It also resulted in increases in the ductility and upper shelf energy (USE) of the 9Cr–1Mo steel, but had little effect on these properties in the 2·25Cr–1Mo steel. The difference in behaviour of each steel, including the different response to aging in each condition, is explained in terms of the microstructural changes caused by the different heat treatments.MST/1232b