AbstractCombined quenching and tempering induced phosphorus segregation to prior austenite grain boundaries inα0·077 wt-%P doped 2·25Cr–1Mo steel was examined using field emission gun scanning transmission electron microscopy. The results indicate that combined equilibrium and non-equilibrium phosphorus segregation may play an important part in temper embrittlement of the steel caused by direct tempering after quenching. Non-equilibrium segregation requires the formation of sufficient quantities of vacancy–impurity complexes and their migration to grain boundaries is of great importance in the segregation. For this reason, the mechanism for migration of the complexes is discussed in detail.MST/3419