Seven turbine generating rotors, either currently in service or recently retired, have been metallurgically examined using nondestructive techniques, including hardness testing and replication for optical and transmission electron microscopy. The aim has been to understand the microstructural changes occurring in a range of rotors due to service exposure. A decrease in hardness of up to 7% was noted, possibly arising from carbide-induced loss of molybdenum from solid solution. Optical microscopy was unable to detect service-induced microstructural changes. However, fine scale secondary precipitation of MC and M2C were observed using transmission electron microscopy. A range of carbide compositional parameters associated with M3C were found to vary with service exposure time.