MORPHOLOGICAL AND COMPOSITIONAL CHANGES OF THE CARBIDES IN CR-MO-V FERRITIC STEEL

The morphological and compositional changes of the carbide particles in a ferritic grade of steel have been studied using electron metallography. Specimens were prepared from a main steam pipe in the virgin condition as well as after service exposure in a thermal power plant. The predominantly plate-like vanadium-rich carbide precipitates present in the virgin state showed no significant morphological and compositional changes as a result of the service exposure. On the contrary, a previous investigation of a somewhat different grade of steel reports that as a result of identical service exposure the plate-shaped vanadium-rich carbide transforms into a molybdenum-rich carbide, giving rise to a typical H-type carbide morphology. This difference in the morphological and compositional behaviour of the carbide is attributed to the difference between the compositions of the two steels studied. It appears that the long-term creep strength in some of the Cr-Mo-V ferritic steels, currently in use in the power generation industry, could be improved by a small change in the composition of the steel.