Evolution of carbide morphology and composition in Cr-Mo-V steel after service exposure

Low alloy Cr-Mo-V steels are often used in steam power generation units. The evolution of carbide often leads to embrittlement during elongated service. Therefore, the determination of carbide evolution mechanism during long time service is important to understand and prevent premature failures such as temper embrittlement. In this study, low alloy Cr-Mo-V steels used as main steam pipes in a thermal power plant were studied after various service times as well as in the as fabricated condition. Electron microscopic analyses were carried out on extraction replicas to observe and analyse the morphology and composition of the carbides. Predominant plate-like vanadium rich carbides were observed in the as fabricated condition. When exposed to high temperature as a result of the use, the V rich carbides transformed to Mo rich carbides, which have a typical H type morphology. The change in morphology and composition of the carbide is mainly due to the gradual depletion of Mo from the solid solution, and a non-destructive carbide extraction method was established for the examination of the precipitates in the working turbine rotor.