Changes occurring in the chemical composition of the precipitates in the steel X 20 CrMoV 12 1 during long-term high temperature service

The steel X 20 CrMoV 12 1 exhibits a tempered martensitic microstructure, when it is hardened and tempered in accordance with the appropriate material standards. At very low austenitizing temperatures the amount of carbon going into solution is small. The undissolved, relatively coarse carbides at these temperatures inhibit grain growth of the austenite. The martensite forming from such an austenite is very fine and often cannot be recognized as such, when examined by optical microscopy. The nonequilibrium structures resulting from the heat treatment will be progressively transformed to thermally more stable structures during high temperature service. The reactions taking place at high temperatures lead to an enrichment of the carbides with the alloying elements, the equilibrium compositions of the carbides being dependent on the temperature. Thus, for example, the composition of the carbide M23C6 is characteristic of heat treatment or service temperatures. The initial arrangement of various phases in the microstructure remains largely unaltered, despite the various precipitation reactions occurring during service. Metallographic examination enable one to determine the heat treatment of the components but they are not sufficient to assess the service conditions and, especially, the residual life of the components. (orig.)