Carbide diagrams and precipitation of alloying elements during aging of low-alloy steels

Low-alloy Cr-Mo-V steels with carbon content up to 0.3 to 0.4% are used for steam turbine components. The effect of exposure to high temperatures under long-term aging or service causes carbide reactions, the kinetics and type of which depend on aging and alloying conditions and metallurgical history. In this research, carbide transformations were investigated in several low-alloy Cr-Mo-V steels during prolonged aging and after service duration by means of physical-chemical phase analysis. The results and conclusions are summarized as follows. Generally, carbide reactions in all steels during martensite aging proceed as follows: M3C → MC + M2C + M7C3 → M23C6 → M6C. Carbide diagrams showing the regions of existence of different carbides have been developed. The type of carbide diagram for each steel is determined by alloying. The rate of preliminary cooling from γ region causes the carbide diagram type to change. A decrease in the rate of cooling leads to the formation of more stable carbides. It was concluded that the M3C → M7C3 reaction is replaced with M3C → M23C6. All the carbides typical for the given alloying are formed during martensite aging, since the metal proceeds through the successive stages of carbide transformations. After long-term service at 520 C to 560 C, Mo is mainly in carbides independent of its original content. Molybdenum precipitation in carbides during service occurs in the first 30,000 to 40,000 hours, and for times up to 200,000 hours, there is no significant change in Mo content in carbides. A carbide of M6C was formed in the steel with 1% Mo, and M23C6 carbide was found in all steels investigated