Compositional changes in minor phases present in 12CrMoVNb steels during thermal exposure at 550°C and 600°C

Abstract

One of the techniques considered promising in the assessment of the time–temperature history of creep resistant power plant steels is based on studies of compositional changes occurring in the minor phases during service exposure. Chemical compositions of alloy carbides and/or nitrides precipitated during quality heat treatment of high alloy steels are expected to be very close to equilibrium. However, since in practice the operating temperatures for these steels are usually much lower than the tempering temperatures used in their original heat treatments, changes in the compositions of their minor phases are likely to occur during service exposure. These changes depend on the differences between the respective equilibrium compositions of the minor phases at the tempering and service temperatures. The greater the difference between these temperatures, the greater will be the expected changes in the compositions of the minor phases during exposure at the service temperature. Since these changes are dependent on the time and temperature of exposure, any minor phase, which is present in the microstructure in the as received condition, as well as after prolonged service exposure, represents a potential indicator of time–temperature history of the material. In this paper the results of quantitative microstructural studies are reported on a series of martensitic 12CrMoVNb steels in their as received heat treated condition and following long term exposure at 550 and 600°C. The results of minor phase composition changes are discussed in terms of the equilibrium minor phase compositions calculated using the MTDATA computer program and the SGTE database, and their potentials as in service time–temperature history indicators for the materials are evaluated.