A model has been developed to enable the kinetics of precipitation reactions in power plant steels to be predicted. The model allows for simultaneous precipitation or dissolution of up to six phases and has been verified using steels ranging from 2.25Cr1Mo to 10CrMoV. Predictions have been made to investigate how changes in the concentration of carbon and nitrogen are expected to influence the precipitation kinetics in a 10 wt% Cr steel. An increase in nitrogen is shown to intensify and stabilise M2X precipitates whilst having little effect on the other phases. A decrease in carbon also results in an increase in M2X precipitation whilst reducing the volume fraction of M23C6 and delaying the onset of its formation. Apart from volume fractions, the model also gives other useful information, such as the growth rate of each phase as a function of time and temperature. The growth rate of Laves phase is compared with that of the other phases, and the influence of molybdenum and tungsten level on its rate is calculated. These predictions suggest that, provided Laves phase is stable, addition of both these elements increases the growth rate by a comparable amount.
Microstructural stability of creep resistant alloys for high temperature power plant applications, edited by A. Strang, J. Cawley and G. W. Greenwood, Institute of Materials, London, (1998) pp. 395-404.