Thermodynamic modelling of the microstructure of high Cr ferritic creep resistant steam pipe steels

Thermodynamic modelling of the microstructure and microstructural stability during high temperature creep of 9-12%Cr steam pipe steels is discussed. Precipitate particles obstruct the movement of dislocations and subgrain boundaries, and control the development of creep strain as a function of time, which means that the stability of the precipitates govern the creep strength of 9-12%Cr steels. M23C6 carbides precipitate during tempering and coarsen during creep. The coarsening rate of M23C6 carbides is modelled, and it seems to depend mainly on the Mo content and on the diffusion rate of substitutional elements. The diffusion rates at around 600 degreesC in 9-12%Cr ferritic steels is controlled by the Curie temperature, which depend on the chemical composition. Ni, Si, Mn and Cr accelerate diffusion and Co and C retard diffusion in 9-12%Cr steels. Laves phase particles precipitate in the steels during the first 10 000 h to 30 000 h at 600 degreesC. Only after this period will coarsening of Laves phase take over as the particle size controlling process. It is predicted that the coarsening rate of Laves phase particles is 2-4 times lower than for M23C6 carbides, so they can be expected to contribute significantly to long term creep strength. A significant number of MX particles precipitate during creep, but due to difficulties in predicting the MX compositions with thermodynamic calculations the stability of these particles can not be estimated accurately.