Microstructure-based assessment of creep rupture strength in 9Cr steels

A microstructure-based model to assess the long-term creep strength in 9Cr steels is proposed. The model takes into account a number of different key issues, including the presence and evolution of the most important families of precipitates (M23C6, MX, Laves and Z phases), the subgrain recovery process, the different strengthening mechanisms (solid solution strengthening and particle strengthening), and is able to give realistic values of the long-term creep strength in P9, P91 and P911 steels. If properly tuned to describe the mid/long-term precipitation of the Z-phase, and the concurrent dissolution of MX precipitates, the model can also predict the sigmoidal behaviour which leads to the early rupture of single heats of P91 steel.

► Creep response at 600 °C of 9% Cr steels.
► Important effect of the different families of precipitates.
► The effect is described by introducing the grain size term in a previously developed model.
► Degradation of particle strengthening effect is considered by calculating the coarsening of the particles.