Influence of aluminium on creep strength of 9–12% Cr steels

The influence of aluminium on creep strength of 9% Cr steels is predicted by a fundamental model for creep. Through thermodynamic modelling the particle structure is determined for a temperature and composition range. This shows how AlN is formed at the expense of MX carbonitrides of VN character when the aluminium content is increased. The remaining MX particles are of NbC type and have approximately one fifth of the original phase fraction. The evolution in microstructure such as particle coarsening is included in the model as well as the recovery. Rupture time is predicted using a modified Norton equation including back-stresses calculated from microstructure. The predictions show correspondence to some of the creep data for the steel P91 over a temperature and stress range. Furthermore, simulation with high Al content verifies the observed early failure of Al rich components. Overall, the simulations show a decrease in rupture time by a factor 6 due to Al additions of up to 0.2%.