Relationship between microstructure and mechanical behaviour of thermomechanically optimised 9-12% Cr steels

International audienceThe development of Generation IV fission nuclear reactors and fusion nuclear reactorsrequires materials able to resist to high temperature (650°C) creep, but also to creep-fatigue.Martensitic 9-12%Cr steels are candidate materials for these applications.Recent studies on commercial P91 steel showed that cyclic loadings coupled to high temperature creep loadings lead to a strong softening effect, which affects the steelmechanical strength. This effect is due to the decrease of the dislocations density and thecoarsening of martensitic microstructure.Thermomechanical treatments, including warm-rolling in austenitic phase and tempering,have been applied to P91 in order to refine its microstructure and to improve its precipitationstate. The temperature of rolling was set at 600°C, and those of annealing at 650°C and700°C, thanks to MatCalc calculations.Microstructural observations proved that the warm-rolling and the following tempering lead toa finer martensite pinned with numerous small precipitates. In terms of mechanical propertiesimprovement, the hardness of thermomechanically treated P91 is higher than that of asreceivedP91. The yield strengths are higher than that of P91 (around 400 MPa at 20°C; andmore than 200 MPa at 550°C). Preliminary creep resu lts show that these treatments improvethe creep lifetime by at least a factor 8