Microtomographic assessment of damage in P91 and E911 steels after long-term creep

Two flat hollow cylinders made of martensitic 9 wt.% Cr steels were creep deformed under in-service conditions typical of steam pipes at fossil-fuel fired power plants. Damage in the tubes was assessed through synchrotron X-ray microtomography by evaluating the shape, size and spatial-distribution of voids. The analysis of the size distribution of non-coalesced voids suggested that void growth is controlled by the plasticity constrained diffusional mechanism, a hypothesis verified by micromechanical simulations. A much higher void density was found in steel grade P91 compared to E911.