Microstructural degradation of Gr.91 steel during creep under low stress

► In Gr.91 steel, premature creep failure occurred in the long-term at 600 °C and 650 °C. ► Dislocation structure slightly recovered during creep up to later stage of tertiary creep. ► Number density of MX particles abruptly decreased in tertiary stage due to the Z-phase formation. ► The decrease in number density of MX particles causes decrease in creep resistance.