Long-term microstructural degradation and creep strength in Gr.91 steel

The cause of the breakdown of creep strength has been studied in Gr.91 steel. The results show that the contribution of the static recovery of subgrains to creep deformation causes the breakdown of creep strength. The subgrain boundaries are mainly stabilized by M23C6 and MX precipitates. MX precipitates are thermally stable even in the time range when coarsening of subgrains takes place. Whereas M23C6 precipitates are not thermally stable and the aggregation of M23C6 precipitates takes place in the time range when coarsening of subgrains happens. Therefore, loss of pinning force from M23C6 precipitates is responsible for the static recovery of subgrains. MX has nothing to do with the static recovery. The microstructural stability of Gr.91 steel in the time range longer than 105 h at 600 °C has also been assessed based on the creep data of Gr.91 steel tested up to 90,408 h at 600 °C and 22,900 h at 650 °C. The aggregation of precipitates and the coarsening of subgrains start to take place at round 105 h at 600 °C. Therefore, the breakdown of creep strength is expected to happen in the time range a little longer than 105 h at 600 °C.