**5. Conclusions**

**Effect of austenitization temperature:** compared to the conventional heat treatments, the use of a higher austenitization temperature (1225°C rather than 1040°C), combined with an ausforming processing step at 900°C, allows the increase of the number density of MX precipitates up to three orders of magnitude after the tempering step, which raises the strengthening capability of the MX at 700°C up to 6.5 times. These microstructures have reduced considerably the minimum disk deflection rate and showed greater time to rupture during the SPCT carried out at 700°C. By contrast, such elevated austenitization temperature induces an important drop in ductility.

**Effect of ausforming:** the SPCT was applied to evaluate the creep behavior of G91 steel after different TMT and heat treatments. The minimum disk deflection rate was lower, and the time to rupture was longer for G91 after the TMT than with the conventional G91 heat treatment (AR). The improvement in creep rupture strength is attributed to the fine and homogeneous distribution of MX carbonitrides. The number density and average precipitate size of MX carbonitrides after the TMT are similar to the oxide particles in ODS steels. These latter steels possess high creep strength due to the high number density of oxides distributed in the

matrix. Considering the MX carbonitrides as a substitute for oxides, 9Cr FM steels after the TMT are a potential replacement of ODS steels, which are fabricated by expensive powder metallurgy and mechanical alloying processing routes.

**Creep failure:** based on the results presented above and taking into account the different stages of the TMT, the loss of creep ductility that enhances the change in fracture mechanism would be promoted by the coarsening of M23C6 carbides at the vicinity of the prior austenite grains. The coarse M23C6 carbides located on prior austenite grain boundaries favor the nucleation of the cavities at the vicinity of the prior austenite grains. Besides, in the TMT samples, the high austenitization temperature produces an enormous prior austenite grain sizes with concomitant large grain boundary surfaces, facilitating an earlier formation of the critical crack length that causes the brittle fracture.
