Fig. 1. Stress vs. rupture life curve at 625 ℃. The black dots are experimental data, and colored lines are regression curves of different regions.

Fig. 2. Equilibrium phase components at various temperatures: (a) whole plot, (b) partial enlarged plot.

Fig. 3. Microstructure of the tempered sample: (a) SEM micrograph, (b) TEM micrograph of an unrecrystallized region showing the distribution of carbides on lath boundaries. (c) TEM micrograph of a fully recrystallized region with carbides locating along equiaxed grain boundaries. (d) Another fully recrystallized region with rod shaped carbides distributing in the matrix.

Fig. 4. Illustration of lattice of Fe matrix and M23C6 carbides observed from different directions: (a) and (b) along [$\bar{1}$10]Fe direction and (c) from [001]Fe direction. The blue spheres represent Cr atoms in M23C6, red spheres represent Fe atoms, the enlarged spheres represent the atoms on the top layer.

Fig. 5. Microstructure of the samples crept at 625℃ for different time: (a-c) SEM micrograph of samples crept for 340 h, 4,470 h and 10,700 h, respectively, (d-f) TEM micrograph of samples crept for 340 h, 4,470 h and 10,700 h, respectively.

Fig. 6. EBSD boundary map superimposed on inverse pole figure, (a) crept for 4,470 h, (b) crept for 10,700 h. The colors represent different orientations as illustrated in the standard triangle legend. Silver and black lines show the boundaries with misorientation angles between 2° and 15°, and larger than 15°, respectively.

Fig. 7. Histogram of the relative frequencies of misorientation angles. The misorientations belonging to KS or NW relationships are marked in the figure.

Fig. 8. Size distribution of M23C6 carbides: (a) as received, (b) crept for 340 h, (c) crept for 4,470 h, (d) crept for 10,700 h. The major axis and minor axis are plotted separately in each state.

Fig. 9. Distribution of aspect ratio of M23C6 carbides after different creep time.

Fig. 10. Coarsening plots of M23C6 carbides by fitting Eq. (4) with different d. Dots are experimental data and solid lines are regression curves.

Fig. 11. TEM micrograph of the investigated steel crept for 10,700 h: (a) bright field image, (b) dark field image with an inset of SAED pattern.

Fig. 12. Illustration of the recovery of martensite and evolution of precipitates, where black dots represent M23C6 carbides, red lines represent recrystallized grain boundaries, black lines represent martensite lath boundaries, blue dots represent Laves phase.

Fig. 13. Contributions of microstructural features to the internal stress. σprecipitate: internal stress due to precipitates, σsubgrain: internal stress due to subgrain boundaries, σdislocation: internal stress due to mobile dislocations.