Based on the aforementioned finding, cladding manufacturing tests were conducted using cold­rolling pilger mill in 12Cr-ODS ferritic steels with the limited Y2O3 content <0.25 mass % to induce the recrystallized structure, and their internal creep rup­ture properties were evaluated at 700 °C, not at 650 °C. The chemical composition of the manufac­tured cladding is listed in Table 1, where the speci­mens are denoted as F1 to F4. The four levels of Y2O3 contents were selected in the range of <0.25 mass %, and the titanium content ranged from 0.13 to 0.31 mass %. Cold rolling by a pilger mill was

Figure 19 Optical microstructure of the F1, F2, F3, and F4 specimens in the final claddings. Reproduced from Ukai, S.; Okuda, T.; Fujiwara, M.; Kobayashi, T.; Mizuta, S.; Nakashima, H. J. Nucl. Sci. Technol. 2002, 39(8), 872-879.

repeated twice with a reduction ratio of about 50% per rolling. The intermediate heat-treatment to soften the cold-rolled cladding was performed at 1100 °C for 30 min, and the final heat-treatment was performed at 1150 °C for 0.5 h.

Figure 19 shows the optical microstructures of the manufactured claddings in the longitudinal and transverse directions.36 All of the specimens seem to be recrystallized. However, the extent of recrys­tallization depends on the yttria and titanium con­tents. In the transverse cross-section, the grain size becomes slightly finer with increasing yttria and titanium contents from F1 to F4. In the F4 speci­men, the elongated grains can be still seen and the aspect ratio in the longitudinal (L) and transverse (T) directions is large, whereas the aspect ratio of specimen F1 appears to be nearly unity. These findings show that F4 specimen with higher yttria and titanium contents did not attain the perfectly recrystallized grain structure by the annealing of 1150 °C for 0.5 h.