4.08.4.1 Strength Anisotropy

When JAEA started to develop ODS steels in 1985, the ferritic type of ODS steels was applied.3, These are similar to MA957,34 which is single ferrite phase and does not include the martensite. Based on the results of R&D conducted for several years, three kinds of claddings, 63DSA, 1DK, and 1DS, were manufactured in 1990. Their chemical compositions

are 13Cr-0.02C-3W-0.7Ti-0.46Y2O3 (63DSA), 13Cr — 0.05C-3W-0.5Ti-0.34Y2O3 (1DK), and 11Cr-0.09C- 3W-0.4Ti-0.66Y2O3 (1DS). The manufacturing process is almost the same as the process shown in Figure 13, except for the rolling process and intermediate heat treatment, because cold-rolling processing can be hardly applied to these ODS steels. In the case of the 1DK cladding, six warm drawings at 800-850 °C, followed by four warm rolling passes at 500 °C with intermediate annealing at 1080 ° C, were repeated to manufacture the thin-walled cladding in the dimension of 7.5 mm outer
diameter, 0.4 mm thickness, and 1 m length. In the case of the 63DSA and IDS claddings, only six warm rolling passes at 650-700 °C with intermediate annealing at 1100 ° C were conducted. The temperature of the final heat treatment of 1DK cladding was 1150°C for 60s, and 63DSA and 1DS claddings at 1100 for 3.6 ks.

The uni — and bi-axial creep rupture strengths of the manufactured claddings at 650 °C are shown in Figure 17, where the uni-axial corresponds to the hot working direction and bi-axial belongs to the internal hoop direction.3 It was found that

there is strong strength anisotropy, and the bi-axial creep rupture strength is considerably lower than that of the uni-axial direction. Microstructure obser­vations of these claddings exhibited the elongated grains like a bamboo structure in parallel to the working direction. The strength degradation in the bi-axial/internal hoop direction, which is essential for the fuel elements, should be mainly attributed to the grain boundary sliding and crack propagation due to stress concentration.