22.08.2015   Comprehensive nuclear materials

Martensitic 9Cr-ODS Steels

4.08.3.1 Chemical Composition and Microstructure

9Cr-ODS steels are being developed by the JAEA (Japan Atomic Energy Agency) for application to SFR fuel cladding. Their standard chemical compo­sition is 9Cr-0.13C-0.2Ti-2W-0.35Y2O3 (wt%). The chromium concentration was determined to be 9wt% in terms of ductility, fracture toughness, and corrosion resistance based on a series of irradiation data of ferrite steels. The addition of titanium pro­duces the nanoscale dispersion of oxide particles, which leads to a markedly improved high-temperature strength. If titanium is added to excess, however, it creates too much strength, which negatively impacts

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Figure 4 Microstructure of 9Cr-ODS steel showing residual ferrite and tempered martensite.

cold rolling and cold workability. To achieve a bal­ance between strength and workability, a value of 0.2 wt% was selected. Tungsten of 2 wt% is also added in order to improve high-temperature strength by means of solid solution hardening.

The microstructure of 9Cr-ODS steels13-18 can be easily controlled by a reversible a-g transformation with a remarkably high driving force of a few hundred MJ m~3, as compared with a driving force of irrevers­ible recrystallization with a few MJ m~3 for 12Cr-ODS steels. By inducing reversible a-g transformations, 9Cr-ODS steel cladding for fast-reactor fuel elements is currently being manufactured at the JAEA.

The microstructure of 9Cr-ODS steel cladding is basically tempered martensite. However, it has been recognized that 9Cr-ODS steel cladding manufactured in an engineering process possesses a dual-phase struc­ture that comprises both tempered martensite and ferrite phases. An example of their microstructure is shown in Figure 4. The ferrite phase appears white, and the elongated phase is indicated by arrows. Their size is about 30-60 pm in length x 3-10 pm in width. The formation of a ferrite phase in 9Cr-ODS steel is somewhat unusual, because only the full martensite phase can be expected in 9Cr-ferritic steel without yttria under normalizing and air-cooling conditions. Moreover, the high-temperature strength of manu­factured 9Cr-ODS steel is significantly improved by the presence of the ferrite phase.1 — 1 This is obvious from the creep rupture data shown in Figure 5.20

Подпись: 10 100 1000 10000 Time to rupture (h) Figure 5 Uni-axial creep rupture strength of 9Cr-ODS steels at 700 °C after the normalizing-and-tempering (1050°C x 1 h, Ar-gas cooling (AC) = > 780 °C x 1 h, AC) with and without residual ferrite. Reproduced from Ohtsuka, S.; Ukai, S.; Fujiwara, M.; Kaito, T.; Narita, T. Mater. Trans. 2005, 46, 487. Подпись: Figure 6 Computed phase diagram with respect to carbon content for 9Cr-xC-0.2Ti-2W system without Y2O3.

Therefore, the control of ferrite phase formation is a key to the realization of high-temperature strength in 9Cr-ODS steel cladding.