In recent years, an attempt to increase the high tem­perature creep life of ferritics to 973 K and target burn-up of the fuel to 250 dpa, has enabled a ‘revisit’ to the concept of strengthening the steel using 5 nm particles of yttria (see Chapter 4.08, Oxide Disper­sion Strengthened Steels), leading to the ODS fer­ritic steels. ODS ferritic steels are prospective candidate materials for sodium cooled fast reactors with peak burn-up of 250 dpa as well as GenlV and fusion reactors. Earliest developments ofODS steels can be traced to the efforts65 of Belgium in 1960s, followed by Japan66 since 1987, and France67 in the last decade. The ODS steels for fast and fusion reac — tors68,69 are in the R&D stage.

The design of ODS steels for fast and fusion reactor applications is based on Fe-Cr-W-Ti — Y2O3, either the martensitic 9 or 12Cr or the ferritic 12Cr steels. The dispersoids which confer the high temperature creep life to the ferrite matrix are70

(Figure 13) in the size range of around 5 nm with a volume fraction around 0.3%. The yttria dissolves in it some amount oftitanium, leading to the formation of mixed, complex oxide, namely TiO2Y2O3.

The rationale for the choice of the matrix compo­sition is as follows:

Chromium: Choice of 9% Cr and 0.1% C ensures 100% martensite, during normalization ofthe steel. It is possible to ensure 100% martensite in 12% chro­mium steel by ensuring the carbon content to be above 0.1%. Ferritic ODS steels can be obtained in 12% chromium steels by lowering the carbon content to be less than 0.03%. Higher chromium provides the corrosion and decarburization resistance in sodium at 973 K, with acceptable oxidation resistance.

Carbon. Addition of 0.1% carbon ensures 100% martensite in 9% Cr steels, thus ensuring absence of anisotropy during g! a transformation. Higher amount of carbon would promote precipitation of M23C6, thus reducing the toughness. On the other hand, M23C6 along the lath boundaries offers the long-term microstructural stability of the lath structure.

Nitrogen: The solubility of nitrogen in ferrite is very low. This is useful in non-ODS ferritic steels like T91, due to enhanced creep resistance by forma­tion of V or Nb carbides/carbonitrides. But, in ODS steels, Ti is used for refining yttria. Hence, nitrogen content is restricted to 0.01%, preventing the forma­tion of deleterious TiN compound.

Tungsten-. Tungsten is a more effective solid solution strengthener than Mo, but at the cost of ductility. Tungsten stabilizes 8-ferrite and accelerates formation of Laves phase, both of which cause reduc­tion in toughness. Hence, it is optimized to 2.0%.

Yttria — The most important constituent of ODS steels is the yttria, which enhances high temperature creep strength by pinning mobile dislocations and delays void swelling by acting as sinks for point defects produced during irradiation. The strength increase is accompanied by a concomitant loss of ductility and saturates around 0.4% yttria. Hence, it is optimized to 0.35%.

Titanium — The major role of titanium in ODS steels is to refine the yttria particles (20 nm after mechanical alloying) to ultra-fine (2-3 nm) particles. The complex Y-Ti-O particle imparts the necessary high temperature creep strength. The beneficial effect of titanium saturates around 0.2%. Further increase introduces manufacturing problems of the tubes and hence titanium is chosen as 0.2%.

Excess oxygen — Oxygen is present during processing of the ODS steels. The oxygen present in excess of the amount required for formation of required amount of Y-Ti-O complex leads to increase in tensile and creep strength. The Y-Ti-O complex oxides requires about 0.07 + 0.01% excess oxygen.

Argon — A strict control of argon (<0.002%) during processing of ODS steels is essential to avoid embrit­tlement due to formation of argon bubbles during irradiation.

Minor elements — Nickel and manganese are to be reduced to maintain the A1 (a! g on heating) tem­perature higher than the anticipated hot spot temper­ature. This enables the tempering temperature to be as high as possible. Silicon, phosphorous, and sulfur undergo RIS and cause embrittlement. Silicon also accelerates formation of deleterious phases like the Laves phase. Hence, their amounts are reduced to

0. 05 and less.

The processing route used worldwide is the powder metallurgy route of mechanically alloying prealloyed powders of Fe-Cr-W-Ti-C + Ti2O3, followed by hot extrusion and rolling or hipping with final heat treatments. Commercial ODS steels (Table 7) have been developed demonstrating the standardizing of fabrication technologies.

The ferritic-martensitic ODS steels have been developed by adjusting the contents of chromium and carbon. The ferritic ODS steels, with 12% chro­mium and carbon content less than 0.02%, derive71 their high temperature creep strength basically from

the dispersoids. The ferrite matrix offers72 superior resistance against oxidation and corrosion while the major challenge appears to be the anisotropy73 of properties. The martensitic steels based on either 12% chromium with 0.1—0.2% carbon or the 9% chromium, derive their strength from the martensitic matrix and the dispersoids. The 9% chromium steel displays isotropic properties while it suffers from inferior corrosion resistance.

The conventional joining technologies pose sig­nificant problems leading to coalescence of oxide particles. Hence, solid state bonding techniques74 like the pressurized resistance welding have been developed for joining the clad tube with the end plug of a fast reactor. Postweld heat treatments (PWHT) have also been developed to match the strength levels of the clad and the end plug.

The in-service performance of ODS steels in flowing sodium has been found to be satisfactory, despite the formation of austenite layer on the sur­face of the clad tube due to the deposition of nickel. The thickness of the oxide layer in 12%Cr ODS steel was found to be only 50% of that in 9Cr ODS steels.

Reactor irradiation experiments have been per — formed75,76 on ODS ferritic steels. The dispersoids were found74 to be stable up to a dose of 10 dpa in JOYO. The mechanical properties at 573 K after neutron irradiation were reported to be same as con­ventional ferritic martensitic steels. The studies on long-term in-service behavior and postirradiation behavior are being studied.

Presently, the main challenges in this variety of ODS steels are the anisotropy observed in steels with chromium more than 12%, less oxidation resistance in steels with 9% chromium, fabrication procedure with cost-effectiveness, uniformity of dispersoids in all regions of the clad tube, stability of the dispersoids under irradiation, and the joining technologies. It is hoped that the above problems would be overcome in the near future and the ODS ferritic steels can be used in fast reactor core as clad and wrapper, for burn-up
beyond 250 dpa, at temperatures exceeding 973 K. Additionally, ODS ferritic steels are also being consid­ered for fusion reactor applications. The rich experi­ence in the development of fast reactor materials would enable launching the advanced ferritic steels for fusion technology, in a shorter time span.