Takanari Okuda

Development of 9Cr-ODS Martensitic Steel Claddings for Fuel Pins by means of Ferrite to Austenite Phase Transformation

For use as fuel cladding of liquid metal fast reactors, Fe-0.12C-9Cr-2W ODS martensitic steel claddings were developed by cold-rolling under the softened ferrite phase induced by slow cooling from austenite phase, subsequently by ferrite to austenite phase transformation to break up substantially elongated grains produced by cold-rolling at the final heat-treatment. The produced claddings showed noticeable improvement in tensile and creep rupture strength that are considerably superior to PNC-FMS and even austenitic PNC316 at higher temperature and extended time to rupture. The strength improvement is mainly attributed to titanium addition in ODS martensitic steels through its reduction of Y2O3 particle size and shortening inter-particles spacing. The behavior of oxide particle size reduction is associated with stoichiometry between Y2O3 and TiO2.

Characterization of High Temperature Creep Properties in Recrystallized 12Cr-ODS Ferritic Steel Claddings

The high temperature strengthening mechanism of previously manufactured 12Cr-ODS ferritic steel claddings was clarified. In the recrystallized 12Cr-2W-0.3Ti-0.24Y2O3-ODS ferritic steel cladding, αY2TiO5 type complex oxide formation was responsible for the drastic reduction of oxide particle size and the resulting shortened distance between particles, which led to superior internal creep rupture strength at 973 K because of the high resistance to gliding dislocation. Internal creep deformation was considered to be controlled by the grain boundary sliding associated with grain morphology: the near Σ11, Σ and Σ19 coincidence boundaries with a (110) common axis.

Tube manufacturing and characterization of oxide dispersion strengthened ferritic steels

Abstract Oxide dispersion strengthened (ODS) ferritic steels have an advantage in radiation resistance and superior creep rupture strength at elevated temperature due to finely distributed Y2O3 particles in the ferritic matrix. Using a basic composition of low activation ferritic steel (Fe–12Cr–2W–0.05C), cladding tube manufacturing by means of pilger mill rolling and subsequent recrystallization heat-treatment was conducted while varying titanium and yttria contents. The recrystallization heat-treatment, to soften the tubes hardened due to cold-rolling and to subsequently improve the degraded mechanical properties, was demonstrated to be effective in the course of tube manufacturing. For a titanium content of 0.3 wt% and yttria of 0.25 wt%, improvement of the creep rupture strength can be attained for the manufactured cladding tubes. The ductility is also adequately maintained.

R&D of oxide dispersion strengthened ferritic martensitic steels for FBR

As prospective cladding material for the long-life core of a Fast Breeder Reactor (FBR), we developed oxide dispersion strengthened (ODS) ferritic/martensitic steels, which have more swelling resistance than austenitic steels and are expected to have a superior creep strength at elevated temperatures. In order to improve the inferior strength in the hoop direction of manufactured ODS cladding tubes, recrystallization and martensitic phase transformation techniques have been developed, and the strength anisotropy was successfully improved in laboratory scale tests. It is also demonstrated that cold rolling manufacturing for the ODS ferritic cladding was possible using the recrystallization technique.

Development of Oxide Dispersion Strengthened Ferritic Steels for FBR Core Application, (I): Improvement of Mechanical Properties by Recrystallization Processing

As to an oxide dispersion strengthened (ODs) ferritic steel cladding as the promising candidate for long-life core materials of the fast reactors, previously fabricated claddings had inferior internal creep rupture strength in hoop direction and inferior formability due to less ductility. Those unexpected features of ODs claddings are substantially ascribed to the needle-like grain structure excessively elongated along the forming direction. Controlling the grain morphology by applying the recrystallization method to ODs ferritic steel made possible to improve those inferior features. The ranges of Y2O3 and excessive oxygen contents for possibly cold-rolling and recrystallization were revealed, and the effects of extruded temperature and deformation texture on recrystallization characteristics were evaluated. The recrystallized ODs ferritic steel showed superior internal creep rupture strength and ductility. It was demonstrated from those results that cold-rolling manufacturing of ODs cladding at room tem...

Development of oxide dispersion strengthened steels for FBR core application, (II). Morphology improvement by martensite transformation

Previously manufactured oxide dispersion strengthened (ODS) ferritic steel cladding tubes had inferior internal creep rupture strength in the circumferential hoop direction. This unexpected feature of ODS cladding tubes was substantially ascribed to the needle-like grain structure aligned with the forming direction. In this study, the grain morphology was controlled by using the martensite transformation in ODS martensitic steels to produce an equi-axial grain structure. A major improvement in the strength anisotropy was successfully achieved. The most effective yttria addition was about 1 mass% in improving the strength of the ODS martensitic steels. A simple addition of titanium was particularly effective in increasing the strength level of the ODS martensitic steels to that of ODS ferritic steels.

Consolidation process study of 9Cr-ODS martensitic steels

Abstract As a practical method to produce a large-scale annular billet made of 9Cr-ODS martensitic steels, a hollow capsule with outer and inner diameter of 147 and 32 mm, respectively, was filled with mechanical milled (MM) powders and were extruded by a 2000 ton press at 1423 K. Hot isostatic pressing was applied as a consolidation process of MM powders at 1423 K and 190 MPa for 3.6×10 3 s. The microstructure and formability of the HIPed product were investigated. The hot-extrusion behavior of the HIP-ODS alloy was also tested using a 2000 ton press. The compaction by the cold isostatic pressing with subsequent sintering is insufficient as a starting material for hot-extrusion.

Effect of Oxide Species and Thermomechanical Treatments on the Strength Properties of Mechanically Alloyed Fe - 17% Cr Ferritic ODS Materials

The effects of several oxide species, such as Y203, Zr02 and MgO, and the thermomechanical treatment (TMT) after the mechanical alloying (MA) process on the strength properties of Fe-17%Cr ferritic ODS (oxide dispersion strengthening) MA materials were investigated. Y20, showed the most uniform dispersion of the finest particles among me above oxides, but the microstructural evolution during the TMT had a larger effect on the strengthening of the alloys than the fine and uniform dispersion of the Y2O3 particles had.

Microstructure and Tensile Properties of ODS Ferritic Steels Produced by Mechanical Alloying in Argon and Hydrogen Gas Environments

Two types of oxide dispersion strengthened (ODS) ferritic steels have been produced by mechanical alloying (MA) either in argon or in hydrogen atmosphere, and vacuum hot pressing (VHP). A drastic reduction in the oxygen and nitrogen contents after VHP was strongly affected by hydrogen gas used as the MA atmosphere. MA in hydrogen was found to be effective for refining the steel matrix and enhancing the tensile ductility of the ODS ferritic steels.