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Oxide dispersion strengthened (ODS) steels are being developing for application as advanced fast reactor cladding and fusion blanket materials, in order to allow increased operation temperature. Oxidation testing of ODS steel was conducted under a controlled dry air atmosphere to evaluate the high temperature oxidation behavior. This showed that 9Cr-ODS martensitic steels and 12Cr-ODS ferritic steels have superior high temperature oxidation resistance compared to 11 mass% Cr PNC-FMS and 17 mass% Cr ferritic stainless steel. This high temperature resistance is attributed to earlier formation of the protective α-Cr2O3 on the outer surface of ODS steels. [Copyright &y& Elsevier]

Ultra-high temperature ring tensile tests were performed to investigate the tensile behavior of oxide dispersion strengthened (ODS) steel claddings and wrapper materials under severe accident conditions with temperatures ranging from room temperature to 1400 °C which is close to the melting point of core materials. The experimental results showed that the tensile strength of 9Cr-ODS steel claddings was highest in the core materials at ultra-high temperatures of 900 – 1200 °C, but there was significant degradation in the tensile strength of 9Cr-ODS steel claddings above 1200 °C. This degradation was attributed to grain boundary sliding deformation with γ/δ transformation, which is associated with reduced ductility. By contrast, the tensile strength of recrystallized 12Cr-ODS and FeCrAl-ODS steel claddings retained its high value above 1200 °C, unlike the other tested materials. [ABSTRACT FROM AUTHOR]

Increasing Cr content from 9 to 12 mass% leads to superior corrosion and high-temperature oxidation resistances, and usually changes microstructure from martensite to a ferrite. To make transformable martensitic type of 12CrODS steels that have superior processing capability by using α/γ phase transformation, alloy design was conducted through varying nickel content. The structure of 12CrODS steels was successfully modified from full ferrite to a transformable martensite-base matrix containing ferrite. This ferrite consists of both equilibrium ferrite and a metastable residual ferrite. It was shown that the fraction of the equilibrium ferrite is predictable by computed phase diagram and formation of the residual ferrite was successfully evaluated through pinning of α/γ interfacial boundaries by oxide particles. [ABSTRACT FROM AUTHOR]

Abstract: The effects of fast neutron irradiation on ring tensile properties of oxide dispersion strengthened (ODS) steel claddings for fast reactor were investigated. Specimens were irradiated in the experimental fast reactor Joyo using the material irradiation rig at temperatures between 693 and 1108K to fast neutron doses ranging from 16 to 33dpa. The post-irradiation ring tensile tests were carried out at irradiation temperatures. The experimental results showed that there was no significant change in tensile strengths after neutron irradiation below 923K, but the tensile strengths at neutron irradiation above 1023K up to 33dpa were decreased by about 20%. On the other hand, uniform elongation after irradiation was more than 2% at all irradiation conditions. The ring tensile properties of these ODS claddings remained excellent within these irradiation conditions compared with conventional 11Cr ferritic/martensitic steel (PNC-FMS) claddings. [Copyright &y& Elsevier]

Abstract: This paper describes the effect on creep strength and microstructure of 9Cr-oxide dispersion strengthened martensitic steel (9Cr-ODS steel) produced by differences in titanium concentration and consolidation temperature. The increase of titanium concentration to 0.30–0.35wt% was shown to provide considerable improvement of creep strength accompanied by the increase of residual-α ferrite. The elevation of hot-extrusion temperature degraded the creep strength, however, it appeared to increase the volume fraction of residual-α ferrite. The creep deformation process of 9Cr-ODS steel was discussed to explain these results based on microstructure observations. [Copyright &y& Elsevier]

Abstract: The effects of mechanical alloying (MA) conditions on oxygen contamination were investigated to establish the manufacturing technology of oxygen-controlled oxide dispersion strengthened (ODS) martensitic steel. High-temperature mechanical tests and microstructure observations were performed in oxygen-and-titanium-controlled steels in order to demonstrate high-temperature strength improvement using oxygen control. Oxygen content was drastically reduced by the purification of MA atmosphere, Ar gas. It was shown that the lower speed agitation at the beginning of MA was effective for the reduction of oxygen contamination because oxygen was mixed mainly at the beginning of mechanical alloying due to the imperfect replacement of Ar gas. Dense distribution of oxide particle and significant improvement in high-temperature strength were achieved by applying 99.9999wt% Ar in MA atmosphere and reducing oxygen contents. It was revealed that the merely increasing Y2O3 and titanium addition cannot improve the high-temperature strength. The excess oxygen is an important parameter for making oxide particles finely and densely distributed as well as improving high-temperature strength. [Copyright &y& Elsevier]

Welding technology between 9Cr-ODS martensitic cladding and end-plugs has been developed by means of the pressurized resistance welding (PRW) method under solid state condition. The method is based on electrical resistance heating of the components while maintaining a continuous force sufficient to forge weld without melting. The appropriate conditions, e.g., electric current, voltage and contact force, were selected. For the PRW welded specimens, the tensile, internal burst and creep rupture tests were conducted and its integrity was confirmed. In addition, a non-destructive ultrasonic inspection method has been developed to assure the integrity of the weld between cladding and end-plug. The PRW was conducted to fabricate the ODS fuel pins in a JNC-Russia collaboration work, and the integrity accomplishment of the welded parts was verified through dimensional and leak testing, mechanical testing and micrographic examination. [Copyright &y& Elsevier]

The effects of different percentages of oxygen and titanium on the mechanical properties and microstructure of 9Cr-ODS steel were investigated. It was shown that the high-temperature strength was drastically improved by controlling the atomic ratio between excess oxygen and titanium (x in TiOx) around 1.0. When x is around 1.0, the elongated alpha-grains containing the ultra-fine and close oxide particle dispersion were generated in addition to equiaxed martensite grains. Furthermore, the number density of oxide particle in the martensite grains increased. The control of phase transformation and oxide particle dispersion by adjusting oxygen and titanium contents should be a key technology to achieve superior high-temperature strength. [Copyright &y& Elsevier]