Effect of nitrogen concentration on creep strength and microstructure of 9Cr-ODS ferritic/martensitic steel

Oka, Hiroshi*; Tanno, Takashi; Yano, Yasuhide; Otsuka, Satoshi; Kaito, Takeji; Hashimoto, Naoyuki*

Journal of Nuclear Materials, 572, p.154032_1 - 154032_8, 2022/12

https://doi.org/10.1016/j.jnucmat.2022.154032

9Cr oxide dispersion strengthened steels with slightly different nitrogen concentrations (0.0034 - 0.029 wt%) were prepared and their creep property at 973 K was investigated with microstructural characterization before and after the creep test. The creep strength decreased significantly as the nitrogen concentration increased. Microstructural observation revealed that, in the higher nitrogen concentration specimen, coarse Y-rich inclusions were found along the boundary between transformed ferrite region and residual ferrite region. The solubility difference of nitrogen in and phase would induce the localized increment of nitrogen concentration in the boundary region during the austenitizing process, resulting in the thermodynamic destabilization and subsequent coarsening of the dispersed oxide particles. The rows of creep voids were found near the rupture part of the crept specimen, suggesting that the coarse inclusions were the starting point of creep void formation and the subsequent premature fracture.

Fabrication of tubed functionally graded material by slurry dipping process; Thickness control of Dip-coated layer

Watanabe, Ryuzo*

PNC TJ9601 98-004, 79 Pages, 1997/03

PNC-TJ9601-98-004.pdf:2.56MB

In order to obtain long life fuel cladding tubes for the fast breeder reactor, the concept of functionally graded material was applied for the material combination of Molybdenum/stainless steel/ Titanium, in which Titanium and Molybdenum were placed at the inner and outer sides, respectively. Slurry dipping method was employed because of its capability of shape forming and microstructural control. We have hitherto reported the design criteria for the graded layers, preparation of the slurry, and microstructural control of the dip-coated layers. In the present report, the thickness control of the dip-coated layer is described in detail. The thickness of the dip-coated layer depends primarily on the viscosity of the slurry. Nevertheless, for the stable dispersion of the powder in the slurry, which dominates the microstructural homogeneity, an optimum viscosity value is present for the individual slurries. With stable slurries of Ti, Mo, stainless steel powders and their mixtures, the thicknesses of dip-coated layers were controlled in dependence of their viscosities and yield values. For Ti and stainless steel powders and their mixture a PAANa was used as a dispersing agent. A NaHMP was found to be effective for the dispersion of Mo powder and Mo/stainless steel powder mixture. For all slurries tested in the present investigation PVA addition was helpful for the viscosity control. Dip-coating maps have been drawn for the stabilization of the slurries and for the formation of films with a sufficient strength for further manipulation for the slurries with low viscosity (10 mPas). The final film thickness for the low-viscosity slurry with the optimum condition was about 200m. The slurries with high viscosities of several hundreds mPa s had a good stability and the yield value was easy to be controlled. The film thickless was able to be adjusted in the size range between several tens and several hundreds m. The final thickness of the graded layer ...

Evaluation of nano-sized oxide dispersion condition in ODS steel by atom probe tomography

Tanno, Takashi; Yano, Yasuhide; Oka, Hiroshi*

no journal, , 

3-dimensional atom prove tomography (3D-APT) was applied to JAEA 9Cr-ODS steel as trial, and the dispersion condition of nano-sized oxide was evaluated. 3D-APT was successfully carried out and the dispersion condition in tiny tip was obtained. 3D-APT mapping showed that the nano-sized oxide is Y-Ti-O complexed, and there could be chemical distribution.

Structure change of ODS steel by creep annealing

Yamazaki, Jin*; Onuma, Masato*; Otsuka, Satoshi; Tanno, Takashi; Toyama, Takeshi*; Mitsuhara, Masatoshi*; Nakashima, Hideharu*

no journal, , 

X-ray small angle scattering (SAXS) analysis was carried out in order to evaluate the stability of nano-sized dispersed oxide particles in oxide dispersed strengthened (ODS) steels after creep test. The size of oxide particles was not changed after creep tests at 800C and below. On the other hand, the size tended to increase in the sampled tested at over 800C. However, the SAXS analysis showed that the nano-sized YTiO oxide particle still existed in the sample after the creep test at 1000C. The result shows that dispersed nano-sized oxide particles in ODS steels have superior stability under creep test at high temperature.