High temperature mechanical properties and microstructure in 9Cr or 12Cr oxide dispersion strengthened steels

Mitsuhara, Masatoshi*; Kurino, Koichi*; Yano, Yasuhide; Otsuka, Satoshi; Toyama, Takeshi*; Onuma, Masato*; Nakashima, Hideharu*

Tetsu To Hagane, 109(3), p.189 - 200, 2023/03

https://doi.org/10.2355/tetsutohagane.TETSU-2022-080

Oxide Dispersion Strengthened (ODS) ferritic steel, a candidate material for fast reactor fuel cladding, has low thermal expansion, good thermal conductivity, and excellent resistance to irradiation damage and high temperature strength. The origin of the excellent high-temperature strength lies in the dispersion of fine oxides. In this study, creep tests at 700 or 750C, which are close to the operating temperatures of fast reactors, and high-temperature tensile tests at 900 to 1350 C, which simulate accident conditions, were conducted on 9Cr ODS ferritic steels, M11 and MP23, and 12Cr ODS ferritic steel, F14, to confirm the growth behavior of oxides. In the M11 and F14 creep test samples, there was little oxide growth or decrease in number density from the initial state, indicating that dispersion strengthening by oxides was effective during deformation. After creep deformation of F14, the development of dislocation substructures such as dislocation walls and subgrain boundaries was hardly observed, and mobile dislocations were homogeneously distributed in the grains. The dislocation density increased with increasing stress during the creep test. In the high-temperature ring tensile tests of MP23 and F14, the strength of both steels decreased at higher temperatures. In MP23, elongation decreased with increasing test temperature from 900 to 1100 C, but increased at 1200 C, decreased drastically at 1250 C, and increased again at 1300 C. In F14, elongation decreased with increasing temperature. It was inferred that the formation of the -ferrite phase was responsible for this complex change in mechanical properties of MP23 from 1200 to 1300 C.

Three-dimensional microstructure and high temperature strength in ODS 9Cr ferritic heat resistant steels

Mitsuhara, Masatoshi*; Harada, Erika*; Yamasaki, Shigeto*; Ikeda, Kenichi*; Hata, Satoshi*; Nakashima, Hideharu*; Otsuka, Satoshi; Kaito, Takeji

Kashika Joho Gakkai-Shi, 31(122), p.98 - 103, 2011/07

https://doi.org/10.3154/jvs.31.98

no abstracts in English

EDS analyses and 3D observations of precipitates on lath boundaries in V-bearing heat resistant steels

Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Hata, Satoshi*; Nakashima, Hideharu*; Wakai, Takashi

Materia, 47(6), P. 301, 2008/06

http://ci.nii.ac.jp/naid/40016094186

no abstracts in English

Creep strengthening mechanisms in high Cr heat resistant steel

Mitsuhara, Masatoshi*; Yoshida, Yu*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

Kyushu Daigaku Cho Koatsu Denkenshitsu Kenkyu Hokoku, 31, p.78 - 79, 2007/00

The effects of V and Nb on creep strength of high Cr ferritic steel were investigated based on the observation results using STEM. As a result, it is clarified that Nb contributes to the improvement of creep strength by encumber the movement of the lath boundary by fine dispersed precipitation as MX particles and V contributes as film precipitation on the lath boundary.

Study on grain boundary structure analysis and control of oxide dispersion strengthened (ODS) martensitic steel cladding

Nakashima, Hideharu*; Ikeda, Kenichi*; Yoshida, Fuyuki*; Otsuka, Satoshi; Ukai, Shigeharu

JNC TY9400 2004-019, 56 Pages, 2004/08

JNC-TY9400-2004-019.pdf:5.22MB

The grian boundary structure of Oxide Dispersion Strengthened (ODS) steel was analysed. Based on the analysis results, the grain boundary control procedure for creep strength improvement was discussed.

Character evaluation of strength in dispersion strengthened ferritic steel (3)

Yoshida, Fuyuki*; *; *; Nakashima, Hideharu*; *

PNC TJ9606 95-002, 62 Pages, 1995/04

PNC-TJ9606-95-002.pdf:2.69MB

In order to clarify the origin of strength anisotropy in ODS ferritic steel with YO particles at high temperatures, a threshold stress measured by stress abruptly loading test (SAL test) were compared with the Orowan and the void-hardening stresses calculated from dispersion parameters and subjected to the high-temperature compression test at temperature from 600 to 700 C and at strain rates from 110 to 110s, and the effect of grain-boundary sliding on the high-temperature strength was clarified. The results are summarized as follows. (1) The Orowan stress and the void-hardening stress calculated from dispersion parameters approximately agreed with the threshold stress obtained by SAL test. In addition, as the time under load elapsed, the transition of the threshold stress obtained by SAL test was observed, it is concluded that the originating mechanism of the threshold stress in ODS ferritic steel is the Srolovitz's one (2) The grain-boundary sliding was clearly observed for grain boundaries inclined by 45 to the stress axis. The flow stress was changed significantly by aspect ratio of grain size. Therefore, it is concluded that the strength anisotropy in ODS ferritic steel induces the grain-boundary sliding behaviour.

Character evaluation of strength in dispersion strengthened ferritic steel

Nakashima, Hideharu*; *

PNC TJ9606 93-001, 36 Pages, 1993/03

PNC-TJ9606-93-001.pdf:0.88MB

It has been examined about mechanisms of strength anisotropy and ductility dip of ODS ferritic steel induded with YO particles at high temperatures. The results are summarized as follows. (1)Above 650C, 0.2% proof stress measured by tensile test and axial cleep rupture strength agree with the threshold stress for dislocation motion evaluated by the theory of the dispersion strengthening. But the bi-axial creep rupture stress is smaller than the threshold stress. It is presumed that grain boundaries in transverse section of cladding tube slide by the hoop stress below the threshold stress. It is considered that the strength anisotropy comes from an elongated grains formed by fabrication. (2)The ductility decreases in two different temperature ranges which are around 600C and 1100C respectively. The ductility-minimum temperature in the lower temperature range is independent of the strain rate in a high strain rate range, whereas that temperature is dependent on the strain rate in a low strain rate range. The reason why the strain rate dependence of the ductility-minimum temperature is different between high and low strin rate ranges has not been clarified. But it is suggested that the dip in the lower temperature range should be related to the grain boundary sliding and the grain-boundary precipitation. In the higher ductility-dip temperature range, this alloy has an - duplex structure. Therefore, it is considered that the ductility dip arises from the concentration of the deformation in phase.

Effect of V/Nb content on microstructural changes in high Cr ferritic steels during creep deformation

Yoshida, Yu*; Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

no journal, , 

no abstracts in English

Relationship between strengthening mechanism and omega parameter of high-Cr ferritic steels containing V and Nb

Yoshida, Yu*; Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

no journal, , 

no abstracts in English

Effect of Vanadium and Niobium on creep strength in 10% Chromium steel analyzed by STEM-EDS

Mitsuhara, Masatoshi*; Yoshida, Yu*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

no journal, , 

The effect of V and Nb on creep strengthening was studied in 10% Cr steels. In a steel added 0.06% V where no particles were observed in the lath, creep strength was slightly increased, indicating that it was not strengthened by particle hardening effect in the lath. Lump-shaped precipitates and film-shaped precipitates including Cr and V, which were observed on lath boundaries in the steel added V, are expected to inhibit the lath boundaries from migrating. This inhibition is the mechanism of the improvement in the steel added V. In the case of steels added Nb individually, creep strength was found to be increased by dispersion hardening due to fine precipitates in the lath. These results revealed that V and Nb have each role to improve creep strength. In steels added both V and Nb, the effect on creep strength was expressed by the sum of the effects due to each element, which were the retardation of the lath boundary migrations and the pinning of the dislocations in the lath.