22D09
Mechanical properties and aging effects of 9Cr ODS steel
9Cr ODS鋼 の強度特性と時効効果
Takeo Muroga1, Yanfen Li1*, and Takuya Nagasaka1
室賀健夫1，李艶芬1*，長坂琢也1，1

National Institute for Fusion Science322-6 Oroshi, Toki, Gifu 509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6*Present address : Institute for Materials Research, Tohoku University
2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan*現在の所属 東北大学金属材料研究所 〒980-8577 仙台市青葉区片平2-1-1
High-temperature thermal creep properties and microstructure of a 9Cr-ODS steel were examined. The properties werecompared with those of normal reduced activation ferritic/martensitic steels (RAFMs) of CLAM and JLF-1. The
9Cr-ODS steel exhibited a superior creep strength. Prediction with Larson-Miller parameter for the typical blanket
conditions suggested that the 9Cr-ODS steel can successfully extend the maximum operation temperature to ~923 – 973
K. The effects of thermal aging on microstructure and mechanical properties were also investigated.

1. Introduction
The oxide dispersion strengthened (ODS) steels
are advanced alloys produced by MechanicalAlloying methods with high density ofnano-particle dispersion and fine grains [1]. ODSsteels with 9%Cr are recognized as promising
candidates for fusion reactors since good joining
capability with RAFMs (9Cr-W steels) is expected
because of similar chemical compositions.
In 2009, an ODS alloy with 9%Cr was produced
and delivered for examinations under a cooperation
program among Japanese universities and National
Institute for Fusion Science (NIFS). In this work,
the mechanical properties of this alloy, called as the9Cr-ODS in this paper, were investigated focusing
on creep behavior. These properties were evaluatedand compared with those of the normal RAFMs of
CLAM and JLF-1, Chinese and Japanese candidates,respectively. Thermal aging effects are also
investigated with respect to microstructures andmechanical properties.
2. Experimental
The 9Cr-ODS alloy was manufactured bymechanical alloying and further extruded into bar[2]. Then, the hot-extruded bar (length=380mm,diameter=27mm) was forged at 1423 K to a plate of
8 mm thickness. Finally, the plate was heat treated.
The chemical compositions are shown in Table 1
and heat treatment conditions are given in Table 2
in comparison with CLAM and JLF-1.
To understand the thermal stability of microstructure,
the as-received 9Cr-ODS was heat treated from 973 K to1423 K with a step of 50 K in vacuum and kept for one
hour at each temperature followed by fast furnace

cooling. After the heat treatments, the Vickers hardnessof the samples was measured with a load of 300 g and aloading time of 30 s at room temperature (RT).
The creep tests were performed using the SSJ-type
specimens with a gauge section of 5 mm (L) X 1.2 mm(W) X 0.25 mm (T), which were machined along theextrusion and forging direction. The surface of the SSJ
samples were ground with alumina papers in grit sizes up
to #4000 and mechanically polished. Uniaxial creep tests
were performed from 823 to 1073 K in a vacuum of 10-4
Pa. Microstructural observations were carried out withJEM-2000FX Transmission Election Microscope inRIAM, Kyushu University.

3. Results and Discussion
The uni-axial creep experiments for the9Cr-ODS were carried out from 823 to 1073 K withstresses from 120 to 400 MPa.
Figure 1 shows the typical creep curves of the9Cr-ODS at 973 K with different stresses. As
Table 1: Chemical compositions of the 9Cr-ODS,
CLAM (0603 HEAT) and JLF-1 (JOYO-2-HEAT).
9Cr-ODS
CLAM
JLF-1

Cr
9.08
8.94
9.00

C
0.14
0.13
0.09

W
1.97
1.45
1.98

Y
0.29
-Ti
0.23-

O
0.16
0.00170.0019Note: the Fe is the balance

Table 2: Heat treatments of the 9Cr-ODS, CLAM
(0603 HEAT) and JLF-1 (JOYO-2-HEAT).
(Note: AC: air cooling)
NormalizationTempering
9Cr-ODS 1323 K/60 min /AC 1073 K/60 min /ACCLAM
1253 K/30 min /AC 1033 K/90 min /AC
JLF-11323 K/60 min /AC 1053 K/60 min /AC520

9Cr-ODS steel

500Hardness480460440420400
load: 300g
Time: 30s

380
360

Fig.1 Strain – time creep curves for the 9Cr-ODS
tested at 973 K with different applied stresses: 250,200, 175 and 150 MPa.

25 700 800 900 1000 1100 1200 1300 1400 1500
600

Anealing temperature(keeping for 1hr), K

Fig.3 Hardness after annealing for 1 hour at various
temperatures.
N&T

973 K /1h1073 K /1h1173 K /1h1273 K /1h

50nm-5μ mFig.4 TEM microstructure (upper) and SEM surface
morphology (lower) after annealing.

Fig.2 Dependence of stress on Larson-Millerparameter for the 9Cr-ODS, comparing with CLAM,
JLF-1 and ODS-EUROFER [3] steels.
(T: temperature, K; tr: rupture time, hr).

shown in the figure, the minimum creep rateincreased and the rupture time decreased with theincrease in stress. The sample with the stress of 250
MPa ruptured after several hours. While with 150
MPa, the rupture time exceeded 230 hours.
All the results of creep rupture tests aresummarized in the Larson-Miller (L-M) plot using a
constant of 30, as shown in Fig.2. For comparison,
the data for CLAM, JLF-1 and ODS-EUROFER97
are also presented in the figure.
Obviously, the L-M parameter of the 9Cr-ODSsteels are much higher than those of CLAM andJLF-1. In the typical blanket condition for RAFMs,
the operation limit is set at 823 K for 100000 h,
which corresponds to the L-M parameter of 28.8
with rupture stress of ~140 MPa. By increasing theblanket temperature to 923 K and 973 K with thesame service life of 100000 h, where the L-M
parameter are equal to 32.3 and 34.0, the rupture
stress for 9Cr-ODS was predicted to be ~173 and~120 MPa, respectively, which are comparable tothose of RAFMs at 823 K for 100000 h. The
present data suggest that the maximum operation
temperature can be extended to about 923 – 973 K
for the 9Cr-ODS steel from the creep viewpoint.
Fig. 3 and 4 show the hardness and TEM/SEM
microstructure of the 9Cr-ODS after the heat
treatments at various temperatures for 1 h.
Remarkable hardening and change in SEM images
occurred at >1200 K, but no significant change wasobserved for the nano-particles by TEM, suggestinghigh thermal stability of the particles.
Acknowledgments
The authors are grateful to Profs. S. Ukai ofHokkaido University and A. Kimura of Kyoto
University for manufacturing the 9Cr-ODS. They
are also thankful to Prof. H. Watanabe of Kyushu
University for the use of TEM.
This study was supported by NIFS Budget CodeNIFS12UFFF022
References[1] A. Kimura, et al., J. Nucl. Mater., 367-370, 60(2007)..
[2] S. Ukai, et al., Mater. Sci. Eng. A 510-511, 115(2009).
[3] R. Lindau, Fus. Eng. Des., 61-62, 659 (2002).