Low Cycle Fatigue Behavior of 9Cr-ODS Steel as a Fusion Blanket Structural Material at Room Temperature
WANG Qitao1, LI Yanfen2,3(), ZHANG Jiarong2,3, LI Yaozhi1, FU Haiyang1, LI Xinle1, YAN Wei2,3, SHAN Yiyin2,3
1 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China 2 Shi -changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 3 CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
WANG Qitao, LI Yanfen, ZHANG Jiarong, LI Yaozhi, FU Haiyang, LI Xinle, YAN Wei, SHAN Yiyin. Low Cycle Fatigue Behavior of 9Cr-ODS Steel as a Fusion Blanket Structural Material at Room Temperature. Acta Metall Sin, 2025, 61(2): 323-335.
Adequate studies have not been conducted on the low cycle fatigue properties of oxide dispersion strengthened (ODS) steels that are typically used for fusion reactors worldwide. Moreover, the majority of the fatigue properties are examined with a small sample size due to the restricted manufacturing capacity, which is insufficient for determining the comprehensive properties of bulk materials. Research on the fatigue properties of Chinese ODS steels was conducted recently. However, it is quite uncommon to report the fatigue properties of self-produced 9Cr-ODS steel. Consequently, this work is the first to examine the effect of a cyclic strain on the low cycle fatigue behavior of a representative low-activation 9Cr-ODS martensitic steel. Hence, the strain control test method was employed here in a strain amplitude range of 0.3%-0.8% at room temperature. The cyclic stress response curves, hysteresis loops, relationships between strain amplitude and life, stress amplitude and plastic strain were obtained, and the corresponding fatigue parameters were summarized. Furthermore, the microstructural evolution, fatigue fracture morphology, and crack propagation characteristics during the fatigue process were analyzed. The results revealed that the cyclic stress response behavior of the 9Cr-ODS steel was related to the strain amplitude. With an increase in the strain amplitude, the peak stress in the tension zone of 9Cr-ODS steel increased and the fatigue life decreased. The relationship between cyclic strain and life agreed well with the Coffin-Manson model. Additionally, the 9Cr-ODS steel had no obvious cyclic hardening but revealed a cyclic softening under higher strain amplitudes of 0.5%-0.8%. The microstructure analysis showed that for higher cyclic strain amplitudes, the average grain size and the fraction of the large-angle grain boundaries increased gradually with a reduction in the dislocation density, leading to the cyclic softening of the material. The fatigue crack initiated at the surface and propagated inward by the transgranular mode. The fine grain boundaries and subgrain boundaries of the 9Cr-ODS steel could induce crack deflection, reduce crack propagation rate, and increase fatigue crack propagation life. Moreover, under the same strain amplitude, the peak stress in the tension zone of the steel was almost twice that of the China low activation martensitic (CLAM) steel without a reduction in the fatigue life, indicating a superior low cycle fatigue resistance of the 9Cr-ODS steel.
Fig.1 Specimen dimension of 9Cr-ODS steel for fatigue experiment (unit: mm. ODS—oxide dispersion strengthened)
Fig.2 OM (a) and SEM (b) images of the heat-treated 9Cr-ODS steel
Fig.3 Cyclic stress response curves of 9Cr-ODS steel (a) relationship between peak stress in tension and fatigue life (Nf) (Arrow indicates not fractured sample, Δεt / 2—total strain amp-litude) (b) relationship between peak stress in tension at Nf / 2 and Nf (Diamonds are data corres-ponding to the arrow in Fig.3a)
Fig.4 Stress-strain hysteresis loops at Nf / 2
Fig.5 Relationship between cyclic strain amplitude and number of reversals to failure 2Nf (Δεe / 2—elastic strain amplitude, Δεp / 2—plastic strain amplitude) (a) and relationship between stress amplitude at Nf / 2 (Δσ / 2) and Δεp / 2 (b)
Fig.6 Inverse pole figures of 9Cr-ODS steel after normalizing & tempering treatment (a) and under Δεt / 2 = 0.3% (b), Δεt / 2 = 0.5% (c), and Δεt / 2 = 0.8% (d) (Black lines are large angle grain boundaries (HAGBs) greater than 15°, white lines are low angle grain boundaries (LAGBs) less than 15°, the same below)
Fig.7 Kernel average misorientation (KAM) maps of 9Cr-ODS steel after normalizing & tempering treatment (a) and under Δεt / 2 = 0.3% (b), Δεt / 2 = 0.5% (c), and Δεt / 2 = 0.8% (d)
Fig.8 Grain average misorientation (GAM) maps of 9Cr-ODS steel after normalizing & tempering treatment (a) and under Δεt / 2 = 0.3% (b), Δεt / 2 = 0.5% (c), and Δεt / 2 = 0.8% (d)
Fig.9 SEM images of macrofractography (a-c), crack source zones (d-f), and crack propagation zones (g-i) under Δεt / 2 = 0.4% (a, d, g), Δεt / 2 = 0.5% (b, e, h), and Δεt / 2 = 0.8% (c, f, i) (Source of fatigue crack is the place where the circle and square circle arrive, the fatigue crack propagation directions are indicated by the red arrows)
Fig.10 Fatigue response curves of 9Cr-ODS, China low activation martensitic (CLAM)[6], and Eurofer ODS[26,27] steels under different Δεt / 2 (a); and relationship between Δεt / 2 and 2Nf of several typical reduced activation ferritic/martensitic (RAFM) steels[1,3,6,26,27] at room temperature (b)
Steel
T
σ / E
ε
b
c
K'
n'
K
MPa
9Cr-ODS
300
0.7331
12.12
-0.0722
-0.5268
1628.40
0.113
CLAM[6]
300
0.5396
59.18
-0.0930
-0.6154
514.66
0.128
Eurofer 97[1]
300
-
-
-
-0.5520
-
-
F82H[3]
300
0.4722
67.72
-0.0850
-0.7805
-
-
JLF-1[27]
300
1.0230
91.02
-0.0946
-0.5956
-
-
Table 1 Low cycle fatigue parameters of 9Cr-ODS steel and several typical 9Cr RAFM steels[1,3,6,27] at room temperature
Fig.11 Results of microstructural evolution for 9Cr-ODS steel after normalizing & tempering and under different Δεt / 2 (a) average grain sizes and amounts (b) volume fractions of HAGBs and LAGBs
Fig.12 Results of microstructural evolution for 9Cr-ODS steel after normalizing & tempering and under different Δεt / 2 (a) KAM (b) geometrically necessary dislocation (GND) (c) GAM
Fig.13 Fatigue crack initiation and propagation morphology of 9Cr-ODS steel under Δεt / 2 = 0.8% (a) SEM image of a main crack (Inset show the locally enlarged image. LD—loading direction) (b) local magnified EBSD-KAM image of a secondary crack of square area in the inset of Fig.13a (c) high magnified grain boundary + band contrast (GB + BC) image of square area in Fig.13b
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