Low Cycle Fatigue Behavior of 9Cr-ODS Steel as a Fusion Blanket Structural Material at Room Temperature

doi:10.11900/0412.1961.2023.00034

Acta Metall Sin

2025, Vol. 61

Issue (2): 323-335 DOI: 10.11900/0412.1961.2023.00034

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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.

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Abstract

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.

Key words: ODS steel low cycle fatigue fatigue life microstructure fracture characteristic

Received: 31 January 2023

ZTFLH:

TG142.1

Fund: National Key Research and Development Program of China(2019YFE03130000);National Natural Science Foundation of China(51971217)

Corresponding Authors: LI Yanfen, professor, Tel: (024)23978990, E-mail: yfli@imr.ac.cn

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	WANG Qitao
	LI Yanfen
	ZHANG Jiarong
	LI Yaozhi
	FU Haiyang
	LI Xinle
	YAN Wei
	SHAN Yiyin

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https://www.ams.org.cn/EN/10.11900/0412.1961.2023.00034 OR https://www.ams.org.cn/EN/Y2025/V61/I2/323

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 (N_f) (Arrow indicates not fractured sample, Δε_t / 2—total strain amp-litude)
(b) relationship between peak stress in tension at N_f / 2 and N_f (Diamonds are data corres-ponding to the arrow in Fig.3a)

Fig.4 Stress-strain hysteresis loops at N_f / 2

Fig.5 Relationship between cyclic strain amplitude and number of reversals to failure 2N_f (Δε_e / 2—elastic strain amplitude, Δε_p / 2—plastic strain amplitude) (a) and relationship between stress amplitude at N_f / 2 (Δσ / 2) and Δε_p / 2 (b)

Fig.6 Inverse pole figures of 9Cr-ODS steel after normalizing & tempering treatment (a) and under Δε_t / 2 = 0.3% (b), Δε_t / 2 = 0.5% (c), and Δε_t / 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 Δε_t / 2 = 0.3% (b), Δε_t / 2 = 0.5% (c), and Δε_t / 2 = 0.8% (d)

Fig.8 Grain average misorientation (GAM) maps of 9Cr-ODS steel after normalizing & tempering treatment (a) and under Δε_t / 2 = 0.3% (b), Δε_t / 2 = 0.5% (c), and Δε_t / 2 = 0.8% (d)

Fig.9 SEM images of macrofractography (a-c), crack source zones (d-f), and crack propagation zones (g-i) under Δε_t / 2 = 0.4% (a, d, g), Δε_t / 2 = 0.5% (b, e, h), and Δε_t / 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 Δε_t / 2 (a); and relationship between Δε_t / 2 and 2N_f of several typical reduced activation ferritic/martensitic (RAFM) steels^{[1,3,6,26,27]} at room temperature (b)

Steel	T	σ $f'$ / E	ε $f'$	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 Δε_t / 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 Δε_t / 2
(a) KAM
(b) geometrically necessary dislocation (GND)
(c) GAM

Fig.13 Fatigue crack initiation and propagation morphology of 9Cr-ODS steel under Δε_t / 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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