Effect of Si on Microstructure and Mechanical Properties of a 9Cr Ferritic/Martensitic Steel
ZHANG Qiankun1,2, HU Xiaofeng1(), JIANG Haichang1, RONG Lijian1
1.CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2.School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
Cite this article:
ZHANG Qiankun, HU Xiaofeng, JIANG Haichang, RONG Lijian. Effect of Si on Microstructure and Mechanical Properties of a 9Cr Ferritic/Martensitic Steel. Acta Metall Sin, 2024, 60(9): 1200-1212.
Due to the worsening of environmental pollution and energy shortage, nuclear energy has become an increasingly important energy source, offering clean, high-energy, safe, and stable power to meet the needs of modern life and production. P91 steel is an excellent heat-resistant steel, widely used in nuclear power plants for its good mechanical properties, high thermal conductivity, and low irradiation swelling rate. However, with the emergence of lead and lead-bismuth eutectic (LBE) cooled fast reactors (LFRs), P91 steel's inferior compatibility with LBE has limited its use in LFR construction. To address this issue, 9Cr ferritic/martensitic steel with high Si content (H-Si steel) was designed in this study to develop a compatible structural material for LFR heat exchange tubes. In addition, the effects of Si on the microstructure and mechanical properties of as-cast, homogenized, tempered, and aged H-Si steel were investigated using various techniques, including OM, FESEM, TEM, EBSD, tensile, and impact tests. The results show that the increase in Si content leads to 3.9% δ ferrite (volume fraction) in as-cast H-Si steel, which can be eliminated completely by homogenization heat treatment. Tempering produces a Si-enriched layer around M23C6 due to the high Si content in the H-Si steel, which can suppress the rapid growth of M23C6 and maintain a smaller size of M23C6 than that of M23C6 in P91 steel. Meanwhile, the Si-enriched layer can slow down the growth of M23C6 and promote the nucleation and growth of the Laves phase during aging at 550°C, which induces smaller M23C6 and larger Laves phase in aged H-Si steel than that in aged P91. Due to the higher Si content and smaller M23C6, the tempered H-Si steel exhibits significantly higher strength than P91. After aging, the strength of H-Si steel further increases because of the precipitation of the Laves phase. Both tempered alloy steels have similar impact energy (about 210 J) and the fracture mode is ductile fracture. However, the precipitation of the Laves phase after aging decreases the impact energy of both alloy steels. The aged H-Si steel had a larger and more Laves phase, resulting in a partial cleavage area in the impact fracture surface and inducing a lower impact energy compared to H-Si steel.
Fund: LingChuang Research Project of China National Nuclear Corporation, Youth Innovation Promotion Association CAS(Y2021059);Strategic Priority Research Program of the Chinese Academy of Sciences(XDA28040200)
Corresponding Authors:
HU Xiaofeng, professor, Tel: (024)23971981, E-mail: xfhu@imr.ac.cn
Table 1 Tensile properties and impact energies at room temperature for P91 and H-Si alloy steels at tempered and aged states
Fig.1 OM images of as-cast P91 (a) and H-Si (b) steels and homogenized H-Si steel (c)
Fig.2 OM images of tempered P91 (a) and H-Si (b) steels
Fig.3 SEM images of tempered P91 (a, c) and H-Si (b, d) steels at lower (a, b) and higher (c, d) magnifications, and the EDS analyses (e, f) of particles A (e) and B (f) in tempered P91 steel as shown in Fig.3c
Fig.4 TEM images of tempered P91 (a) and H-Si (b) steels (Insets in Fig.4b show the selected area electron diffraction (SAED) patterns of particles C and D )
Fig.5 SEM images of aged P91 (a, c) and H-Si (b, d) steels at lower (a, b) and higher (c, d) magnifications
Fig.6 TEM image of aged P91 (a) and H-Si (b) steels and the elements mapping of the area denoted by the rectangle in Fig.6b (c) (Inset in Fig.6b is the SAED pattern of particle E)
Fig.7 EBSD phase distribution images of aged P91 (a) and H-Si (b) steels
Fig.8 Changes of phase transition temperature of P91 and H-Si alloy steels
Precipitate
Tempered state
Aged state
P91
H-Si
P91
H-Si
M23C6
106 ± 35
82 ± 32
135 ± 38
98 ± 38
NbC
515 ± 12
512 ± 14
512 ± 14
514 ± 21
Laves phase
-
-
79 ± 32
129 ± 47
Table 2 Precipitate sizes of P91 and H-Si alloy steels at tempered and aged states
Fig.9 TEM image of M23C6 precipitated at boundaries of tempered H-Si steel (a), elements mapping image of the area denoted by the rectangle in Fig.9a (b), and line scanning image of M23C6 along the line in Fig.9b (c)
Fig.10 TEM image of M23C6precipitated at boundaries of tempered P91 steel (a) and elements mapp-ing image of the area denoted by the rectangle in Fig.10a (b)
Fig.11 SEM images of impact fracture surfaces of aged P91 (a) and H-Si (b) steels at room temperature (Inset in Fig.11b is the magnified image of the rectangle area, and dotted line shows the cleavage zone)
Fig.12 Cross section SEM images of aged P91 (a) and H-Si (b) steels near the impact fracture surfaces (Inset in Fig.12b is the EBSD phase distribution image of the impact fracture surface denoted by rectangle)
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