Effect of δ-Ferrite on Hot Deformation and Recrystallization of 316KD Austenitic Stainless Steel for Sodium-Cooled Fast Reactor Application
CHEN Shenghu1, WANG Qiyu1,2, JIANG Haichang1, RONG Lijian1()
1CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
Cite this article:
CHEN Shenghu, WANG Qiyu, JIANG Haichang, RONG Lijian. Effect of δ-Ferrite on Hot Deformation and Recrystallization of 316KD Austenitic Stainless Steel for Sodium-Cooled Fast Reactor Application. Acta Metall Sin, 2024, 60(3): 367-376.
The sodium-cooled fast reactor is the most mature reactor among generation-IV nuclear reactors. A carbon/nitrogen-controlled 316KD austenitic stainless steel has been developed for the construction of pressure vessels and internals in Chinese CFR600 demonstration reactor. During their industrial production, δ-ferrite is present in large-scale billets because of the combined effect of non-equilibrium segregation and low cooling rate. For large-scale billets containing δ-ferrite, inhomogeneous grain-size distributions are observed in the product after hot working. Extensive studies on the recrystallization of the austenite phase in austenitic stainless steels during hot deformation were conducted. However, the effect of δ-ferrite on the recrystallization behavior of the austenite phase remains unclear. In this study, uniaxial hot compression tests of 316KD austenitic stainless steels involving as-cast and homogenized conditions were conducted at 1423 K and 0.1 s-1 using a Gleeble-3800 thermal-mechanical simulator, and the effect of δ-ferrite on hot deformation and recrystallization was analyzed by SEM, EBSD, and TEM. Results showed that δ-ferrite could be nearly eliminated through δ-ferrite→austenite transformation after homogenization at 1473 K for 14 h, whereas austenite grain showed evident growth. The elimination of δ-ferrite was a Cr-diffusion-controlled process through kinetic analysis. Plastic deformation occurred preferentially in δ-ferrite and at the δ-ferrite/austenite interface, and subsequently in austenite during hot deformation. The flow stress of as-cast samples was much lower than that of homogenized samples at the same strain because of the presence of soft δ-ferrite. Dynamic recovery occurred easier in δ-ferrite, and the resulting dynamic softening remarkably reduced flow stress with an increase in strain. Discontinuous dynamic recrystallization characterized by original austenite grain boundary bulging was the dominant mechanism in homogenized samples. However, the presence of δ-ferrite promoted the occurrence of continuous dynamic recrystallization in austenite near the δ-ferrite/austenite interface in as-cast samples. Compared with the homogenized samples, a higher degree of recrystallization was observed in as-cast samples because of the combined effects of continuous dynamic recrystallization and discontinuous dynamic recrystallization.
Fund: National Natural Science Foundation of China(51871218);Youth Innovation Promotion Association CAS(2018227);CNNC Science Fund for Talented Young Scholars
Corresponding Authors:
RONG Lijian, professor, Tel: (024)23971979, E-mail: ljrong@imr.ac.cn
Fig.1 Changes in the area fraction of δ-ferrite across the thickness (a) and OM image of as-cast samples at the center position (b) of 180 mm thick 316KD austenitic stainless steel billet (Inset in Fig.1b shows the SEM image of precipitate in δ-ferrite)
Fig.2 Area fraction curves of δ-ferrite as a function of homogenization temperature and time of as-cast samples at the center position of 180 mm thick slab
Fig.3 OM (a, c, d) and SEM (b) images of as-cast samples at the center position of 180 mm thick slab after homogenization treatment at 1473 K for 2 h (a, b), 4 h (c), and 14 h (d) (The black arrows indicate the δ-ferrite)
Fig.4 lg{ln[1 / (1 - X(t))]} as a function of lg{lnt} (a) and lnk as a function of 1 / T (b) (T—homogenization temperature, X—area fraction of δ-ferrite, t—time, k—reaction rate constant)
T / K
n
k
r
1323
1.173
8.77 × 10-6
0.993
1423
1.079
4.82 × 10-5
0.984
1473
1.035
8.76 × 10-5
0.995
Table 1 Time exponent (n) and rate constant (k) values in Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation obtained from Fig.4a during isothermal transformation of δ-ferrite
Fig.5 True stress-true strain curves of as-cast and homogenized samples after deformation at 1423 K and 0.1 s-1
Fig.6 Grain boundary maps (a, c, e), corresponding local misorientation maps (b, d), and inverse pole figure (IPF) (f) of as-cast samples with the strains of 0.22 (a, b), 0.36 (c, d), and 0.92 (e, f) (In Figs.6a, c, and e, the black and green lines indicate the high angle grain boundaries (misorientation angle > 15°) and low angle grain boundaries (misorientation angle 2°-15°), respectively; pink color represents the δ-ferrite; the same in figures below. CD—compression direction)
Fig.7 Grain boundary maps (a, b) and IPF (c) of homogenized samples with the strains of 0.36 (a, c) and 0.92 (b)
Fig.8 Grain boundary map of as-cast samples with the strain of 0.36 (The black arrows indicate the newly formed high angle grain boundaries)
Fig.9 TEM bright field images (a, b) and corresponding elemental mapping (c) from the selected rectangular area indicated in Fig.9a of as-cast samples with the strain of 0.36 (Fig.9b shows the high magnification image at the lower left corner of the selected rectangular area in Fig.9a)
Fig.10 Schematics about the continuous dynamic recrystallization processes in austenite near the δ-ferrite/austenite interface (a) dislocation pile-up at interface (b) formation of dislocation walls/cells (c) formation of recrystallized grain
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