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Acta Metall Sin  2018, Vol. 54 Issue (11): 1705-1714    DOI: 10.11900/0412.1961.2018.00361
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Precipitate Evolution in a Modified 25Cr-20Ni Austenitic Heat Resistant Stainless Steel During CreepRupture Test at 750 ℃
Guodong HU1,2, Pei WANG1, Dianzhong LI1(), Yiyi LI1
1 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
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Abstract  

25Cr-20Ni austenitic heat resistant stainless steels are widely used as structural materials in nuclear industries and power plants for their excellent corrosion resistance and creep properties at elevated temperature. It is generally accepted that the precipitation during creep is a key factor influencing the creep properties. However, the evolution of precipitates is complicated due to the interaction of the alloy elements. To investigate the precipitation behaviors, a modified 25Cr-20Ni austenitic heat resistant stainless steel has been crept at 750 ℃ under different stresses varying from 100 MPa to 180 MPa. The microstructure observation indicates that M23C6 and (Nb, V)(C, N) precipitates are formed during 32.6 h creeping deformation under 180 MPa. M23C6 precipitates are mainly generated at grain boundaries and (Nb, V)(C, N) particles are dispersively distributed in austenitic matrix. The grain boundary M23C6 carbides are significantly coarsened and Ostwald ripening process happens during 98.1 h creeping deformation under the stress of 150 MPa and 353.0 h creeping deformation under stress of 120 MPa, while (Nb, V)(C, N) carbonitrides show high dimensional stability. With the creep rupture time further prolonging to 353.0 h and 752.3 h under the creep stress of 120 and 100 MPa, respectively, σ-phases are generated first at grain boundaries and then at inner grains. Meanwhile, large amounts of σ-phases are formed around (Nb, V)(C, N) particles, indicating the σ-phase precipitation is accelerated by (Nb, V)(C, N) carbonitrides. Composition analysis and thermodynamic calculation are subsequently performed to elucidate the precipitation mechanism of σ-phase. Carbon and nitrogen depleted zone is detected at the interface between (Nb, V)(C, N) precipitates and austenitic matrix. A correlation between σ-phase and C/N contents has been calculated by Thermo-Calc, which shows that the mass fraction of σ-phase increases with the decreasing C/N contents. According to the thermodynamic calculations and experimental results, it is reasonably inferred that the formation of σ-phase is induced by the carbon and nitrogen depletion in austenitic matrix. Additionally, the fracture surfaces of creep specimens show intergranular fracture under all creep stresses. When the creep time is comparatively short, cracks are inclined to propagate along grain boundaries owing to the low cohesion between grain boundary M23C6 precipitates and austenitic matrix, resulting in intergranular creep fracture. With the precipitation of σ-phase at grain boundaries after long time creep, the cracks are primarily generated from σ-phase, further deteriorating the creep elongation.

Key words:  austenitic heat resistant stainless steel      creep rupture      precipitate evolution      σ-phase      MX     
Received:  01 August 2018     
ZTFLH:  TG142.1  
Fund: Supported by National Natural Science Foundation of China (No.U1708252)

Cite this article: 

Guodong HU, Pei WANG, Dianzhong LI, Yiyi LI. Precipitate Evolution in a Modified 25Cr-20Ni Austenitic Heat Resistant Stainless Steel During CreepRupture Test at 750 ℃. Acta Metall Sin, 2018, 54(11): 1705-1714.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2018.00361     OR     https://www.ams.org.cn/EN/Y2018/V54/I11/1705

Fig.1  SEM image (a) and EDS (b) of precipitates in solid solution state of the investigated 25Cr-20Ni austenitic heat resistant stainless steel (White arrows in Fig.1a indicate MX precipitates)
Fig.2  SEM images of precipitates in the 25Cr-20Ni austenitic heat resistant stainless steel after crept at 750 ℃ under stresses of 180 MPa (a) and 150 MPa (c); TEM images of grain boundary M23C6 under stresses of 180 MPa (b) and 150 MPa (d) (Inset in Fig.2b shows the SAED pattern of M23C6 and austenitic matrix)
Fig.3  SEM images of precipitates in the 25Cr-20Ni austenitic heat resistant stainless steel after crept at 750 ℃ under stresses of 120 MPa (a) and 100 MPa (c); TEM images of grain boundary M23C6 under stresses of 120 MPa (b) and 100 MPa (d)
Fig.4  TEM images of intragranular precipitates in the 25Cr-20Ni austenitic heat resistant stainless steel after crept at 750 ℃ under stresses of 180 MPa (a), 150 MPa (b), 120 MPa (c) and 100 MPa (d) (Insets in Figs.4a and b show the SAED patterns of M23C6 and austenitic matrix)
Fig.5  Bright field (a) and dark field (b) TEM images of σ-phase formed with MX precipitate in 750 ℃, 100 MPa crept specimen (Inset in Fig.5a shows SAED pattern of σ-phase)
Fig.6  TEM images (a, c) and EDS concentration profiles (b, d) across MX precipitate (a, b) and σ-phase (c, d) in 750 ℃, 100 MPa crept specimen
Fig.7  Thermodynamic calculation of the variation of σ-phase fraction with different carbon and nitrogen contents (a), mass fraction of constitute phases as well as solute carbon and nitrogen content in austenitic matrix at different temperatures (b)
Fig.8  Creep strain-time curves for the 25Cr-20Ni austenitic heat resistant stainless steel at 750 ℃ under different applied stresses
Fig.9  Low (a~c) and high (d~f) magnified intergranular fracture morphologies of the 25Cr-20Ni austenitic heat resistant stainless steels after crept at 750 ℃ under 180 MPa (a, d), 120 MPa (b, e) and 100 MPa (c, f)
Fig.10  Intergranular cracks formed at grain boundaries with M23C6 carbides in 750 ℃, 180 MPa crept specimens (a) and the grain boundaries with σ-phases in 750 ℃, 120 MPa crept specimens (b) (Inset in Fig.10a shows the crack under high magnification)
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