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Acta Metall Sin  2018, Vol. 54 Issue (11): 1705-1714    DOI: 10.11900/0412.1961.2018.00361
Mechanical Properties Current Issue | Archive | Adv Search |
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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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.

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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)
[1] Zhang Z, Hu Z F, Tu H Y, et al.Microstructure evolution in HR3C austenitic steel during long-term creep at 650 ℃[J]. Mater. Sci. Eng., 2017, A681: 74
[2] Bai X, Pan J, Chen G, et al.Effect of high temperature aging on microstructure and mechanical properties of HR3C heat resistant steel[J]. Mater. Sci. Technol., 2014, 30: 205
[3] Cao T S, Fang X D, Cheng C Q, et al.Creep behavior of two kinds of HR3C heat resistant steels based on stress relaxation tests[J]. Acta Metall. Sin., 2014, 50: 1343(曹铁山, 方旭东, 程从前等. 应力松弛方法研究2种HR3C耐热钢的高温蠕变行为[J]. 金属学报, 2014, 50: 1343)
[4] Fang Y Y, Zhao J, Li X N.Precipitates in HR3C steel aged at high temperature[J]. Acta Metall. Sin., 2010, 46: 844(方园园, 赵杰, 李晓娜. HR3C钢高温时效过程中的析出相[J]. 金属学报, 2010, 46: 844)
[5] Yang Y H, Zhu K H, Wang Q J, et al.Microstructural evolution and the effect on hardness and plasticity of S31042 heat-resistant steel during creep[J]. Mater. Sci. Eng., 2014, A608: 164
[6] Sourmail T.Precipitation in creep resistant austenitic stainless steels[J]. Mater. Sci. Technol., 2001, 17: 1
[7] Peng B C, Zhang H X, Hong J, et al.Effect of aging on the impact toughness of 25Cr-20Ni-Nb-N steel[J]. Mater. Sci. Eng., 2010, A527: 1957
[8] Tavares S S M, Moura V, da Costa V C, et al. Microstructural changes and corrosion resistance of AISI 310S steel exposed to 600-800 ℃[J]. Mater. Charact., 2009, 60: 573
[9] Padilha A F, Rios P R.Decomposition of austenite in austenitic stainless steels[J]. ISIJ Int., 2002, 42: 325
[10] K?llqvist J, Andrén H O.Microanalysis of a stabilised austenitic stainless steel after long term ageing[J]. Mater. Sci. Eng., 1999, A270: 27
[11] Park D B, Huh M Y, Jung W S, et al.Effect of vanadium addition on the creep resistance of 18Cr9Ni3CuNbN austenitic stainless heat resistant steel[J]. J. Alloys Compd., 2013, 574: 532
[12] Erneman J, Schwind M, Andrén H O, et al.The evolution of primary and secondary niobium carbonitrides in AISI 347 stainless steel during manufacturing and long-term ageing[J]. Acta Mater., 2006, 54: 67
[13] Ni Z F, Sun Y S, Xue F, et al.Study on fabrication, microstructure and properties of in situ TiC particle on dispersion-strengthened 304 stainless steel[J]. Acta Metall. Sin., 2010, 46: 935(倪自飞, 孙扬善, 薛烽等. 原位TiC颗粒弥散强化304不锈钢的制备及组织性能研究[J]. 金属学报, 2010, 46: 935)
[14] Chastell D J, Flewitt P E J. The formation of the σ-phase during long term high temperature creep of type 316 austenitic stainless steel[J]. Mater. Sci. Eng., 1979, A38: 153
[15] Anburaj J, Nazirudeen S S M, Narayanan R, et al. Ageing of forged superaustenitic stainless steel: Precipitate phases and mechanical properties[J]. Mater. Sci. Eng., 2012, A535: 99
[16] Di Gianfrancesco A.Materials for Ultra-Supercritical and Advanced Ultra-Supercritical Power Plants [M]. Duxford: Woodhead Publishing, 2017: 376
[17] Hsieh C C, Wu W T.Overview of intermetallic sigma (σ) phase precipitation in stainless steels[J]. ISRN Metall., 2012, 2012: 732471
[18] Schwind M, K?llqvist J, Nilsson J O, et al.σ-phase precipitation in stabilized austenitic stainless steels[J]. Acta Mater., 2000, 48: 2473
[19] Garcés G R, Le Coze J, Garin J L, et al.σ-phase precipitation in two heat-resistant steels-influence of carbides and microstructure[J]. Scr. Mater., 2004, 50: 651
[20] Barcik J.Mechanism of σ-phase precipitation in Cr-Ni austenitic steels[J]. Mater. Sci. Technol., 1988, 4: 5
[21] Lee T H, Oh C S, Lee C G, et al.Precipitation of σ-phase in high-nitrogen austenitic 18Cr-18Mn-2Mo-0.9N stainless steel during isothermal aging[J]. Scr. Mater., 2004, 50: 1325
[22] Minami Y, Kimura H, Ihara Y.Microstructural changes in austenitic stainless steels during long-term aging[J]. Mater. Sci. Technol., 1986, 2: 795
[23] West D, Hulance J, Higginson R L, et al.σ-phase precipitation in 347HFG stainless steel[J]. Mater. Sci. Technol., 2013, 29: 835
[24] Perron A, Toffolon-Masclet C, Ledoux X, et al.Understanding sigma-phase precipitation in a stabilized austenitic stainless steel (316Nb) through complementary CALPHAD-based and experimental investigations[J]. Acta Mater., 2014, 79: 16
[25] Kaneko K, Fukunaga T, Yamada K, et al.Formation of M23C6-type precipitates and chromium-depleted zones in austenite stainless steel[J]. Scr. Mater., 2011, 65: 509
[26] Liu Q D, Chu Y L, Peng J C, et al.3D atom probe characterazation of alloy carbides in tempering martenite III. Coarsening[J] Acta Metall. Sin., 2009, 45: 1297(刘庆冬, 褚于良, 彭剑超等. 回火马氏体中合金碳化物的3D原子探针表征 Ⅲ. 粗化[J]. 金属学报, 2009, 45: 1297)
[27] Shinohara K, Seo T, Kumada K.Recrystallization and sigma phase formation as concurrent and interacting phenomena in 25%Cr-20%Ni steel[J]. Trans. Jpn. Inst. Met., 1979, 20: 713
[28] Wang Y Q, Yang B, Li N, et al.Embrittlement of σ phase in stainless steel for primary coolant pipes of nuclear power plant[J]. Acta Metall. Sin., 2015, 52: 17(王永强, 杨滨, 李娜等. σ相在核电一回路主管道不锈钢中的脆化机理[J]. 金属学报, 2015, 52: 17)
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