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Effects of prior solution treatment on thermal aging behavior of duplex stainless steels
Journal of Nuclear Materials 441 (2013) 337–342
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effects of prior solution treatment on thermal aging behavior of duplex stainless steels Shilei Li a, Yanli Wang a, Hailong Zhang a, Shuxiao Li a, Genqi Wang b, Xitao Wang a,⇑ a b
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China Yantai Taihai Marnoir Nuclear Equipment Co. Ltd., Yantai 264003, China
a r t i c l e
i n f o
Article history: Received 12 March 2013 Accepted 12 June 2013 Available online 21 June 2013
a b s t r a c t The influence of solution temperature on thermal aging behavior was studied in duplex stainless steels. With increasing solution temperature, the ferrite contents remarkably increase, Cr and Ni elements redistribute. During thermal aging, the impact properties of higher solution temperature treated materials suffer a serious degradation, which is not only related with ferrite content but also the alloy compositions in ferrite. Enrichment of Ni in ferrite can accelerate the spinodal decomposition kinetics. Thermal aging-inducing strain fields in ferrite cause the embrittlement of DSS. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Duplex stainless steels (DSS), containing the dual phases of ferrite and austenite, are widely used in many industrial fields, such as nuclear, oil and chemical industries. It has long been recognized that the microstructures and mechanical properties of DSS change a lot after many years of service at moderate temperature because of thermal aging embrittlement in ferrite [1–3]. This embrittlement is characterized by an increase in hardness and a loss of impact toughness associated with a shift of the ductile to brittle transition temperature [4–6]. It has been an agreement that spinodal decomposition in ferrite caused this thermal aging embrittlement [7–9]. Thermal aging behavior of DSS is closely related to the ferrite content and the chemical compositions in ferrite. For a particular grade of steel, these factors are controlled by the fabrication process, including the solidification and the following heat treatment, especially by the solution treatment process. Shiao et al. [10] have studied the effects of solution temperature on the ferrite content and the degradation of mechanical properties in the aging process. Lemoine et al. [11] have discussed the influence of the quenching rate on the spinodal decomposition in a duplex stainless steel. Elements redistribution in ferrite during the solution treatment may also affects the thermal aging behavior, which is still not very clear. In the present work, the DSS materials, solution treated at the temperatures range from 980 °C to 1380 °C and further thermal aged for up to 3000 h at 400 °C, are investigated, in order to evaluate the effects of prior solution treatment on the thermal aging behavior. The microstructural evolution, element redistribution, ⇑ Corresponding author. Tel.: +86 10 82375280; fax: +86 10 62333447. E-mail address: [email protected] (X. Wang). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.06.017
micromechanical properties in ferrite and impact fracture behavior of DSS after solution treatment and thermal aging were systematically studied. Also, the embrittlement in ferrite was characterized by the transmission electron microscopy and geometric phase analysis. 2. Experimental The material being studied had a composition in wt.% of 20.45Cr, 10.2Ni, 1.15Si, 1.02Mn, 0.2Mo, 0.031C, 0.026P, 0.0032S, and balance Fe. After centrifugal casting, the materials were solution treated at different temperatures: 980, 1080, 1180, 1280 and 1380 °C, for 8 h followed by water quenched. The solution treated DSS were further thermal aged at 400 °C for as long as 3000 h. In particular, the 1080 °C solution treated material was thermal aged at 400 °C for 20,000 h. The heat treatment process is shown in Fig. 1. Microstructural evolutions during solution treatment were observed by a scanning electron microscope (SEM, Zeiss Supra 55, Germany). The alloy compositions of ferrite in DSS after different temperature solution treated were detected by an electron probe microanalyzer (EPMA, JEOL JXA 8100, Japan). Micromechanical properties of ferrite after thermal aging were studied by a nanoindenter (Nano Indenter DCM, MTS, USA) with an indentation depth of 500 nm. An instrumented Charpy V-notch impact tester (NI 500, NCS, China) was used to evaluate the impact behavior after thermal aged for 3000 h. The impact fracture surfaces were examined under the previously mentioned SEM. Microstructures in both the unaged and long-term aged DSS were observed by a field emission high-resolution transmission electron microscope (HRTEM, FEI Tecnai F20-ST, Netherlands) operated at 200 kV. The HRTEM specimens were prepared by a focused ion beam-scanning electron microscope system (FIB-SEM, Zeiss Auriga, German).
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A metallographic method was used to measure the ferrite contents, and the phase fraction in this steel was also calculated by JMatPro (version 6.1), as shown in Fig. 3a. When solution temperature rises from 980 to 1380 °C, ferrite content increases from 9% to 43.5%. Fig. 3b shows the change of element distribution in the solution process. With the increase of solution temperature, Cr and Ni redistribute, leading to decrease of Cr and increase of Ni in ferrite. Comparing with Cr and Ni, Mn, Si and Mo contents seem to have no obvious change in the solution process. After solution treated at 1380 °C for 8 h, the compositions in the austenite and ferrite phases tend to be close. After long-term thermal aging at 400 °C, no changes in ferrite content and element distribution are found, as the low diffusion coefficient at this aging temperature. Vitek [12] also reports this phenomenon in DSS after thermal aging at 475 °C for up to 10,000 h. Fig. 1. The heat treatment of solution + thermal aging process.
3.2. Impact fracture behavior 3. Results and discussion 3.1. Microstructural evolution and element distribution Fig. 2 shows the SEM observation of the microstructures after solution treated. With increasing solution temperature, the ferrite contents remarkably increase, and the ferrite phases become larger and connected.
Fig. 4a shows the impact behavior for the materials after solution treated at different temperatures for 8 h and thermal aged at 400 °C for 3000 h. For the unaged materials, impact properties increase with increasing solution temperature. The 1380 °C solution treated DSS, with almost the same ferrite and austenite fraction, has the best impact property. Thermal aging changes the effects of solution temperature on the impact properties. For the thermal
Fig. 2. Microstructures of materials after solution treated at different temperatures for 8 h. (a) 980 °C, (b) 1080 °C, (c) 1180 °C, (d) 1280 °C and (e) 1380 °C.
Fig. 3. Variation of ferrite content (a) and element distribution and (b) with solution temperature.
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Fig. 4. Impact properties of materials after solution treatment and further thermal aging. (a) Variation of impact energy with aging time. (b) Force–displacement curves of the materials thermal aged for 3000 h.
Fig. 5. Impact fracture morphologies of materials solution treated at different temperatures for 8 h and thermal aged at 400 °C for 3000 h. (a) 980 °C, (b) 1080 °C, (c) 1180 °C, (d) 1280 °C and (e) 1380 °C.
Fig. 6. Longitudinal section morphologies near the fractures of materials solution treated at 980 °C for 8 h and thermal aged at 400 °C for 3000 h. (a and b) were taken from different angles. Additional understanding of this observation is available in the inserted illustration.
aged materials, the higher solution temperature rises, the worse of aging resistance becomes. The force–displacement curves of the materials thermal aged for 3000 h change a lot with increasing solution temperature, shown in Fig. 4b. The area enclosed by the curve and displacement axis is the total impact energy. The impact energy of 1380 °C solution treated material has a dramatically degradation in the thermal aging process. Thus, the unaged DSS materials with the best mechanical properties do not mean good performance when they are thermal aged for a long-term period.
Fig. 5 shows the impact fracture morphologies of materials solution treated and further thermal aged. After solution treated at 980 and 1080 °C, many ductile features in the thermal aged specimens can be observed, corresponding to the lowest degree of deterioration in impact energies. When solution treated above 1180 °C, the impact energies of thermal aged DSS decline and the proportion of brittle cleavages increases with increasing solution temperature. The DSS material, solution treated at 1380 °C and thermal aged for 3000 h, showing a totally brittle fracture as shown Fig. 5e.
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Fig. 7. Longitudinal section morphologies near the fractures of materials solution treated at 1280 °C for 8 h and thermal aged at 400 °C for 3000 h.
400 °C for 3000 h, the average content of ferrite in the impact fracture is approximate the same as that in the cross-section. The deformation bands in ferrite in Fig. 6b indicates that ferrite has obvious plastic deformation in the impact process. Cracks are not preferred to propagate in ferrite or austenite for the non-serious thermal aging embrittled DSS. With the increase of solution temperature, the embrittled degree of thermal aged DSS become more and more serious. For the material solution treated at 1280 °C for 8 h and thermal aged at 400 °C for 3000 h, the average content of ferrite in the impact fracture is obviously more than that in the cross-section, as shown in Fig. 7. No deformation bands but secondary cracks are found in ferrite, this indicates that ferrite is difficult to plastically deform in the impact process. Some ferrite phases nearby the notch will first fracture, and the principal crack inclines to extend throughout the fractured ferrite phases. Thus, there are more ferrite on the impact fracture than that of the cross-section. The ferrite phases remarkably increase and become the matrix of the material after solution treated at 1380 °C for 8 h. After a further thermal aging at 400 °C for 3000 h, the impact energy reduces to only 9 J, and the wavy profiles of fracture become flat, as shown in Fig. 8. Under impact loading, the prior fractured big-size ferrite phases generate high enough stress concentration, which causes the localised tearing of austenite. That is why both ferrite and austenite on the fracture surfaces show brittle features. 3.3. Micromechanical properties of ferrite
Fig. 8. Longitudinal section morphologies near the fractures of materials solution treated at 1380 °C for 8 h and thermal aged at 400 °C for 3000 h.
Longitudinal section morphologies near the fractures, cut along the vertical direction of notch from the fractured impact specimens, are investigated in SEM. The connected slim ferrite phases present a three-dimensional network, as shown in Fig. 6a. For the material solution treated at 980 °C for 8 h and thermal aged at
It is well known that, thermal aging behavior of DSS is closely related to the ferrite content. Timofeev and Nikolaev [13] point out that the higher the ferrite content, the faster the impact properties of DSS degrade. In addition, Yamada et al. [2] consider that the saturated impact energy increases linearly with the decrease of ferrite content. In the present study, the raise of solution temperature obviously increases the ferrite content. But besides that, the properties of ferrite could have a major impact on the aggravation of embrittlement. A nanoindenter was carried out to investigate the influence of solution temperature on the micromechanical properties in ferrite. Fig. 9 shows the nanoindentation curves and nanohardness in ferrite of DSS after solution treated at different temperatures for 8 h and further thermal aged at 400 °C for 3000 h. With the indentation depth limited to 500 nm, the maximum force of the indentation increases with increasing solution temperature, as shown in Fig. 9a. This indicates that raise the solution temperature can enhance the deformation resistance of the ferrite phases. Fig. 9b shows the nanohardness in ferrite of the thermal aged DSS has a significant increase with increasing solution
Fig. 9. Nanoindentation curves (a) and nanohardness and (b) in ferrite of materials after solution treated at different temperatures for 8 h and thermal aged at 400 °C for 3000 h.
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Fig. 10. (a) HRTEM image of ferrite along [0 0 1] direction in DSS solution treated at 1380 °C for 8 h and thermal aged at 400 °C for 3000 h. (b) Corresponding fast Fourier transform (FFT) pattern of (a). (c) Corresponding inverse FFT image by masking (1 1 0) reflection, with the highlighted ovals showing the dislocations along [0 0 1]. (d–f) 0) lattice fringes. Geometric phase images of (1 1 0) and (1 1
Fig. 11. (a) HRTEM image of ferrite along [0 0 1] direction in DSS solution treated at 1080 °C for 8 h and unaged. (b) Corresponding geometric phase image of (a). (c) HRTEM image of ferrite along [0 0 1] direction in DSS solution treated at 1080 °C for 8 h and thermal aged at 400 °C for 20,000 h. (d) Corresponding geometric phase image of (c).
temperature, from 4.0 GPa of 980 °C to 7.4 GPa of 1380 °C. Significant hardening in ferrite will cause brittle fracture under the impact loading.
Fig. 3b shows that, With increasing solution temperature, the Cr content reduces form 27.8 at.% to 22.1 at.%, and the Ni content increases by up to 73.7%, from 4.3 at.% to 7.4 at.%. The reduction of Cr
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content will slow down the rate of spinodal decomposition in ferrite. But this may be overshadowed by the more intense influence induced by the Ni enrichment in ferrite. By studying a series of Fe–Cr–Ni alloys with different Ni contents, Brown and Smith [14] point out that the final amplitude of the spinodal fluctuations decrease with increasing Ni content although the rate of evolution of the spinodal increase with increasing Ni content. Miller and Russell [15] report the same effect of Ni on the thermal aging kinetics. Chung and Leax [16] also consider that kinetics of spinodal are influenced strongly by ferrite chemical composition, notably Ni, Mo, Si and C content. So, both the ferrite content and the decomposition dynamics in ferrite caused by solution treatment affect the followed thermal aging behavior. 3.4. Characterization of embrittlement in ferrite A TEM was carried out to characterize the embrittlement in ferrite. Fig. 10a shows the HRTEM image of ferrite along [0 0 1] direction in DSS solution treated at 1380 °C for 8 h and thermal aged at 400 °C for 3000 h. The corresponding fast Fourier transform (FFT) pattern in Fig. 10b shows only one set of diffraction spots. The Cr-rich and Fe-rich domains produced by spinodal decomposition in the thermal aging process have the same lattice structure and the very close lattice parameter with the ferrite matrix. The corresponding inverse FFT image by masking (1 1 0) reflection is shown in Fig. 10c, and the dislocations along [0 0 1] are marked by the highlighted ovals. These dislocations are considered to be brought in by the ferrite decomposition, which leads to the severe strain 0) lattice fringes fields. Geometric phase images of (1 1 0) and (1 1 are used to characterized the strain fields, as shown in Fig. 10d–f. To evaluate the influence of solution temperature on the embrittlement in ferrite, another DSS treated under a low solution temperature but much longer aging time was observed by HRTEM along the same direction. Fig. 11a shows the HRTEM image of ferrite in DSS solution treated at 1080 °C for 8 h and thermal aged at 400 °C for 20,000 h. In spite of aging for a much extension of time, there have relatively slight strain fields in ferrite (Fig. 11b). The unaged DSS was also observed by HRTEM along the same [0 0 1] direction (Fig. 11c). Fig. 11d shows no strong strain fields in ferrite of unaged DSS. This proves that thermal aging-inducing strain fields cause the hardening and reduction of deformability of the ferrite phases. Solution treatment is an important process in the production of duplex stainless steels. It is generally used to re-dissolve the harmful precipitates and eliminate the macro-segregation. In addition, the content, shape and distribution of the ferrite phases, as well as the alloy compositions in ferrite and austenite, can be adjusted by altering the solution temperature and time. The decomposition kinetics in ferrite is closely related to the chemical compositions, especially the Ni content, which could be changed in the solution treatment process. Although higher temperature solution can improve the mechanical properties at room temperature by
increasing the ferrite content, it could lead to a severe embrittlement when DSS components working for a long time at the moderate temperatures (280–500 °C). 4. Conclusions The effects of prior solution treatment on thermal aging behavior of duplex stainless steels were studied and the following conclusions can be drawn: (1) With increasing solution temperature, ferrite content remarkably increases, leading to the reduction of Cr content and the increase of Ni content in ferrite. (2) The higher solution temperature raises, the more seriously of the impact properties degenerate after long-term thermal aging. (3) Both the ferrite content and elements redistribution in ferrite caused by solution treatment affect the thermal aging behavior. (4) Raising the Ni content in ferrite can accelerate the rate of spinodal decomposition in ferrite during thermal aging. (5) Thermal aging-inducing strain fields cause the hardening and reduction of deformability of the ferrite phases, as well as the embrittlement.
Acknowledgements This work was financially supported by the National High-Tech Research and Development Program of China (863 Program) through Grant Nos. 2012AA050901 and 2012AA03A507 and the National Science and Technology Major Project of China through Grant No. 2011ZX06004. References [1] S.L. Li, Y.L. Wang, H.L. Zhang, S.X. Li, K. Zheng, F. Xue, X.T. Wang, J. Nucl. Mater. 433 (2013) 41–49. [2] T. Yamada, S. Okano, H. Kuwano, J. Nucl. Mater. 350 (2006) 47–55. [3] H.M. Chung, Int. J. Pres. Ves. Pip. 50 (1992) 179–213. [4] K. Chandra, R. Singhal, V. Kain, V.S. Raja, Mater. Sci. Eng.: A 527 (2010) 3904– 3912. [5] V. Calonne, A.F. Gourgues, A. Pineau, Fatigue Fract. Eng. M 27 (2004) 31–43. [6] J.B. Vogt, K. Massol, J. Foct, Int. J. Fatigue 24 (2002) 627–633. [7] S.L. Li, H.L. Zhang, Y.L. Wang, S.X. Li, K. Zheng, F. Xue, X.T. Wang, Mater. Sci. Eng.: A 564 (2013) 85–91. [8] C. Pareige, S. Novy, S. Saillet, P. Pareige, J. Nucl. Mater. 411 (2011) 90–96. [9] S. Kawaguchi, N. Sakamoto, G. Takano, F. Matsuda, Y. Kikuchi, L. Mráz, Nucl. Eng. Des. 174 (1997) 273–285. [10] J.J. Shiao, C.H. Tsai, J.J. Kai, J.H. Huang, J. Nucl. Mater. 217 (1994) 269–278. [11] C. Lemoine, A. Fnidiki, J. Teillet, M. Hédin, F. Danoix, Scripta Mater. 39 (1998) 61–66. [12] J. Vitek, S. David, D. Alexander, J. Keiser, R. Nanstad, Acta Metall. Mater. 39 (1991) 503–516. [13] B.T. Timofeev, Y.K. Nikolaev, Pres. Ves. Pip. 76 (1999) 849–856. [14] J.E. Brown, G.D.W. Smith, Surf. Sci. 246 (1991) 285–291. [15] M.K. Miller, K.F. Russell, Appl. Surf. Sci. 94–95 (1996) 398–402. [16] H. Chung, T. Leax, Mater. Sci. Technol. 6 (1990) 285–292.
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effects of prior solution treatment on thermal aging behavior of duplex stainless steels Shilei Li a, Yanli Wang a, Hailong Zhang a, Shuxiao Li a, Genqi Wang b, Xitao Wang a,⇑ a b
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China Yantai Taihai Marnoir Nuclear Equipment Co. Ltd., Yantai 264003, China
a r t i c l e
i n f o
Article history: Received 12 March 2013 Accepted 12 June 2013 Available online 21 June 2013
a b s t r a c t The influence of solution temperature on thermal aging behavior was studied in duplex stainless steels. With increasing solution temperature, the ferrite contents remarkably increase, Cr and Ni elements redistribute. During thermal aging, the impact properties of higher solution temperature treated materials suffer a serious degradation, which is not only related with ferrite content but also the alloy compositions in ferrite. Enrichment of Ni in ferrite can accelerate the spinodal decomposition kinetics. Thermal aging-inducing strain fields in ferrite cause the embrittlement of DSS. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Duplex stainless steels (DSS), containing the dual phases of ferrite and austenite, are widely used in many industrial fields, such as nuclear, oil and chemical industries. It has long been recognized that the microstructures and mechanical properties of DSS change a lot after many years of service at moderate temperature because of thermal aging embrittlement in ferrite [1–3]. This embrittlement is characterized by an increase in hardness and a loss of impact toughness associated with a shift of the ductile to brittle transition temperature [4–6]. It has been an agreement that spinodal decomposition in ferrite caused this thermal aging embrittlement [7–9]. Thermal aging behavior of DSS is closely related to the ferrite content and the chemical compositions in ferrite. For a particular grade of steel, these factors are controlled by the fabrication process, including the solidification and the following heat treatment, especially by the solution treatment process. Shiao et al. [10] have studied the effects of solution temperature on the ferrite content and the degradation of mechanical properties in the aging process. Lemoine et al. [11] have discussed the influence of the quenching rate on the spinodal decomposition in a duplex stainless steel. Elements redistribution in ferrite during the solution treatment may also affects the thermal aging behavior, which is still not very clear. In the present work, the DSS materials, solution treated at the temperatures range from 980 °C to 1380 °C and further thermal aged for up to 3000 h at 400 °C, are investigated, in order to evaluate the effects of prior solution treatment on the thermal aging behavior. The microstructural evolution, element redistribution, ⇑ Corresponding author. Tel.: +86 10 82375280; fax: +86 10 62333447. E-mail address: [email protected] (X. Wang). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.06.017
micromechanical properties in ferrite and impact fracture behavior of DSS after solution treatment and thermal aging were systematically studied. Also, the embrittlement in ferrite was characterized by the transmission electron microscopy and geometric phase analysis. 2. Experimental The material being studied had a composition in wt.% of 20.45Cr, 10.2Ni, 1.15Si, 1.02Mn, 0.2Mo, 0.031C, 0.026P, 0.0032S, and balance Fe. After centrifugal casting, the materials were solution treated at different temperatures: 980, 1080, 1180, 1280 and 1380 °C, for 8 h followed by water quenched. The solution treated DSS were further thermal aged at 400 °C for as long as 3000 h. In particular, the 1080 °C solution treated material was thermal aged at 400 °C for 20,000 h. The heat treatment process is shown in Fig. 1. Microstructural evolutions during solution treatment were observed by a scanning electron microscope (SEM, Zeiss Supra 55, Germany). The alloy compositions of ferrite in DSS after different temperature solution treated were detected by an electron probe microanalyzer (EPMA, JEOL JXA 8100, Japan). Micromechanical properties of ferrite after thermal aging were studied by a nanoindenter (Nano Indenter DCM, MTS, USA) with an indentation depth of 500 nm. An instrumented Charpy V-notch impact tester (NI 500, NCS, China) was used to evaluate the impact behavior after thermal aged for 3000 h. The impact fracture surfaces were examined under the previously mentioned SEM. Microstructures in both the unaged and long-term aged DSS were observed by a field emission high-resolution transmission electron microscope (HRTEM, FEI Tecnai F20-ST, Netherlands) operated at 200 kV. The HRTEM specimens were prepared by a focused ion beam-scanning electron microscope system (FIB-SEM, Zeiss Auriga, German).
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A metallographic method was used to measure the ferrite contents, and the phase fraction in this steel was also calculated by JMatPro (version 6.1), as shown in Fig. 3a. When solution temperature rises from 980 to 1380 °C, ferrite content increases from 9% to 43.5%. Fig. 3b shows the change of element distribution in the solution process. With the increase of solution temperature, Cr and Ni redistribute, leading to decrease of Cr and increase of Ni in ferrite. Comparing with Cr and Ni, Mn, Si and Mo contents seem to have no obvious change in the solution process. After solution treated at 1380 °C for 8 h, the compositions in the austenite and ferrite phases tend to be close. After long-term thermal aging at 400 °C, no changes in ferrite content and element distribution are found, as the low diffusion coefficient at this aging temperature. Vitek [12] also reports this phenomenon in DSS after thermal aging at 475 °C for up to 10,000 h. Fig. 1. The heat treatment of solution + thermal aging process.
3.2. Impact fracture behavior 3. Results and discussion 3.1. Microstructural evolution and element distribution Fig. 2 shows the SEM observation of the microstructures after solution treated. With increasing solution temperature, the ferrite contents remarkably increase, and the ferrite phases become larger and connected.
Fig. 4a shows the impact behavior for the materials after solution treated at different temperatures for 8 h and thermal aged at 400 °C for 3000 h. For the unaged materials, impact properties increase with increasing solution temperature. The 1380 °C solution treated DSS, with almost the same ferrite and austenite fraction, has the best impact property. Thermal aging changes the effects of solution temperature on the impact properties. For the thermal
Fig. 2. Microstructures of materials after solution treated at different temperatures for 8 h. (a) 980 °C, (b) 1080 °C, (c) 1180 °C, (d) 1280 °C and (e) 1380 °C.
Fig. 3. Variation of ferrite content (a) and element distribution and (b) with solution temperature.
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Fig. 4. Impact properties of materials after solution treatment and further thermal aging. (a) Variation of impact energy with aging time. (b) Force–displacement curves of the materials thermal aged for 3000 h.
Fig. 5. Impact fracture morphologies of materials solution treated at different temperatures for 8 h and thermal aged at 400 °C for 3000 h. (a) 980 °C, (b) 1080 °C, (c) 1180 °C, (d) 1280 °C and (e) 1380 °C.
Fig. 6. Longitudinal section morphologies near the fractures of materials solution treated at 980 °C for 8 h and thermal aged at 400 °C for 3000 h. (a and b) were taken from different angles. Additional understanding of this observation is available in the inserted illustration.
aged materials, the higher solution temperature rises, the worse of aging resistance becomes. The force–displacement curves of the materials thermal aged for 3000 h change a lot with increasing solution temperature, shown in Fig. 4b. The area enclosed by the curve and displacement axis is the total impact energy. The impact energy of 1380 °C solution treated material has a dramatically degradation in the thermal aging process. Thus, the unaged DSS materials with the best mechanical properties do not mean good performance when they are thermal aged for a long-term period.
Fig. 5 shows the impact fracture morphologies of materials solution treated and further thermal aged. After solution treated at 980 and 1080 °C, many ductile features in the thermal aged specimens can be observed, corresponding to the lowest degree of deterioration in impact energies. When solution treated above 1180 °C, the impact energies of thermal aged DSS decline and the proportion of brittle cleavages increases with increasing solution temperature. The DSS material, solution treated at 1380 °C and thermal aged for 3000 h, showing a totally brittle fracture as shown Fig. 5e.
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Fig. 7. Longitudinal section morphologies near the fractures of materials solution treated at 1280 °C for 8 h and thermal aged at 400 °C for 3000 h.
400 °C for 3000 h, the average content of ferrite in the impact fracture is approximate the same as that in the cross-section. The deformation bands in ferrite in Fig. 6b indicates that ferrite has obvious plastic deformation in the impact process. Cracks are not preferred to propagate in ferrite or austenite for the non-serious thermal aging embrittled DSS. With the increase of solution temperature, the embrittled degree of thermal aged DSS become more and more serious. For the material solution treated at 1280 °C for 8 h and thermal aged at 400 °C for 3000 h, the average content of ferrite in the impact fracture is obviously more than that in the cross-section, as shown in Fig. 7. No deformation bands but secondary cracks are found in ferrite, this indicates that ferrite is difficult to plastically deform in the impact process. Some ferrite phases nearby the notch will first fracture, and the principal crack inclines to extend throughout the fractured ferrite phases. Thus, there are more ferrite on the impact fracture than that of the cross-section. The ferrite phases remarkably increase and become the matrix of the material after solution treated at 1380 °C for 8 h. After a further thermal aging at 400 °C for 3000 h, the impact energy reduces to only 9 J, and the wavy profiles of fracture become flat, as shown in Fig. 8. Under impact loading, the prior fractured big-size ferrite phases generate high enough stress concentration, which causes the localised tearing of austenite. That is why both ferrite and austenite on the fracture surfaces show brittle features. 3.3. Micromechanical properties of ferrite
Fig. 8. Longitudinal section morphologies near the fractures of materials solution treated at 1380 °C for 8 h and thermal aged at 400 °C for 3000 h.
Longitudinal section morphologies near the fractures, cut along the vertical direction of notch from the fractured impact specimens, are investigated in SEM. The connected slim ferrite phases present a three-dimensional network, as shown in Fig. 6a. For the material solution treated at 980 °C for 8 h and thermal aged at
It is well known that, thermal aging behavior of DSS is closely related to the ferrite content. Timofeev and Nikolaev [13] point out that the higher the ferrite content, the faster the impact properties of DSS degrade. In addition, Yamada et al. [2] consider that the saturated impact energy increases linearly with the decrease of ferrite content. In the present study, the raise of solution temperature obviously increases the ferrite content. But besides that, the properties of ferrite could have a major impact on the aggravation of embrittlement. A nanoindenter was carried out to investigate the influence of solution temperature on the micromechanical properties in ferrite. Fig. 9 shows the nanoindentation curves and nanohardness in ferrite of DSS after solution treated at different temperatures for 8 h and further thermal aged at 400 °C for 3000 h. With the indentation depth limited to 500 nm, the maximum force of the indentation increases with increasing solution temperature, as shown in Fig. 9a. This indicates that raise the solution temperature can enhance the deformation resistance of the ferrite phases. Fig. 9b shows the nanohardness in ferrite of the thermal aged DSS has a significant increase with increasing solution
Fig. 9. Nanoindentation curves (a) and nanohardness and (b) in ferrite of materials after solution treated at different temperatures for 8 h and thermal aged at 400 °C for 3000 h.
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Fig. 10. (a) HRTEM image of ferrite along [0 0 1] direction in DSS solution treated at 1380 °C for 8 h and thermal aged at 400 °C for 3000 h. (b) Corresponding fast Fourier transform (FFT) pattern of (a). (c) Corresponding inverse FFT image by masking (1 1 0) reflection, with the highlighted ovals showing the dislocations along [0 0 1]. (d–f) 0) lattice fringes. Geometric phase images of (1 1 0) and (1 1
Fig. 11. (a) HRTEM image of ferrite along [0 0 1] direction in DSS solution treated at 1080 °C for 8 h and unaged. (b) Corresponding geometric phase image of (a). (c) HRTEM image of ferrite along [0 0 1] direction in DSS solution treated at 1080 °C for 8 h and thermal aged at 400 °C for 20,000 h. (d) Corresponding geometric phase image of (c).
temperature, from 4.0 GPa of 980 °C to 7.4 GPa of 1380 °C. Significant hardening in ferrite will cause brittle fracture under the impact loading.
Fig. 3b shows that, With increasing solution temperature, the Cr content reduces form 27.8 at.% to 22.1 at.%, and the Ni content increases by up to 73.7%, from 4.3 at.% to 7.4 at.%. The reduction of Cr
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content will slow down the rate of spinodal decomposition in ferrite. But this may be overshadowed by the more intense influence induced by the Ni enrichment in ferrite. By studying a series of Fe–Cr–Ni alloys with different Ni contents, Brown and Smith [14] point out that the final amplitude of the spinodal fluctuations decrease with increasing Ni content although the rate of evolution of the spinodal increase with increasing Ni content. Miller and Russell [15] report the same effect of Ni on the thermal aging kinetics. Chung and Leax [16] also consider that kinetics of spinodal are influenced strongly by ferrite chemical composition, notably Ni, Mo, Si and C content. So, both the ferrite content and the decomposition dynamics in ferrite caused by solution treatment affect the followed thermal aging behavior. 3.4. Characterization of embrittlement in ferrite A TEM was carried out to characterize the embrittlement in ferrite. Fig. 10a shows the HRTEM image of ferrite along [0 0 1] direction in DSS solution treated at 1380 °C for 8 h and thermal aged at 400 °C for 3000 h. The corresponding fast Fourier transform (FFT) pattern in Fig. 10b shows only one set of diffraction spots. The Cr-rich and Fe-rich domains produced by spinodal decomposition in the thermal aging process have the same lattice structure and the very close lattice parameter with the ferrite matrix. The corresponding inverse FFT image by masking (1 1 0) reflection is shown in Fig. 10c, and the dislocations along [0 0 1] are marked by the highlighted ovals. These dislocations are considered to be brought in by the ferrite decomposition, which leads to the severe strain 0) lattice fringes fields. Geometric phase images of (1 1 0) and (1 1 are used to characterized the strain fields, as shown in Fig. 10d–f. To evaluate the influence of solution temperature on the embrittlement in ferrite, another DSS treated under a low solution temperature but much longer aging time was observed by HRTEM along the same direction. Fig. 11a shows the HRTEM image of ferrite in DSS solution treated at 1080 °C for 8 h and thermal aged at 400 °C for 20,000 h. In spite of aging for a much extension of time, there have relatively slight strain fields in ferrite (Fig. 11b). The unaged DSS was also observed by HRTEM along the same [0 0 1] direction (Fig. 11c). Fig. 11d shows no strong strain fields in ferrite of unaged DSS. This proves that thermal aging-inducing strain fields cause the hardening and reduction of deformability of the ferrite phases. Solution treatment is an important process in the production of duplex stainless steels. It is generally used to re-dissolve the harmful precipitates and eliminate the macro-segregation. In addition, the content, shape and distribution of the ferrite phases, as well as the alloy compositions in ferrite and austenite, can be adjusted by altering the solution temperature and time. The decomposition kinetics in ferrite is closely related to the chemical compositions, especially the Ni content, which could be changed in the solution treatment process. Although higher temperature solution can improve the mechanical properties at room temperature by
increasing the ferrite content, it could lead to a severe embrittlement when DSS components working for a long time at the moderate temperatures (280–500 °C). 4. Conclusions The effects of prior solution treatment on thermal aging behavior of duplex stainless steels were studied and the following conclusions can be drawn: (1) With increasing solution temperature, ferrite content remarkably increases, leading to the reduction of Cr content and the increase of Ni content in ferrite. (2) The higher solution temperature raises, the more seriously of the impact properties degenerate after long-term thermal aging. (3) Both the ferrite content and elements redistribution in ferrite caused by solution treatment affect the thermal aging behavior. (4) Raising the Ni content in ferrite can accelerate the rate of spinodal decomposition in ferrite during thermal aging. (5) Thermal aging-inducing strain fields cause the hardening and reduction of deformability of the ferrite phases, as well as the embrittlement.
Acknowledgements This work was financially supported by the National High-Tech Research and Development Program of China (863 Program) through Grant Nos. 2012AA050901 and 2012AA03A507 and the National Science and Technology Major Project of China through Grant No. 2011ZX06004. References [1] S.L. Li, Y.L. Wang, H.L. Zhang, S.X. Li, K. Zheng, F. Xue, X.T. Wang, J. Nucl. Mater. 433 (2013) 41–49. [2] T. Yamada, S. Okano, H. Kuwano, J. Nucl. Mater. 350 (2006) 47–55. [3] H.M. Chung, Int. J. Pres. Ves. Pip. 50 (1992) 179–213. [4] K. Chandra, R. Singhal, V. Kain, V.S. Raja, Mater. Sci. Eng.: A 527 (2010) 3904– 3912. [5] V. Calonne, A.F. Gourgues, A. Pineau, Fatigue Fract. Eng. M 27 (2004) 31–43. [6] J.B. Vogt, K. Massol, J. Foct, Int. J. Fatigue 24 (2002) 627–633. [7] S.L. Li, H.L. Zhang, Y.L. Wang, S.X. Li, K. Zheng, F. Xue, X.T. Wang, Mater. Sci. Eng.: A 564 (2013) 85–91. [8] C. Pareige, S. Novy, S. Saillet, P. Pareige, J. Nucl. Mater. 411 (2011) 90–96. [9] S. Kawaguchi, N. Sakamoto, G. Takano, F. Matsuda, Y. Kikuchi, L. Mráz, Nucl. Eng. Des. 174 (1997) 273–285. [10] J.J. Shiao, C.H. Tsai, J.J. Kai, J.H. Huang, J. Nucl. Mater. 217 (1994) 269–278. [11] C. Lemoine, A. Fnidiki, J. Teillet, M. Hédin, F. Danoix, Scripta Mater. 39 (1998) 61–66. [12] J. Vitek, S. David, D. Alexander, J. Keiser, R. Nanstad, Acta Metall. Mater. 39 (1991) 503–516. [13] B.T. Timofeev, Y.K. Nikolaev, Pres. Ves. Pip. 76 (1999) 849–856. [14] J.E. Brown, G.D.W. Smith, Surf. Sci. 246 (1991) 285–291. [15] M.K. Miller, K.F. Russell, Appl. Surf. Sci. 94–95 (1996) 398–402. [16] H. Chung, T. Leax, Mater. Sci. Technol. 6 (1990) 285–292.