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Mechanical properties and microstructure of 9Cr-ODS-CLF-1 steel
Fusion Engineering and Design 151 (2020) 111406
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Mechanical properties and microstructure of 9Cr-ODS-CLF-1 steel a,
a
a
a
b
Shuang Yang *, Jiming Chen , Haiying Fu , Pinghuai Wang , Zhangjian Zhou , Pengfei Zheng a b
T a
Southwestern Institute of Physics, Chengdu 610041, China University of Science and Technology Beijing, Beijing 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oxide dispersion strengthening 9Cr-ODS-CLF-1 Mechanical properties Creep Microstructure observation
A kind of oxide dispersion strengthened (ODS) steel with addition of 0.3 % Y2O3 and 0.3 % Ti was manufactured via mechanical alloying (MA) process from CLF-1 steel raw powder that was prepared by Ar gas atomization. This 10 kg ingot of 9Cr-ODS-CLF-1 steel was obtained by hot isostatic pressing (HIP) and subsequent forging. The as-HIPed and forged 9Cr-ODS-CLF-1 steel exhibits high Vicker hardness to 384 Hv which is much higher than that of CLF-1 steel (224 Hv). The ultimate tensile strength (UTS) is about 1100 MPa with elongation of 7 % at room temperature (RT). After 90 min tempering at 1013 K, the RT elongation increased to 11 % and the ultimate tensile strength decreased to 670 MPa. The creep results showed that the minimum strain rate for 9Cr-ODS-CLF-1 steel as received and 1013 K tempered state tested at 873 K under 160 MPa were low as 1.6 × 10-8 /s and 8.42 × 10-9 /s, respectively. Typical small dispersion particles were observed in the matrix. The grains showed inhomogeneous size distribution for both the as received and the 1013 K tempering state. The method of mechanical alloying combined with HIP proved to be an effective way to strengthen CLF-1 steel in large scale.
1. Introduction Reduced activation ferritic/martensitic (RAFM) steel is a promising structural material in fusion application due to its good thermal mechanical properties, low irradiation swelling and helium embrittlement, as well as good compatibility with major coolants and breeding materials [1,2]. As a typical reduced activation ferritic/martensitic (RAFM) steel, CLF-1 steel was researched and developed as the structural material for Chinese helium-cooled solid Tritium Breeding Modules (TBM) program and a promising candidate material for DEMO fusion reactors. With more than ten years of research and development in Southwestern Institute of Physics (SWIP), scale of the steel fabrication has been enlarged from kgs to tons by the vacuum induction melting (VIM) and consumable electron re-melting (CER) [3]. However, the upper application temperature for RAFM steel is 823 K which severely limits its application [4–9]. In order to increase its high temperature strength, oxide particles like yttrium oxide were used as dispersion strengthening particles to increase its mechanical strength as well as irradiation resistance [10–13]. In the past decades, 9Cr-ODS steel has been widely researched. In Japan, the study of 9Cr-ODS steel is not based on RAFM steel without the chemical composition of Mn, V and Ta which shall act as an important role in the strengthening of the steel [22]. Differently, our study focuses on the RAFM steel based 9Cr-ODS steel with the same chemical
⁎
composition of CLF-1 steel before adding Y2O3 and Ti.Additionally, in previous study of 9Cr-ODS steel [18–20], only small scale of ingots were fabricated which limits the real commercial application as structural material.Thus in this study, a 10 kg ingot of oxide dispersion strengthened (ODS) steel (9Cr-ODS-CLF-1) was fabricated via mechanical alloying (MA) followed by hot isostatic pressing (HIP)and hot forging. The large size fabrication process of this steel would affect future designing of fusion reactor materials and components.The thermal mechanical properties and microstructure were characterized in order to test the strengthening effect of yttrium oxide particles and Ti element. The grain size inhomogeneity and particles existence in the matrix related to mechanical properties were discussed in order to avoid substandard mass production. 2. Experimental The pre-alloyed argon-gas-atomized powders (200 mesh) made from CLF-1 bar steel were mixed with 0.3 wt %Y2O3 powders (< 30 nm) and 0.3 wt. % Ti (200 mesh). Ti was over added for scavenging of oxygen during the milling process to form titanium oxide such as TiO2. The mixed powders were then mechanically alloyed in a planetary ball milling machine(QM-QX10) with four 2.5 L vessels under super pure argon atmosphere(O < 10 ppm). Such large capacity milling machine and vessels were utilized for high-efficiency large scale
Corresponding author. E-mail address: [email protected] (S. Yang).
https://doi.org/10.1016/j.fusengdes.2019.111406 Received 10 September 2019; Received in revised form 5 November 2019; Accepted 8 November 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 151 (2020) 111406
S. Yang, et al.
fabrication of ODS steel. The vessels and milling balls are all made of stainless steel with similar chemical composition of ODS steel to avoid contamination. The mass ratio of ball-to-material was 5:1. The rotation speed and the milling time were 300 rpm and 30 h, respectively [18–20]. The as-milled powders were degassed and sealed in a stainless steel container under vacuum conditions of 1 × 10−4 Pa, and then sintered by HIP at 1423 K for 3 h under the pressure of 120 MPa. Then the as-HIPed ingot was forged with a forging ratio of 3:1 at 1423 K. Finally, a 10 kg 9Cr-ODS-CLF-1 steel with the designed chemical composition of Fe-8.5Cr-1.5W-0.5Mn-0.3V-0.1Ta (wt.%) with additional 0.3 wt.% Y2O3 and 0.3 wt.% Ti was obtained (as received state). In order to obtain balance between strength and ductility, heat treatment was done for the as-HIPed and forged steel. In this study 1013 K tempering for 90 min was chosen as a heat treatment to recover the ductility of the steel. For XRD analysis, Cu target was used with 40 kV tube voltage and 150 mA current. Mechanical tests of the 9Cr-ODS-CLF-1 steel were executed on the as-received and tempered small specimens (gauge length: 5 mm). Tensile tests were done at room temperature (RT) with strain rate of 6.67 × 10-4 /s. Thermal creep tests were conducted at 873 K and 923 K under various stresses. Vickers hardness measurements were carried out on the polished surface of bulk specimens with a load of 200 g for 30 s. Microstructure characterization before and after thermal-mechanical treatments were performed by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). TEM investigation was carried out on thin foil samples (diameter: 3 mm). The thin foil samples were polished by standard metallography techniques and subsequently subjected to twin-jet electro-polishing using an electrolyte of 10 % perchloric acid in ethanol with 22 V at 253 K.
Fig. 2. 9Cr-ODS steel powder after milling and high temperature annealing.
temperature increased to 1150 ℃, Y2Ti2O7 particles were confirmed in the XRD results. Therefore, the annealing temperature was determined as the sintering temperature in the HIP process regarding the XRD results and previous literature [23]. 3.2. EBSD observation The grain morphologies of 9Cr-ODS-CLF-1 steel were characterized by EBSD as shown in Fig. 3. The grain sizes of both the as received and tempered state were quite scattered. For both states, many coarse grains were detected with the largest grain size up to 18 μm, while the average grain sizes for the as received and tempered state were 2.38 μm and 2.47 μm, respectively. The average grain sizes for both states of 9CrODS-CLF-1 steel were much smaller than CLF-1 steel(around 14 μm). Grain size strengthening contributes much to the strengthening and hardening effect of 9Cr-ODS-CLF-1 steel. However, the grain size inhomogeneity may severely limit the potential mechanical properties of the steel. The reason for such phenomenon could be improper quality control during the HIP and forging process which should be avoided in the future.
3. Results and discussion 3.1. Mechanical alloying Fig. 1 demonstrates the raw powder morphology change by the high energy ball milling. The original spherical powders changed into lamellar structures after 30 h ball milling. The powder deformation process includes plastic deformation, bonding and fragmentation during the milling. Such process is intended for the mechanical alloying of the gas-atomized powder with yttrium oxide and Ti to form ytrrium-titanium oxide which is typical in the oxide dispersion strengthening procedure [21]. Fig. 2 displays the XRD results of 9Cr-ODS steel powder after milling and high temperature annealing. In the high-energy ball milling process, the added Y2O3 particle were dissolved into the matrix. Hence, no Y2O3 or Y-Ti-O particle were detected in the XRD results. After annealing at 950℃, still no oxide particles were found. Until the annealing
3.3. TEM observation The characteristics of strengthening nanoparticles have a major influence on the mechanical property and irradiation resistance of oxide dispersion strengthened (ODS) steels [14,15]. Thus it’s quite essential to confirm the particle existence and evolution during the high temperature sintering and heat treatment process. Ferrite and martensite phase were confirmed in the TEM observation as demonstrated in Fig. 4. Large size particles existed in the steel and were speculated to be
Fig. 1. 9Cr-ODS-CLF-1 steel powder morphology evolution: (a) before milling, (b) after 30 h milling. 2
Fusion Engineering and Design 151 (2020) 111406
S. Yang, et al.
Fig. 3. Grain size difference of 9Cr-ODS-CLF-1 steel by the method of EBSD: (a)9Cr-ODS-CLF-1 steel as received state, (b) 9Cr-ODS-CLF-1 steel 1013 K tempered state.
carbides or titanium oxides(TiO2). Typical small size oxide particles were observed and distributed homogeneously in the steel which contributes to the dispersion strengthening of 9Cr-ODS-CLF-1 steel. The average particle size is around 10 nm. Dispersed small particles which were speculated to be Y2Ti2O7 or Y2O3 shall act as obstacles for dislocation movement resulting in improving the creep property. The dislocations largely exist in the as-received state which contribute to the creep resistance as well.
3.4. Vicker hardness As shown in Fig. 5, Vicker hardness of the as received 9Cr-ODS-CLF1 steel is 384Hv, which is much higher than that of CLF-1 steel (220 Hv) fabricated by the method of VIM and CER. After tempering at 1013 K for 90 min, the hardness of 9Cr-ODS-CLF-1 steel decreased to 267 Hv, which still shows strengthening effect compared to that of CLF-1 steel. Such obvious hardening effect should be attributable to the high concentration of martensitic packets and laths, small dispersed particles as well as the abnormally small grains as illustrated in Figs. 2 and 3. Even though grain size is inhomogenous, the hardness of 9Cr-ODS-CLF-1 steel is still higher than CLF-1 steel due to the average small grain size.
Fig. 5. Vicker hardness comparison for CLF-1 steel and 9Cr-ODS-CLF-1 steel at two different state.
Fig. 4. TEM characterization of 9Cr-ODS-CLF-1 steel for as received state. 3
Fusion Engineering and Design 151 (2020) 111406
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Fig. 7. Creep curves of 9Cr-ODS-CLF-1 1013 K tempered steel tested at 873 K and 923 K under different stress.
Fig. 6. Tensile test results comparison of CLF-1 steel and 9Cr-ODS-CLF-1 steel as received and 1013 K tempered state.
3.5. Tensile strength Fig. 8. Smallest strain rate comparison of 9Cr-ODS-CLF-1 steel, ODS EUROFER 97 and CLF-1 steel [16,17].
The tensile test results are shown in Fig. 6. The as received 9Cr-ODSCLF-1 steel shows obvious strengthening effect compared with CLF-1 steel at room temperature test with ultimate tensile and yield strength around 1080 MPa and 950 MPa, respectively. The total elongation is 7 % which is smaller than that of CLF-1 steel. After tempering at 1013 K, the 9Cr-ODS-CLF-1 steel became soft, while its ultimate tensile and yield strength are still higher than CLF-1 steel and the elongation increases to 11 % which is lower than CLF-1 steel as well. The yield and ultimate tensile strength of the samples followed the same trend as their Vickers microhardness shown in Fig. 5.
97 [17] at 923 K, the smallest strain rate of 9Cr-ODS-CLF-1 steel 1013 K tempered state is one order higher which should be improved in the future. The grain size inhomogeneity of 9Cr-ODS-CLF-1 steel should be attributed to the creep degradation compared with ODS EUROFER 97. It is also concluded that 1013 K tempering increased the ductility of the steel, but it decreased its strength and creep property. 4. Conclusions The method of mechanical alloying combined with HIP proved to be an effective way to strengthen CLF-1 steel in large scale. However, the grain inhomogeneity of the 9Cr-ODS-CLF-1 steel is hazardous to the mechanical properties. 1013 K tempering did increase the ductility of the 9Cr-ODS-CLF-1 steel, while it decreased its tensile strength and hardness resulting in the degradation of creep property. Other proper heat treatment should be proposed in the future. The properties tested above indicate the evidence for its strengthening effect and related conclusions are as follows:
3.6. Creep properties In fusion reactors, structural materials of blanket and first wall stand high temperature and stress for long period of time, thus for fusion structural material, one of the most important mechanical properties is the thermal creep property. The creep property of 9Cr-ODS-CLF-1 steel for as received and 1013 K tempered state were tested at 873 K and 923 K under different stress. The thermal creep curves of 9Cr-ODS-CLF1 steel 1013 k tempered state are shown in Fig. 7. By calculating the smallest strain rate of each specimen at steady state, the creep property was compared in Fig. 8. By comparing with CLF-1 steel tested at 873 K under 160 MPa, 9Cr-ODS-CLF-1 steel for both as received and 1013 K tempered state exhibit superior smaller strain rate, which demonstrates good creep property. The creep results showed that the minimum strain rate for 9Cr-ODS-CLF-1 steel as received and 1013 K tempered state tested at 873 K under 160 MPa were low as 1.6 × 10-8 /s and 8.42 × 10-9 /s, respectively. However, compared with ODS EUROFER
(1) 9Cr-ODS-CLF-1 steel shows higher hardness and tensile strength compared with CLF-1 steel. (2) Grain size inhomogeneity severely affected the strengthening of 9Cr-ODS-CLF-1 steel. (3) Though the smallest strain rate of 9Cr-ODS-CLF-1 1013 K tempered state is relatively low at 873 K compared with CLF-1 steel, its creep property still needs to be upgraded compared with other ODS steels 4
Fusion Engineering and Design 151 (2020) 111406
S. Yang, et al.
References
(ODS EUROFER 97). Declaration of Competing Interest
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
The contents of this manuscript have not been copyrighted or published previously. The contents of this manuscript are not now under consideration for publication elsewhere. The contents of this manuscript will not be copyrighted, submitted, or published elsewhere while acceptance by Fusion Engineering and Design is under consideration. There are no directly related manuscripts or abstracts, published or unpublished, by any authors of this manuscript. No financial support or incentive has been provided for this manuscript. I am one author signing on behalf of all co-authors of this manuscript and attesting to the above.
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Acknowledgments This work was supported by the project of National Key Research and Development Project (No. 2017YFE0301306) and ITER FW project. Special thanks to Prof Yingmin Wang and Dr. Peng Chen for the TEM and EBSD observation help and fruitful discussion about the paper.
5
S.J. Zinkle, K.A. Terrani, L.L. Snead, Curr. Opin. Solid STM. 20 (2016) 401. J. Knaster, A. Moeslang, T. Muroga, Nat. Phys. 12 (2016) 424. P.H. Wang, J.M. Chen, H.Y. Fu, J. Nucl. Mater. 442 (2013) S9. B. Fournier, A. Steckmeyer, A.L. Rouffie, et al., J.Nucl. Mater. 430 (2012) 142. S.K. Karak, M.J. Dutta, Z. Witczak, et al., Mater. Sci. Eng. A 580 (2013) 231. P. Dou, A. Kimura, R. Kasada, et al., J. Nucl. Mater. 444 (2014) 441. J. Brodrick, D. Hepburn, G. Ackland, J. Nucl. Mater. 445 (2014) 291. S. Ukai, M. Fujiwara, J. Nucl. Mater. 307 (2002) 749. M. Nagini, R. Vijay, K.V. Rajulapati, et al., Mater. Sci. Eng. A 708 (2017) 451. H. Dong, L. Yu, Y. Liu, et al., J. Alloys Compd. 702 (2017) 538. S. Yang, P.F. Zheng, P.H. Wang, et al., Fusion Eng. Des. 136 (2018) 442. E. Sánchez-Gonzáles, P. Miranda, J.J. Meléndez-Martínez, et al., J. Eur. Ceram. Soc. 27 (2007) 3345. L. Li, W. Xu, M. Saber, et al., Mater. Sci. Eng. A 636 (2015) 565. A. Steckmeyer, M. Proud, B. Fournier, et al., J. Nucl. Mater. 405 (2010) 95. Z. Li, Z. Lu, R. Xie, et al., Fusion Eng. Des. 121 (2017) 159. Document NO. HCCB TBM-SWIP/CNDA/IO-SOMD-XA(DID-20101) V1.4. G. Yu, N. Nita, N. Baluc, Fusion Eng. Des. 75 (2005) 1039. M. Wang, Z. Zhou, H. Sun, Mater. Sci. Eng. A. 559 (2013) 287–292. G. Zhang, Z. Zhou, M. Wang, et al., Fusion Eng. Des. 89 (2014) 280–283. G. Zhang, Z. Zhou, K. Mo, et al., J. Alloys Compd. 648 (2015) 223–228. L. Song, S. Liu, X. Mao, Fusion Eng. Des. 112 (2016) 460–467. Y. Li, H. Abe, T. Nagasaka, et al., J. Nucl. Mater. 443 (2013) 200–206. S. Ukai, M. Fujiwara, J. Nucl. Mater. 307-311 (2002) 749–757.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Mechanical properties and microstructure of 9Cr-ODS-CLF-1 steel a,
a
a
a
b
Shuang Yang *, Jiming Chen , Haiying Fu , Pinghuai Wang , Zhangjian Zhou , Pengfei Zheng a b
T a
Southwestern Institute of Physics, Chengdu 610041, China University of Science and Technology Beijing, Beijing 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oxide dispersion strengthening 9Cr-ODS-CLF-1 Mechanical properties Creep Microstructure observation
A kind of oxide dispersion strengthened (ODS) steel with addition of 0.3 % Y2O3 and 0.3 % Ti was manufactured via mechanical alloying (MA) process from CLF-1 steel raw powder that was prepared by Ar gas atomization. This 10 kg ingot of 9Cr-ODS-CLF-1 steel was obtained by hot isostatic pressing (HIP) and subsequent forging. The as-HIPed and forged 9Cr-ODS-CLF-1 steel exhibits high Vicker hardness to 384 Hv which is much higher than that of CLF-1 steel (224 Hv). The ultimate tensile strength (UTS) is about 1100 MPa with elongation of 7 % at room temperature (RT). After 90 min tempering at 1013 K, the RT elongation increased to 11 % and the ultimate tensile strength decreased to 670 MPa. The creep results showed that the minimum strain rate for 9Cr-ODS-CLF-1 steel as received and 1013 K tempered state tested at 873 K under 160 MPa were low as 1.6 × 10-8 /s and 8.42 × 10-9 /s, respectively. Typical small dispersion particles were observed in the matrix. The grains showed inhomogeneous size distribution for both the as received and the 1013 K tempering state. The method of mechanical alloying combined with HIP proved to be an effective way to strengthen CLF-1 steel in large scale.
1. Introduction Reduced activation ferritic/martensitic (RAFM) steel is a promising structural material in fusion application due to its good thermal mechanical properties, low irradiation swelling and helium embrittlement, as well as good compatibility with major coolants and breeding materials [1,2]. As a typical reduced activation ferritic/martensitic (RAFM) steel, CLF-1 steel was researched and developed as the structural material for Chinese helium-cooled solid Tritium Breeding Modules (TBM) program and a promising candidate material for DEMO fusion reactors. With more than ten years of research and development in Southwestern Institute of Physics (SWIP), scale of the steel fabrication has been enlarged from kgs to tons by the vacuum induction melting (VIM) and consumable electron re-melting (CER) [3]. However, the upper application temperature for RAFM steel is 823 K which severely limits its application [4–9]. In order to increase its high temperature strength, oxide particles like yttrium oxide were used as dispersion strengthening particles to increase its mechanical strength as well as irradiation resistance [10–13]. In the past decades, 9Cr-ODS steel has been widely researched. In Japan, the study of 9Cr-ODS steel is not based on RAFM steel without the chemical composition of Mn, V and Ta which shall act as an important role in the strengthening of the steel [22]. Differently, our study focuses on the RAFM steel based 9Cr-ODS steel with the same chemical
⁎
composition of CLF-1 steel before adding Y2O3 and Ti.Additionally, in previous study of 9Cr-ODS steel [18–20], only small scale of ingots were fabricated which limits the real commercial application as structural material.Thus in this study, a 10 kg ingot of oxide dispersion strengthened (ODS) steel (9Cr-ODS-CLF-1) was fabricated via mechanical alloying (MA) followed by hot isostatic pressing (HIP)and hot forging. The large size fabrication process of this steel would affect future designing of fusion reactor materials and components.The thermal mechanical properties and microstructure were characterized in order to test the strengthening effect of yttrium oxide particles and Ti element. The grain size inhomogeneity and particles existence in the matrix related to mechanical properties were discussed in order to avoid substandard mass production. 2. Experimental The pre-alloyed argon-gas-atomized powders (200 mesh) made from CLF-1 bar steel were mixed with 0.3 wt %Y2O3 powders (< 30 nm) and 0.3 wt. % Ti (200 mesh). Ti was over added for scavenging of oxygen during the milling process to form titanium oxide such as TiO2. The mixed powders were then mechanically alloyed in a planetary ball milling machine(QM-QX10) with four 2.5 L vessels under super pure argon atmosphere(O < 10 ppm). Such large capacity milling machine and vessels were utilized for high-efficiency large scale
Corresponding author. E-mail address: [email protected] (S. Yang).
https://doi.org/10.1016/j.fusengdes.2019.111406 Received 10 September 2019; Received in revised form 5 November 2019; Accepted 8 November 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 151 (2020) 111406
S. Yang, et al.
fabrication of ODS steel. The vessels and milling balls are all made of stainless steel with similar chemical composition of ODS steel to avoid contamination. The mass ratio of ball-to-material was 5:1. The rotation speed and the milling time were 300 rpm and 30 h, respectively [18–20]. The as-milled powders were degassed and sealed in a stainless steel container under vacuum conditions of 1 × 10−4 Pa, and then sintered by HIP at 1423 K for 3 h under the pressure of 120 MPa. Then the as-HIPed ingot was forged with a forging ratio of 3:1 at 1423 K. Finally, a 10 kg 9Cr-ODS-CLF-1 steel with the designed chemical composition of Fe-8.5Cr-1.5W-0.5Mn-0.3V-0.1Ta (wt.%) with additional 0.3 wt.% Y2O3 and 0.3 wt.% Ti was obtained (as received state). In order to obtain balance between strength and ductility, heat treatment was done for the as-HIPed and forged steel. In this study 1013 K tempering for 90 min was chosen as a heat treatment to recover the ductility of the steel. For XRD analysis, Cu target was used with 40 kV tube voltage and 150 mA current. Mechanical tests of the 9Cr-ODS-CLF-1 steel were executed on the as-received and tempered small specimens (gauge length: 5 mm). Tensile tests were done at room temperature (RT) with strain rate of 6.67 × 10-4 /s. Thermal creep tests were conducted at 873 K and 923 K under various stresses. Vickers hardness measurements were carried out on the polished surface of bulk specimens with a load of 200 g for 30 s. Microstructure characterization before and after thermal-mechanical treatments were performed by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). TEM investigation was carried out on thin foil samples (diameter: 3 mm). The thin foil samples were polished by standard metallography techniques and subsequently subjected to twin-jet electro-polishing using an electrolyte of 10 % perchloric acid in ethanol with 22 V at 253 K.
Fig. 2. 9Cr-ODS steel powder after milling and high temperature annealing.
temperature increased to 1150 ℃, Y2Ti2O7 particles were confirmed in the XRD results. Therefore, the annealing temperature was determined as the sintering temperature in the HIP process regarding the XRD results and previous literature [23]. 3.2. EBSD observation The grain morphologies of 9Cr-ODS-CLF-1 steel were characterized by EBSD as shown in Fig. 3. The grain sizes of both the as received and tempered state were quite scattered. For both states, many coarse grains were detected with the largest grain size up to 18 μm, while the average grain sizes for the as received and tempered state were 2.38 μm and 2.47 μm, respectively. The average grain sizes for both states of 9CrODS-CLF-1 steel were much smaller than CLF-1 steel(around 14 μm). Grain size strengthening contributes much to the strengthening and hardening effect of 9Cr-ODS-CLF-1 steel. However, the grain size inhomogeneity may severely limit the potential mechanical properties of the steel. The reason for such phenomenon could be improper quality control during the HIP and forging process which should be avoided in the future.
3. Results and discussion 3.1. Mechanical alloying Fig. 1 demonstrates the raw powder morphology change by the high energy ball milling. The original spherical powders changed into lamellar structures after 30 h ball milling. The powder deformation process includes plastic deformation, bonding and fragmentation during the milling. Such process is intended for the mechanical alloying of the gas-atomized powder with yttrium oxide and Ti to form ytrrium-titanium oxide which is typical in the oxide dispersion strengthening procedure [21]. Fig. 2 displays the XRD results of 9Cr-ODS steel powder after milling and high temperature annealing. In the high-energy ball milling process, the added Y2O3 particle were dissolved into the matrix. Hence, no Y2O3 or Y-Ti-O particle were detected in the XRD results. After annealing at 950℃, still no oxide particles were found. Until the annealing
3.3. TEM observation The characteristics of strengthening nanoparticles have a major influence on the mechanical property and irradiation resistance of oxide dispersion strengthened (ODS) steels [14,15]. Thus it’s quite essential to confirm the particle existence and evolution during the high temperature sintering and heat treatment process. Ferrite and martensite phase were confirmed in the TEM observation as demonstrated in Fig. 4. Large size particles existed in the steel and were speculated to be
Fig. 1. 9Cr-ODS-CLF-1 steel powder morphology evolution: (a) before milling, (b) after 30 h milling. 2
Fusion Engineering and Design 151 (2020) 111406
S. Yang, et al.
Fig. 3. Grain size difference of 9Cr-ODS-CLF-1 steel by the method of EBSD: (a)9Cr-ODS-CLF-1 steel as received state, (b) 9Cr-ODS-CLF-1 steel 1013 K tempered state.
carbides or titanium oxides(TiO2). Typical small size oxide particles were observed and distributed homogeneously in the steel which contributes to the dispersion strengthening of 9Cr-ODS-CLF-1 steel. The average particle size is around 10 nm. Dispersed small particles which were speculated to be Y2Ti2O7 or Y2O3 shall act as obstacles for dislocation movement resulting in improving the creep property. The dislocations largely exist in the as-received state which contribute to the creep resistance as well.
3.4. Vicker hardness As shown in Fig. 5, Vicker hardness of the as received 9Cr-ODS-CLF1 steel is 384Hv, which is much higher than that of CLF-1 steel (220 Hv) fabricated by the method of VIM and CER. After tempering at 1013 K for 90 min, the hardness of 9Cr-ODS-CLF-1 steel decreased to 267 Hv, which still shows strengthening effect compared to that of CLF-1 steel. Such obvious hardening effect should be attributable to the high concentration of martensitic packets and laths, small dispersed particles as well as the abnormally small grains as illustrated in Figs. 2 and 3. Even though grain size is inhomogenous, the hardness of 9Cr-ODS-CLF-1 steel is still higher than CLF-1 steel due to the average small grain size.
Fig. 5. Vicker hardness comparison for CLF-1 steel and 9Cr-ODS-CLF-1 steel at two different state.
Fig. 4. TEM characterization of 9Cr-ODS-CLF-1 steel for as received state. 3
Fusion Engineering and Design 151 (2020) 111406
S. Yang, et al.
Fig. 7. Creep curves of 9Cr-ODS-CLF-1 1013 K tempered steel tested at 873 K and 923 K under different stress.
Fig. 6. Tensile test results comparison of CLF-1 steel and 9Cr-ODS-CLF-1 steel as received and 1013 K tempered state.
3.5. Tensile strength Fig. 8. Smallest strain rate comparison of 9Cr-ODS-CLF-1 steel, ODS EUROFER 97 and CLF-1 steel [16,17].
The tensile test results are shown in Fig. 6. The as received 9Cr-ODSCLF-1 steel shows obvious strengthening effect compared with CLF-1 steel at room temperature test with ultimate tensile and yield strength around 1080 MPa and 950 MPa, respectively. The total elongation is 7 % which is smaller than that of CLF-1 steel. After tempering at 1013 K, the 9Cr-ODS-CLF-1 steel became soft, while its ultimate tensile and yield strength are still higher than CLF-1 steel and the elongation increases to 11 % which is lower than CLF-1 steel as well. The yield and ultimate tensile strength of the samples followed the same trend as their Vickers microhardness shown in Fig. 5.
97 [17] at 923 K, the smallest strain rate of 9Cr-ODS-CLF-1 steel 1013 K tempered state is one order higher which should be improved in the future. The grain size inhomogeneity of 9Cr-ODS-CLF-1 steel should be attributed to the creep degradation compared with ODS EUROFER 97. It is also concluded that 1013 K tempering increased the ductility of the steel, but it decreased its strength and creep property. 4. Conclusions The method of mechanical alloying combined with HIP proved to be an effective way to strengthen CLF-1 steel in large scale. However, the grain inhomogeneity of the 9Cr-ODS-CLF-1 steel is hazardous to the mechanical properties. 1013 K tempering did increase the ductility of the 9Cr-ODS-CLF-1 steel, while it decreased its tensile strength and hardness resulting in the degradation of creep property. Other proper heat treatment should be proposed in the future. The properties tested above indicate the evidence for its strengthening effect and related conclusions are as follows:
3.6. Creep properties In fusion reactors, structural materials of blanket and first wall stand high temperature and stress for long period of time, thus for fusion structural material, one of the most important mechanical properties is the thermal creep property. The creep property of 9Cr-ODS-CLF-1 steel for as received and 1013 K tempered state were tested at 873 K and 923 K under different stress. The thermal creep curves of 9Cr-ODS-CLF1 steel 1013 k tempered state are shown in Fig. 7. By calculating the smallest strain rate of each specimen at steady state, the creep property was compared in Fig. 8. By comparing with CLF-1 steel tested at 873 K under 160 MPa, 9Cr-ODS-CLF-1 steel for both as received and 1013 K tempered state exhibit superior smaller strain rate, which demonstrates good creep property. The creep results showed that the minimum strain rate for 9Cr-ODS-CLF-1 steel as received and 1013 K tempered state tested at 873 K under 160 MPa were low as 1.6 × 10-8 /s and 8.42 × 10-9 /s, respectively. However, compared with ODS EUROFER
(1) 9Cr-ODS-CLF-1 steel shows higher hardness and tensile strength compared with CLF-1 steel. (2) Grain size inhomogeneity severely affected the strengthening of 9Cr-ODS-CLF-1 steel. (3) Though the smallest strain rate of 9Cr-ODS-CLF-1 1013 K tempered state is relatively low at 873 K compared with CLF-1 steel, its creep property still needs to be upgraded compared with other ODS steels 4
Fusion Engineering and Design 151 (2020) 111406
S. Yang, et al.
References
(ODS EUROFER 97). Declaration of Competing Interest
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
The contents of this manuscript have not been copyrighted or published previously. The contents of this manuscript are not now under consideration for publication elsewhere. The contents of this manuscript will not be copyrighted, submitted, or published elsewhere while acceptance by Fusion Engineering and Design is under consideration. There are no directly related manuscripts or abstracts, published or unpublished, by any authors of this manuscript. No financial support or incentive has been provided for this manuscript. I am one author signing on behalf of all co-authors of this manuscript and attesting to the above.
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Acknowledgments This work was supported by the project of National Key Research and Development Project (No. 2017YFE0301306) and ITER FW project. Special thanks to Prof Yingmin Wang and Dr. Peng Chen for the TEM and EBSD observation help and fruitful discussion about the paper.
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