- Email: [email protected]
High-strength of modified Ausform 9CrODS steels
Journal of Nuclear Materials 417 (2011) 154–157
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
High-strength of modified Ausform 9CrODS steels Shigeharu Ukai a,⇑, Masahiro Yamamoto a, Nobuyoshi Chikada a, Shigenari Hayashi a, Takeji Kaito b, Satoshi Ohtsuka b, Tsukasa Azuma c, Satoshi Ohsaki c a
Hokkaido University, Faculty of Engineering, Materials Science and Engineering, N-13, W-8, Kita-ku, Sapporo 060-8628, Japan Japan Atomic Energy Agency, 4002 Narita, O-arai, Ibaraki 311-1393, Japan c Japan Steel Works Ltd., 4 Chazu, Muroran 051-8505, Japan b
a r t i c l e
i n f o
Article history: Available online 24 December 2010
a b s t r a c t Hot-rolling at the austenitic c phase (1000 °C) led to a formation of ultra-fine grains and a high dislocation density within grain interiors by an Ausform process. Those heavily accumulated strains accelerated coarsening of prior austenite grains, and block grains transformed from the coarsened prior austenite grains were also coarser in the subsequent normalizing heat treatment. This process is called as modified Ausform process. The modified Ausform 9CrODS steels were improved in the ultimate tensile strength at 700 °C due to the formation of uniformly coarser block grains. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The 9Cr oxide dispersion strengthened (ODS) steels have been developing to apply to the fuel cladding materials of advanced sodium-cooled fast reactors and to the structural materials of the advanced fusion reactor blanket systems [1–3]. In the previous study [4], the effect of grain size on the high-temperature strength of ODS steels was investigated. Fig. 1 shows grain size dependence of Vickers hardness, where the hardness measurement was conducted using 500 gf load under a vacuum of 3 10 5 torr at room temperature (RT), 500 and 700 °C. As raising annealing temperature for 13CrODS ferritic steels, the size of ferrite grains increases from nano-crystals formed by mechanical alloying to ultra-fine grains less than 1 lm and coarser grains up to 5 lm through the primary and secondarily recrystallization. Decreasing grain size induces an increase of hardness at RT, which is so called Hall–Petch relationship. However, it is worth noting that at higher temperature (700 °C) ultra-fine grains induce lower hardness. Similar results are reported by Rao et al. [5], where 0.2% flow stress decreases with decreasing grain size of austenitic steel at higher temperature above 600 °C. They pointed out that this strength degradation is dominated by enhanced grain boundary sliding in the finer grains. In case of 9CrODS steel with finer grains, it is clearly shown by Sakasegawa et al. [6] that the tensile and creep deformation at high-temperature are dominated by grain boundary sliding. Based on their studies and our previous one, the grain size should be coarsened to improve the higher-temperature strength. In this study, coarsening of the grain size in a 9CrODS steel was investi-
⇑ Corresponding author. Tel./fax: +81 11 706 6355. E-mail address: [email protected] (S. Ukai). 0022-3115/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.054
gated using a thermo-mechanical processing, in order to furthermore improve its high-temperature mechanical properties. 2. Experimental procedure Pure elemental powders of iron (99.5 mass%, 45–100 lm), carbon (99.7 mass%, 5 lm), chromium (99.9 mass%, under 250 lm), tungsten (99.9 mass%, 4.5–7.5 lm), and titanium (99.7 mass%, under 150 lm) were mechanically alloyed (MAed) together with Y2O3 powder (99.9 mass%, 20 nm) for 48 h in an argon gas atmosphere using a planetary-type ball mill (Fritsch P-6). The target composition of the 9CrODS is 9Cr-0.2C-2W-0.2Ti-0.35Y2O3 (mass%) with full martensitic structure. The thermo-mechanical processing conducted for the MAed powders is illustrated in Fig. 2. The hot-rolling (HRing) was introduced in 80% reduction ratio at the austenitic c phase (1000 °C) for the consolidated powders by hot-pressing (HPing). This thermo-mechanical processing is so called Ausform process for the martensitic steels [7]. Normalizing (N) and tempering (T) heat-treatments were carried out at 1050 and 800 °C 1 h, respectively. For the manufactured specimens, hardness measurement at room temperature and tensile test by a strain rate of 10 3/ s at 700 °C were conducted, and microstructure was evaluated by means of EBSD (Electron Back Scatter Diffraction) analysis using FE-SEM (Field Emission type-Scanning Electron Microscope). 3. Results and discussion The result of Vickers hardness at room temperature after each processing is shown in Fig. 3. The as-HR condition gives higher hardness as high as 660 Hv, compared with the as-HP condition. Decrease of hardness by N and T heat treatments is slightly enhanced in the HRed specimens, compared to the HPed specimens.
155
S. Ukai et al. / Journal of Nuclear Materials 417 (2011) 154–157
1000 800
Hardness Hv
600
RT
500
400 200
500 ºC
120 100
700 ºC
80
700
60 40 0.5
1.0
2.0
3.0
4.0
5.0
Grain size µm
5 µm
Fig. 1. Grain size vs. Vickers hardness in 13CrODS ferritic steels. Grain size was controlled by the primary and secondary recrystallization.
Fig. 2. Schematic illustration of thermo-mechanical processing in 9CrODS steels.
700
Vickers hardness, HV
600 500 400 300 200 100 0
as HP
as HR
HP=>NT
HR=>T
HR=>NT
Fig. 3. Vickers hardness at room temperature. HP: hot-pressing, HR: hot-rolling, N: normalizing, and T: tempering.
Fig. 4 shows the microstructure of as-HRed specimen by SEM. Pores can be slightly observed at the grain boundaries, however, the density of the HRed specimen was confirmed to be almost 100% of the theoretical density. Fine grains with sub-micron size are distributed among elongated grains along the HRed direction. These fine grains possibly contribute to the hardness increase of
Fig. 4. SEM micrograph of as-hot-rolled specimen.
660 Hv at room temperature for the as-HRed specimen. The inverse pole figures by EBSD analyses after N and T heat treatments are presented in Fig. 5. The HPed-NT specimen contains a large number of sub-micron sized small grains, whilst a number of those small grains decrease in the HRed-T specimen and almost disappear in the HRed-NT specimen. The values of Vickers hardness are also indicated in Fig. 5. The sub-micron sized small grains could be attributed to the increase of room temperature hardness. Based on the previous study exhibited in Fig. 1, it is qualitatively expected that the yield stress can be improved in the HRed-NT specimen that contains a limited number of sub-micron sized small grains. Fig. 6 represents stress–strain curves at 700 °C for the specimens, which were HRed and followed by T and NT heat-treatments. Those curves are typical for ODS ferritic steels. The HRedT specimen has a lower ultimate tensile stress (UTS) of 286 MPa and a uniform elongation of 16%, although the rupture elongation is longer around 30%. On the other hand, the HRed-NT specimen induces a higher UTS of 365 MPa and an adequate uniform elongation of 11%. As a reference, the UTS line of 9CrODS steel with full martensite manufactured by extrusion at 1150 °C in JAEA is also shown; the UTS is 308 MPa and the uniform elongation is 4%. It is important to note that the UTS is significantly improved by N heat-treatment other than T heat-treatment after HR. HR is more effective than hot-extrusion for increasing UTS without losing ductility. Fig. 7 shows a distribution of misorientation angle obtained from the EBSD analyses on the HRed-NT specimen (Fig. 5). Typical peaks can be seen at around 30°, 55° and 60°. The upper figure in Fig. 7 also shows the typical misorientation angle for various kinds of martensitic steels; martensite transformation gives crystalline misorientation between adjacent grains to be around 30°, 55° and 60° with K–S relationship [7]. As results of EBSD analyses, it is confirmed that each grain of the HRed-NT specimen (Fig. 5) was formed by martensite transformation. Prior austenite grains (PAGs) cannot be directly identified by EBSD analysis and SEM, because PAGs readily transform to martensite at room temperature. Nakashima’s group pointed out that austenite grains in a few micron meter size in ODS steels should transform to one block grain without variant selection [8]. Only tempering does not induce a change of grain size and morphology. Based on the above information, we can consider that each grain of the HRed specimen shown in Fig. 5 should correspond to each block grain transformed from each PAG. Fig. 8 schematically illustrates the structure change during the thermo-mechanical processing. The HR processing formed ultra-fine grains and introduce a high density of dislocations within grain interiors. Those heavily
156
S. Ukai et al. / Journal of Nuclear Materials 417 (2011) 154–157
Fig. 5. Inverse pole figures analysed by EBSD measurement.
400
(b) 350
(c)
Stress (MPa)
300
(a)
250 200 150 100 50 0
0
10
20
30
40
Strain (%) Fig. 6. Stress–strain curves of specimens at 700 °C: (a) hot-rolled and T, (b) hotrolled NT and (c) extruded bar of 9CrODS steel with full martensite at 1150 °C by JAEA.
0.20
4. Summary Based on the results of previous study that ultra-fine grains degraded high-temperature strength of 9CrODS steels, thermomechanical processing for grain coarsening was investigated. Hot-rolling at the austenitic c phase (1000 °C) led to a formation of ultra-fine grains and a high dislocation density within grain interiors by an Ausform process. Those heavily accumulated strains accelerated coarsening of prior austenite grains, and block grains
0.15
Relative frequency
accumulated strains induced by the Ausform process accelerated coarsening of prior austenite grains by the subsequent N heat treatment. The block grains transformed from the prior austenite grains are also coarser during cooling in the N heat treatment. We define this process as modified Ausform process, and it was confirmed that coarser block grains produced by the modified Ausform process lead to the improved high-temperature strength as shown in Fig. 6.
0.10
0.05
0.00 10
20
30
40
50
60
70
80
90
Misorientation angle (deg.) Fig. 7. Misorientation angle distribution measured by EBSD in the hot-rolled, subsequently normalized and tempered specimen.
S. Ukai et al. / Journal of Nuclear Materials 417 (2011) 154–157
157
hot pressing Hot rolling
Normalizing
N Tempering
T
Mechanical alloying
Ultra-fine grains and high dislocation density by Ausform process
Coarser prior austenite grains
Coarser block grains transformed from prior austenite grains
Fig. 8. Schematic illustration of the structure change in the process of modified Ausform.
transformed from the coarsened prior austenite grains were also coarser in the subsequent normalizing heat treatment. This process was called as modified Ausform process, and it was revealed that the formation of uniformly coarser block grains improved the ultimate tensile strength of 9CrODS steels at 700 °C. References [1] S. Ukai, S. Ohtsuka, T. Kaito, H. Sakasegawa, N. Chikata, S. Hayashi, Mater. Sci. Eng. A 510–511 (2009) 115–120.
[2] S. Ohtsuka, S. Ukai, H. Sakasegawa, M. Fujiwara, T. Kaito, T. Narita, J. Nucl. Mater. 367–370 (2007) 160–165. [3] S. Ukai, S. Ohtsuka, Energy Mater. 2 (1) (2007) 26–35. [4] N. Chikata, S. Hayashi, S. Ukai, S. Ohtsuka, T. Kaito, in: Annual Meeting on Jpn. Inst. Metals, Tokyo Institute of Technology, Japan, 2008. [5] V.K. Rao, D.M.R. Taplin, P.R. Rao, Metall. Trans. A 6A (1975) 77–86. [6] H. Sakasegawa, S. Ukai, M. Tamura, S. Ohtsuka, H. Tanigawa, H. Ogiwara, A. Kohyama, M. Fujiwara, J. Nucl. Mater. 373 (2007) 82–89. [7] T. Maki, I. Tamura, in: Proceeding of International Conference on Physical Metallurgy of Thermomechanical Processing of Steels and other Metals, ISIJ, 1988. p. 458. [8] H. Nakashima, Tetsu-to-Hagane 90 (2) (2004) 73–78.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
High-strength of modified Ausform 9CrODS steels Shigeharu Ukai a,⇑, Masahiro Yamamoto a, Nobuyoshi Chikada a, Shigenari Hayashi a, Takeji Kaito b, Satoshi Ohtsuka b, Tsukasa Azuma c, Satoshi Ohsaki c a
Hokkaido University, Faculty of Engineering, Materials Science and Engineering, N-13, W-8, Kita-ku, Sapporo 060-8628, Japan Japan Atomic Energy Agency, 4002 Narita, O-arai, Ibaraki 311-1393, Japan c Japan Steel Works Ltd., 4 Chazu, Muroran 051-8505, Japan b
a r t i c l e
i n f o
Article history: Available online 24 December 2010
a b s t r a c t Hot-rolling at the austenitic c phase (1000 °C) led to a formation of ultra-fine grains and a high dislocation density within grain interiors by an Ausform process. Those heavily accumulated strains accelerated coarsening of prior austenite grains, and block grains transformed from the coarsened prior austenite grains were also coarser in the subsequent normalizing heat treatment. This process is called as modified Ausform process. The modified Ausform 9CrODS steels were improved in the ultimate tensile strength at 700 °C due to the formation of uniformly coarser block grains. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The 9Cr oxide dispersion strengthened (ODS) steels have been developing to apply to the fuel cladding materials of advanced sodium-cooled fast reactors and to the structural materials of the advanced fusion reactor blanket systems [1–3]. In the previous study [4], the effect of grain size on the high-temperature strength of ODS steels was investigated. Fig. 1 shows grain size dependence of Vickers hardness, where the hardness measurement was conducted using 500 gf load under a vacuum of 3 10 5 torr at room temperature (RT), 500 and 700 °C. As raising annealing temperature for 13CrODS ferritic steels, the size of ferrite grains increases from nano-crystals formed by mechanical alloying to ultra-fine grains less than 1 lm and coarser grains up to 5 lm through the primary and secondarily recrystallization. Decreasing grain size induces an increase of hardness at RT, which is so called Hall–Petch relationship. However, it is worth noting that at higher temperature (700 °C) ultra-fine grains induce lower hardness. Similar results are reported by Rao et al. [5], where 0.2% flow stress decreases with decreasing grain size of austenitic steel at higher temperature above 600 °C. They pointed out that this strength degradation is dominated by enhanced grain boundary sliding in the finer grains. In case of 9CrODS steel with finer grains, it is clearly shown by Sakasegawa et al. [6] that the tensile and creep deformation at high-temperature are dominated by grain boundary sliding. Based on their studies and our previous one, the grain size should be coarsened to improve the higher-temperature strength. In this study, coarsening of the grain size in a 9CrODS steel was investi-
⇑ Corresponding author. Tel./fax: +81 11 706 6355. E-mail address: [email protected] (S. Ukai). 0022-3115/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.054
gated using a thermo-mechanical processing, in order to furthermore improve its high-temperature mechanical properties. 2. Experimental procedure Pure elemental powders of iron (99.5 mass%, 45–100 lm), carbon (99.7 mass%, 5 lm), chromium (99.9 mass%, under 250 lm), tungsten (99.9 mass%, 4.5–7.5 lm), and titanium (99.7 mass%, under 150 lm) were mechanically alloyed (MAed) together with Y2O3 powder (99.9 mass%, 20 nm) for 48 h in an argon gas atmosphere using a planetary-type ball mill (Fritsch P-6). The target composition of the 9CrODS is 9Cr-0.2C-2W-0.2Ti-0.35Y2O3 (mass%) with full martensitic structure. The thermo-mechanical processing conducted for the MAed powders is illustrated in Fig. 2. The hot-rolling (HRing) was introduced in 80% reduction ratio at the austenitic c phase (1000 °C) for the consolidated powders by hot-pressing (HPing). This thermo-mechanical processing is so called Ausform process for the martensitic steels [7]. Normalizing (N) and tempering (T) heat-treatments were carried out at 1050 and 800 °C 1 h, respectively. For the manufactured specimens, hardness measurement at room temperature and tensile test by a strain rate of 10 3/ s at 700 °C were conducted, and microstructure was evaluated by means of EBSD (Electron Back Scatter Diffraction) analysis using FE-SEM (Field Emission type-Scanning Electron Microscope). 3. Results and discussion The result of Vickers hardness at room temperature after each processing is shown in Fig. 3. The as-HR condition gives higher hardness as high as 660 Hv, compared with the as-HP condition. Decrease of hardness by N and T heat treatments is slightly enhanced in the HRed specimens, compared to the HPed specimens.
155
S. Ukai et al. / Journal of Nuclear Materials 417 (2011) 154–157
1000 800
Hardness Hv
600
RT
500
400 200
500 ºC
120 100
700 ºC
80
700
60 40 0.5
1.0
2.0
3.0
4.0
5.0
Grain size µm
5 µm
Fig. 1. Grain size vs. Vickers hardness in 13CrODS ferritic steels. Grain size was controlled by the primary and secondary recrystallization.
Fig. 2. Schematic illustration of thermo-mechanical processing in 9CrODS steels.
700
Vickers hardness, HV
600 500 400 300 200 100 0
as HP
as HR
HP=>NT
HR=>T
HR=>NT
Fig. 3. Vickers hardness at room temperature. HP: hot-pressing, HR: hot-rolling, N: normalizing, and T: tempering.
Fig. 4 shows the microstructure of as-HRed specimen by SEM. Pores can be slightly observed at the grain boundaries, however, the density of the HRed specimen was confirmed to be almost 100% of the theoretical density. Fine grains with sub-micron size are distributed among elongated grains along the HRed direction. These fine grains possibly contribute to the hardness increase of
Fig. 4. SEM micrograph of as-hot-rolled specimen.
660 Hv at room temperature for the as-HRed specimen. The inverse pole figures by EBSD analyses after N and T heat treatments are presented in Fig. 5. The HPed-NT specimen contains a large number of sub-micron sized small grains, whilst a number of those small grains decrease in the HRed-T specimen and almost disappear in the HRed-NT specimen. The values of Vickers hardness are also indicated in Fig. 5. The sub-micron sized small grains could be attributed to the increase of room temperature hardness. Based on the previous study exhibited in Fig. 1, it is qualitatively expected that the yield stress can be improved in the HRed-NT specimen that contains a limited number of sub-micron sized small grains. Fig. 6 represents stress–strain curves at 700 °C for the specimens, which were HRed and followed by T and NT heat-treatments. Those curves are typical for ODS ferritic steels. The HRedT specimen has a lower ultimate tensile stress (UTS) of 286 MPa and a uniform elongation of 16%, although the rupture elongation is longer around 30%. On the other hand, the HRed-NT specimen induces a higher UTS of 365 MPa and an adequate uniform elongation of 11%. As a reference, the UTS line of 9CrODS steel with full martensite manufactured by extrusion at 1150 °C in JAEA is also shown; the UTS is 308 MPa and the uniform elongation is 4%. It is important to note that the UTS is significantly improved by N heat-treatment other than T heat-treatment after HR. HR is more effective than hot-extrusion for increasing UTS without losing ductility. Fig. 7 shows a distribution of misorientation angle obtained from the EBSD analyses on the HRed-NT specimen (Fig. 5). Typical peaks can be seen at around 30°, 55° and 60°. The upper figure in Fig. 7 also shows the typical misorientation angle for various kinds of martensitic steels; martensite transformation gives crystalline misorientation between adjacent grains to be around 30°, 55° and 60° with K–S relationship [7]. As results of EBSD analyses, it is confirmed that each grain of the HRed-NT specimen (Fig. 5) was formed by martensite transformation. Prior austenite grains (PAGs) cannot be directly identified by EBSD analysis and SEM, because PAGs readily transform to martensite at room temperature. Nakashima’s group pointed out that austenite grains in a few micron meter size in ODS steels should transform to one block grain without variant selection [8]. Only tempering does not induce a change of grain size and morphology. Based on the above information, we can consider that each grain of the HRed specimen shown in Fig. 5 should correspond to each block grain transformed from each PAG. Fig. 8 schematically illustrates the structure change during the thermo-mechanical processing. The HR processing formed ultra-fine grains and introduce a high density of dislocations within grain interiors. Those heavily
156
S. Ukai et al. / Journal of Nuclear Materials 417 (2011) 154–157
Fig. 5. Inverse pole figures analysed by EBSD measurement.
400
(b) 350
(c)
Stress (MPa)
300
(a)
250 200 150 100 50 0
0
10
20
30
40
Strain (%) Fig. 6. Stress–strain curves of specimens at 700 °C: (a) hot-rolled and T, (b) hotrolled NT and (c) extruded bar of 9CrODS steel with full martensite at 1150 °C by JAEA.
0.20
4. Summary Based on the results of previous study that ultra-fine grains degraded high-temperature strength of 9CrODS steels, thermomechanical processing for grain coarsening was investigated. Hot-rolling at the austenitic c phase (1000 °C) led to a formation of ultra-fine grains and a high dislocation density within grain interiors by an Ausform process. Those heavily accumulated strains accelerated coarsening of prior austenite grains, and block grains
0.15
Relative frequency
accumulated strains induced by the Ausform process accelerated coarsening of prior austenite grains by the subsequent N heat treatment. The block grains transformed from the prior austenite grains are also coarser during cooling in the N heat treatment. We define this process as modified Ausform process, and it was confirmed that coarser block grains produced by the modified Ausform process lead to the improved high-temperature strength as shown in Fig. 6.
0.10
0.05
0.00 10
20
30
40
50
60
70
80
90
Misorientation angle (deg.) Fig. 7. Misorientation angle distribution measured by EBSD in the hot-rolled, subsequently normalized and tempered specimen.
S. Ukai et al. / Journal of Nuclear Materials 417 (2011) 154–157
157
hot pressing Hot rolling
Normalizing
N Tempering
T
Mechanical alloying
Ultra-fine grains and high dislocation density by Ausform process
Coarser prior austenite grains
Coarser block grains transformed from prior austenite grains
Fig. 8. Schematic illustration of the structure change in the process of modified Ausform.
transformed from the coarsened prior austenite grains were also coarser in the subsequent normalizing heat treatment. This process was called as modified Ausform process, and it was revealed that the formation of uniformly coarser block grains improved the ultimate tensile strength of 9CrODS steels at 700 °C. References [1] S. Ukai, S. Ohtsuka, T. Kaito, H. Sakasegawa, N. Chikata, S. Hayashi, Mater. Sci. Eng. A 510–511 (2009) 115–120.
[2] S. Ohtsuka, S. Ukai, H. Sakasegawa, M. Fujiwara, T. Kaito, T. Narita, J. Nucl. Mater. 367–370 (2007) 160–165. [3] S. Ukai, S. Ohtsuka, Energy Mater. 2 (1) (2007) 26–35. [4] N. Chikata, S. Hayashi, S. Ukai, S. Ohtsuka, T. Kaito, in: Annual Meeting on Jpn. Inst. Metals, Tokyo Institute of Technology, Japan, 2008. [5] V.K. Rao, D.M.R. Taplin, P.R. Rao, Metall. Trans. A 6A (1975) 77–86. [6] H. Sakasegawa, S. Ukai, M. Tamura, S. Ohtsuka, H. Tanigawa, H. Ogiwara, A. Kohyama, M. Fujiwara, J. Nucl. Mater. 373 (2007) 82–89. [7] T. Maki, I. Tamura, in: Proceeding of International Conference on Physical Metallurgy of Thermomechanical Processing of Steels and other Metals, ISIJ, 1988. p. 458. [8] H. Nakashima, Tetsu-to-Hagane 90 (2) (2004) 73–78.