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Mechanical properties of ARAA steel after electron beam welding
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Mechanical properties of ARAA steel after electron beam welding Jae Sung Yoon a,∗ , Suk-Kwon Kim a , Eo Hwak Lee a , Hyung Gon Jin a , Dong Won Lee a , Seungyon Cho b a b
Korea Atomic Research Energy Institute, Daejeon, Republic of Korea National Fusion Research Institute, Daejeon, Republic of Korea
h i g h l i g h t s • • • •
E-beam welding in ARAA plate. Performed mechanical properties of ARAA for fabrication of KO HCCR TBM. Evaluation of the tensile, Charpy impact, bend tests after PWHT. Evaluation of the hardness and microstructure on the welded area before and after PWHT.
a r t i c l e
i n f o
Article history: Received 3 October 2016 Received in revised form 11 April 2017 Accepted 11 April 2017 Available online xxx Keywords: ITER HCCR TBM Mechanical property E-beam welding PWHT
a b s t r a c t Korea has designed a helium cooled ceramic reflector (HCCR) test blanket module (TBM) that includes a TBM shield, called a TBM set, that will be tested in ITER. The HCCR TBM is composed of four sub-modules and a back manifold. In addition, each sub-module is composed of a first wall (FW), a breeding box with a seven-layer breeding zone (BZ), and side walls with a cooling path. Korean RAFM steel was developed as a structural material for the HCCR TBM, and advanced reduced activation alloy (ARAA) was selected as the primary candidate from various program alloys. Welding technologies for fabrication of the HCCR TBM were developed using ARAA. Tensile, impact, bend tests were performed after post-weld heat treatment, and hardness, microstructure characteristics were determined before and after post-weld heat treatment to evaluate the welded specimen under the chosen welding conditions. © 2017 Elsevier B.V. All rights reserved.
1. Introduction To develop next-generation technologies, one of the primary objectives of the ITER project is an investigation of the heat extraction from a blanket module in a fusion reactor and tritium extraction experiments [1–5]. Korea has developed a helium cooled ceramic reflector (HCCR) test blanket module (TBM) that will be installed and tested in ITER [6]. Fabrication technologies for the HCCR TBM have been developed and confirmed through the fabrication of mockups using ferritic martensitic steel (FMS), SS316L steel, and advanced reduced activation alloy (ARAA) which is a 9Cr–1.2W based ferritic–martensitic steel with 0.01 wt.% Zr [7–10]. Joint technologies, such as tungsten inert gas (TIG) welding, electron beam (E-beam) welding, and hot isostatic pressing (HIP) joining, have also been developed and evaluated for RAFM steel through several mechanical tests [11–13]. The reduced activation
∗ Corresponding author. E-mail address: [email protected] (J.S. Yoon).
ferritic/martensitic (RAFM) steel is the primary candidate structural material for the HCCR TBM. In addition, an ARAA steel with a nominal composition of 9Cr-1.2W-0.2V-0.01Zr is under development and evaluation as a structural material for the HCCR TBM [10]. The purpose of this study was to conduct and analyze tests for the mechanical properties of the E beam weld joints of the ARAA steel and verify these properties through various tests on the mechanical properties. The optimization method of the welding condition was investigated in a previous paper [9], and the results of the previous paper were applied in the present study. In this study, 7-mm thick and 1350-mm long ARAA plates were prepared, and E-beam welding and post weld heat treatment (PWHT) were conducted using the developed weld conditions and process to fabricate the test specimens for the mechanical property tests. Charpy impact tests, and tensile property tests were applied to the heat affected zone (HAZ) and weld metal (WM) of the E-beam welded joints after PWHT. Micro-hardness measurements, microstructural observations of the welded joints were also analyzed at the HAZ and WM areas before and after PWHT.
http://dx.doi.org/10.1016/j.fusengdes.2017.04.046 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: J.S. Yoon, et al., Mechanical properties of ARAA steel after electron beam welding, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.046
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Fig. 1. Configuration of design concept of HCCR TBM and its sub-module dimensions.
Table 1 Chemical composition of ARAA-2 steel (wt%). Element
Concentration (wt%)
C Si Mn Cr W V Ta N Ti Zr Fe
0.10 0.11 0.26 9.04 1.25 0.20 0.06 0.0043 0.0061 0.005 Balanced
2. Design and fabrication procedure of the HCCR TBM sub-module The HCCR TBM set is composed of a TBM body and TBM shield. The TBM body consists of four sub-modules and a back manifold. The design and fabrication process of the HCCR TBM body were developed through iterations of the simulation of the design and fabrication, and through tests of the small mockups. Various joint technologies, such as TIG, an E-beam welding method, and HIP joining, were used to fabricate the HCCR TBM body. The E-beam welding technology was used for many joints during the fabrication process of the TBM body. The manufacturing process of the TBM sub-module mockup was developed and introduced [14]. Fig. 1 shows the configuration of the design concept of the HCCR TBM body and its sub-module dimensions. 3. Mechanical property test of the E-beam welded ARAA plate 3.1. Preparation of the test specimen The material used for the mechanical property test was ARAA steel. The chemical composition of ARAA-2 (2nd large-scale product of ARAA) steel is shown in Table 1. This material was normalized at 1000 ◦ C for 40 min and then air cooled and tempered at 750 ◦ C for 70 min before additional air cooling. To investigate the mechanical property test for the E-beam welding of an ARAA plate, 7-mm thick and 1350-mm long ARAA plates were welded using E-beam weld-
Fig. 2. Microstructure of the optimized E-beam welded joint in a 7-mm thick ARAA plate, L1:welding width on the surface, L2:welding width at the plate, L3:welding penetration depth, L4:plate length without back plate, L5:margined penetration depth, L6:sputtering depth.
ing. The welding conditions were optimized for welding 7 mm thick plates. Welding was performed at a welding speed of 1200 mm/min and a welding current of 80 mA in consideration of the sputtering that occurs at the end of the electron beam welding. The test for optimum conditions of the PWHT were conducted by conducting hardness, impact, and histological tests on the specimens, which were post-heat treated for 1 h, 1.5 h, and 2 h within the temperature range of 710 ◦ C, 730 ◦ C, and 750 ◦ C considering the ARAA tempering conditions (750 ◦ C, 70 min). After conducting the tests for PWHT, the optimized conditions of 730 ◦ C/1 h was determined [9]. Fig. 2 shows the microstructure of the optimized E-beam weld of the 7-mm thick ARAA plate specimens and the measured values of the penetration depth, including the penetration depth of the back plate and the width of the fusion zone. 3.2. Mechanical property test The test specimens were fabricated and the mechanical property tests conducted. To investigate the effect of PWHT for the hardness values, Micro Vickers hardness measurements using a test force of HV0.1-load (0.1kgf) were performed across the E-beam welding
Please cite this article in press as: J.S. Yoon, et al., Mechanical properties of ARAA steel after electron beam welding, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.046
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Fig. 3. Results of hardness test across the E-beam welding joint of an ARAA plate before and after PWHT at 730 ◦ C/1 h.
Fig. 4. Results of the Charpy impact test on the HAZ and WM of the E-beam welding joint of an ARAA plate after PWHT at 730 ◦C/1 h.
joint. Fig. 3 shows the micro-hardness results for the BM, HAZ, and WM regions before and after PWHT. The micro-hardness results indicate that the hardness value of the as-welded (before PWHT) point was much higher than that obtained after conducting the PWHT. The average hardness values before PWHT were 375 HV in the HAZ and 352 HV in the WM, and after PWHT, the values decreased to 242 HV in the HAZ and 263 HV in the WM. The average hardness in the base metal close to the HAZ decreased slightly from 210 HV to 196 HV after PWHT. Charpy impact specimens were fabricated with notches at the center of the HAZ and weld zone after their surfaces were polished and etched parallel to the weld surface. The dimensions of the specimen were 55mmL × 10mmW × 5mmT. Impact tests were performed at temperatures of −80 ◦ C, −70 ◦ C, −60 ◦ C, −55 ◦ C, −50 ◦ C, −40 ◦ C, −30 ◦ C, −20 ◦ C, −10 ◦ C, 0 ◦ C, and room temperature (RT). The maximum Charpy impact values in the HAZ and WM regions, which were calculated using the hyperbolic tangent curve fitting method, are 112 J at 0 ◦ C and 127 J at a RT, respectively. The Charpy impact value in the HAZ and WM regions are shown in Fig. 4. The ductile brittle transition temperatures (DBTT) of all specimens were −42 ◦ C on the HAZ and −43 ◦ C on the WM. The DBTT of the weld metal was slightly less than the DBTT of HAZ, while the upper shelf energy (USE) was greater.
3
Fig. 5. Tensile strength of ARAA with temperature after PWHT at 730 ◦ C/1 h.
Fig. 6. Total elongation and reduction of Area of ARAA with temperature after PWHT at 730 ◦ C/1 h.
The tensile property specimens were fabricated using dog-bone type specimens for room temperature, and cylindrical specimens for higher than room temperature. The dimensions of the dog-bone type specimens and cylindrical specimens were 200 mmL × 25 mmW × 5 mmT, and 100 mmL × dia. 4 mmT, respectively. Tensile tests were conducted at room temperature (RT), 100 ◦ C, 200 ◦ C, 300 ◦ C, 400 ◦ C, 500 ◦ C and 550 ◦ C. Fig. 5 shows the temperature dependence of the tensile strength for welded materials. The tensile strength decreased as the temperature increased. The yield strength (YS) and ultimate tensile strength (UTS) were 553 MPa and 652 MPa, respectively. The elongation value and reduction of the area are shown in Fig. 6. The tensile specimens were fractured at the base metal as, shown in Fig. 7. It appeared that the tensile test would not show any noticeable property deterioration owing to the hardening in the HAZ and weld zone. Bend tests were conducted for the face and root bend tests without any failure (Fig. 7). 3.3. Microstructure observation The microstructures of ARAA in the as-welded and as-PWHTed conditions were investigated. As shown in Fig. 8, the as-welded BM exhibited a tempered martensitic structure with carbides formed along the grain boundaries. Such microstructure was little affected by the post welding heat treatment conducted at 730 ◦ C for 1 h,
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observed in the as-weld WM as the carbides were fully dissolved during the melting, and the subsequent air cooling suppressed the carbide formation. The PWHT of the WM led to the formation of a tempered martensite structure. The carbides, which are finer than those in the as-PWHTed BM, were also observed at the grain boundaries. There is no change in the size of grain before or after heat treatment. This is because the tempering temperature can change the size of the precipitates, but it is not enough to change the grain size. 4. Conclusion
Fig. 7. Results of mechanical property tests: (a) Charpy impact test specimens, (b) tensile test specimens of cylindrical type, (c) tensile test specimens of dog-bone type, and (d) bend test specimens.
E-beam welding technology was applied to many joints during the fabrication process of the HCCR TBM. To conduct mechanical property tests of an E-beam welded ARAA plate, 7-mm thick and 1350-mm long specimens were prepared for testing. The welded specimens were fabricated using the E-beam welding conditions for the ARAA materials used for TBM fabrication. Tensile, impact, and bend tests were performed after post-weld heat treatment, and hardness, microstructure characteristics were studied before and after post-weld heat treatment to evaluate the welded specimens under the determined welding conditions. The micro-hardness results indicate that the hardness value of the as-welded (before PWHT) point was much higher than that obtained after conducting the PWHT. The average hardness values before PWHT were 375 HV in the HAZ and 352 HV in the WM, and after PWHT, the values decreased to 242 HV in the HAZ and 263 HV in the WM. The maximum Charpy impact values in the HAZ and WM regions are 112 J at 0 ◦ C and 127 J at RT, respectively. The ductile brittle transition temperatures (DBTT) of all specimens were −42 ◦ C on the HAZ and −43 ◦ C on the WM. The DBTT of the weld metal was slightly less than the DBTT of HAZ, while the upper shelf energy (USE) was greater. The tensile property specimens were fabricated using dog-bone type specimens for room temperature, and cylindrical specimens for higher than room temperature. The tensile strength decreased as the temperature increased. The yield strength (YS) and ultimate tensile strength (UTS) were 553 MPa and 652 MPa, respectively. Bend tests were conducted for the face and root bend tests without any failure. Acknowledgments This work was supported by the R&D Program through National Fusion Research Institute (NFRI) funded by the Ministry of Education, Science and Technology of the Republic of Korea (NFRIIN1603). References
Fig. 8. Microstructure of E-beam welded ARAA plates (x400) (a) BM as welded, (b) BM after PWHT of 730 ◦ C, (c) WM as welded, (d) WM after PWHT of 730 ◦ C.
except for a slight coarsening of carbides formed throughout the microstructure. On the other hand, a martensite structure with much coarser blocks and packets developed in the as-welded WM, which can be attributed to the coarsened prior austenite grains formed during the solidification of the melt. Carbides were rarely
[1] L.M. Giancarli, et al., Overview of the ITER TBM program, Fusion Eng. Des. 87 (2012) 395–402. [2] E. Rajendra Kumar, et al., Overview of TBM R&D activities in India, Fusion Eng. Des. 87 (2012) 461–465. [3] M. Enoeda, et al., Development of the water cooled ceramic breeder test blanket module in Japan, Fusion Eng. Des. 87 (2012) 1363–1369. [4] K.M. Feng, et al., Progress on design and R&D for helium-cooled ceramic breeder TBM in China, Fusion Eng. Des. 87 (2012) 1138–1145. [5] G. Aiello, et al., HCLL TBM design status and development, Fusion Eng. Des. 86 (2011) 2129–2134. [6] S. Cho, et al., Overview of helium cooled ceramic reflector test blanket module development in Korea, Fusion Eng. Des. 88 (2013) 621–625. [7] J.S. Yoon, et al., Development of fabrication procedure for korean HCCR TBM, Fusion Eng. Des. 89 (2014) 1081–1085. [8] J.S. Yoon, et al., Fabrication of a 1/6-scale mock-up for the Korea TBM first wall in ITER, Fusion Sci. Technol. 62 (2012) 29–33. [9] J.S. Yoon, et al., Evaluation of ARAA steel E-beam welding characteristics for the fabrication of KO HCCR TBM, Fusion Eng. Des. 109–111 (2016) 82–87. [10] Y.B. Chun, et al., Development of Zr-containing advanced reduced-activation alloy (ARAA) as structural material for fusion reactors, Fusion Eng. Des. 109–111 (2016) 629–633.
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[11] D.Y. Ku, et al., TIG and HIP joining of reduced activation ferrite/martensitic steel for the Korean ITER-TBM, J. Nucl. Mater. 417 (2011) 67–71. [12] S. Cho, et al., Development of low activation ferritic/martensitic steel welding technology for the fabrication of KO HCSB TBM, J. Nucl. Mater. 386–388 (2009) 491–494.
[13] J.S. Lee, et al., HIP joining of RAFM/RAFM steel and beryllium/RAFM steel for fabrication of the ITER TBM first wall, Met. Mater. Intern. 15 (2009) 465. [14] J.S. Yoon, et al., Development of fabrication procedure and technology through fabrication of small mock-up for the KO HCCR TBM, in: 22th TOFE, PA, USA, August 22–25, 2016.
Please cite this article in press as: J.S. Yoon, et al., Mechanical properties of ARAA steel after electron beam welding, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.046
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FUSION-9398; No. of Pages 5
Fusion Engineering and Design xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Mechanical properties of ARAA steel after electron beam welding Jae Sung Yoon a,∗ , Suk-Kwon Kim a , Eo Hwak Lee a , Hyung Gon Jin a , Dong Won Lee a , Seungyon Cho b a b
Korea Atomic Research Energy Institute, Daejeon, Republic of Korea National Fusion Research Institute, Daejeon, Republic of Korea
h i g h l i g h t s • • • •
E-beam welding in ARAA plate. Performed mechanical properties of ARAA for fabrication of KO HCCR TBM. Evaluation of the tensile, Charpy impact, bend tests after PWHT. Evaluation of the hardness and microstructure on the welded area before and after PWHT.
a r t i c l e
i n f o
Article history: Received 3 October 2016 Received in revised form 11 April 2017 Accepted 11 April 2017 Available online xxx Keywords: ITER HCCR TBM Mechanical property E-beam welding PWHT
a b s t r a c t Korea has designed a helium cooled ceramic reflector (HCCR) test blanket module (TBM) that includes a TBM shield, called a TBM set, that will be tested in ITER. The HCCR TBM is composed of four sub-modules and a back manifold. In addition, each sub-module is composed of a first wall (FW), a breeding box with a seven-layer breeding zone (BZ), and side walls with a cooling path. Korean RAFM steel was developed as a structural material for the HCCR TBM, and advanced reduced activation alloy (ARAA) was selected as the primary candidate from various program alloys. Welding technologies for fabrication of the HCCR TBM were developed using ARAA. Tensile, impact, bend tests were performed after post-weld heat treatment, and hardness, microstructure characteristics were determined before and after post-weld heat treatment to evaluate the welded specimen under the chosen welding conditions. © 2017 Elsevier B.V. All rights reserved.
1. Introduction To develop next-generation technologies, one of the primary objectives of the ITER project is an investigation of the heat extraction from a blanket module in a fusion reactor and tritium extraction experiments [1–5]. Korea has developed a helium cooled ceramic reflector (HCCR) test blanket module (TBM) that will be installed and tested in ITER [6]. Fabrication technologies for the HCCR TBM have been developed and confirmed through the fabrication of mockups using ferritic martensitic steel (FMS), SS316L steel, and advanced reduced activation alloy (ARAA) which is a 9Cr–1.2W based ferritic–martensitic steel with 0.01 wt.% Zr [7–10]. Joint technologies, such as tungsten inert gas (TIG) welding, electron beam (E-beam) welding, and hot isostatic pressing (HIP) joining, have also been developed and evaluated for RAFM steel through several mechanical tests [11–13]. The reduced activation
∗ Corresponding author. E-mail address: [email protected] (J.S. Yoon).
ferritic/martensitic (RAFM) steel is the primary candidate structural material for the HCCR TBM. In addition, an ARAA steel with a nominal composition of 9Cr-1.2W-0.2V-0.01Zr is under development and evaluation as a structural material for the HCCR TBM [10]. The purpose of this study was to conduct and analyze tests for the mechanical properties of the E beam weld joints of the ARAA steel and verify these properties through various tests on the mechanical properties. The optimization method of the welding condition was investigated in a previous paper [9], and the results of the previous paper were applied in the present study. In this study, 7-mm thick and 1350-mm long ARAA plates were prepared, and E-beam welding and post weld heat treatment (PWHT) were conducted using the developed weld conditions and process to fabricate the test specimens for the mechanical property tests. Charpy impact tests, and tensile property tests were applied to the heat affected zone (HAZ) and weld metal (WM) of the E-beam welded joints after PWHT. Micro-hardness measurements, microstructural observations of the welded joints were also analyzed at the HAZ and WM areas before and after PWHT.
http://dx.doi.org/10.1016/j.fusengdes.2017.04.046 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: J.S. Yoon, et al., Mechanical properties of ARAA steel after electron beam welding, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.046
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Fig. 1. Configuration of design concept of HCCR TBM and its sub-module dimensions.
Table 1 Chemical composition of ARAA-2 steel (wt%). Element
Concentration (wt%)
C Si Mn Cr W V Ta N Ti Zr Fe
0.10 0.11 0.26 9.04 1.25 0.20 0.06 0.0043 0.0061 0.005 Balanced
2. Design and fabrication procedure of the HCCR TBM sub-module The HCCR TBM set is composed of a TBM body and TBM shield. The TBM body consists of four sub-modules and a back manifold. The design and fabrication process of the HCCR TBM body were developed through iterations of the simulation of the design and fabrication, and through tests of the small mockups. Various joint technologies, such as TIG, an E-beam welding method, and HIP joining, were used to fabricate the HCCR TBM body. The E-beam welding technology was used for many joints during the fabrication process of the TBM body. The manufacturing process of the TBM sub-module mockup was developed and introduced [14]. Fig. 1 shows the configuration of the design concept of the HCCR TBM body and its sub-module dimensions. 3. Mechanical property test of the E-beam welded ARAA plate 3.1. Preparation of the test specimen The material used for the mechanical property test was ARAA steel. The chemical composition of ARAA-2 (2nd large-scale product of ARAA) steel is shown in Table 1. This material was normalized at 1000 ◦ C for 40 min and then air cooled and tempered at 750 ◦ C for 70 min before additional air cooling. To investigate the mechanical property test for the E-beam welding of an ARAA plate, 7-mm thick and 1350-mm long ARAA plates were welded using E-beam weld-
Fig. 2. Microstructure of the optimized E-beam welded joint in a 7-mm thick ARAA plate, L1:welding width on the surface, L2:welding width at the plate, L3:welding penetration depth, L4:plate length without back plate, L5:margined penetration depth, L6:sputtering depth.
ing. The welding conditions were optimized for welding 7 mm thick plates. Welding was performed at a welding speed of 1200 mm/min and a welding current of 80 mA in consideration of the sputtering that occurs at the end of the electron beam welding. The test for optimum conditions of the PWHT were conducted by conducting hardness, impact, and histological tests on the specimens, which were post-heat treated for 1 h, 1.5 h, and 2 h within the temperature range of 710 ◦ C, 730 ◦ C, and 750 ◦ C considering the ARAA tempering conditions (750 ◦ C, 70 min). After conducting the tests for PWHT, the optimized conditions of 730 ◦ C/1 h was determined [9]. Fig. 2 shows the microstructure of the optimized E-beam weld of the 7-mm thick ARAA plate specimens and the measured values of the penetration depth, including the penetration depth of the back plate and the width of the fusion zone. 3.2. Mechanical property test The test specimens were fabricated and the mechanical property tests conducted. To investigate the effect of PWHT for the hardness values, Micro Vickers hardness measurements using a test force of HV0.1-load (0.1kgf) were performed across the E-beam welding
Please cite this article in press as: J.S. Yoon, et al., Mechanical properties of ARAA steel after electron beam welding, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.046
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Fig. 3. Results of hardness test across the E-beam welding joint of an ARAA plate before and after PWHT at 730 ◦ C/1 h.
Fig. 4. Results of the Charpy impact test on the HAZ and WM of the E-beam welding joint of an ARAA plate after PWHT at 730 ◦C/1 h.
joint. Fig. 3 shows the micro-hardness results for the BM, HAZ, and WM regions before and after PWHT. The micro-hardness results indicate that the hardness value of the as-welded (before PWHT) point was much higher than that obtained after conducting the PWHT. The average hardness values before PWHT were 375 HV in the HAZ and 352 HV in the WM, and after PWHT, the values decreased to 242 HV in the HAZ and 263 HV in the WM. The average hardness in the base metal close to the HAZ decreased slightly from 210 HV to 196 HV after PWHT. Charpy impact specimens were fabricated with notches at the center of the HAZ and weld zone after their surfaces were polished and etched parallel to the weld surface. The dimensions of the specimen were 55mmL × 10mmW × 5mmT. Impact tests were performed at temperatures of −80 ◦ C, −70 ◦ C, −60 ◦ C, −55 ◦ C, −50 ◦ C, −40 ◦ C, −30 ◦ C, −20 ◦ C, −10 ◦ C, 0 ◦ C, and room temperature (RT). The maximum Charpy impact values in the HAZ and WM regions, which were calculated using the hyperbolic tangent curve fitting method, are 112 J at 0 ◦ C and 127 J at a RT, respectively. The Charpy impact value in the HAZ and WM regions are shown in Fig. 4. The ductile brittle transition temperatures (DBTT) of all specimens were −42 ◦ C on the HAZ and −43 ◦ C on the WM. The DBTT of the weld metal was slightly less than the DBTT of HAZ, while the upper shelf energy (USE) was greater.
3
Fig. 5. Tensile strength of ARAA with temperature after PWHT at 730 ◦ C/1 h.
Fig. 6. Total elongation and reduction of Area of ARAA with temperature after PWHT at 730 ◦ C/1 h.
The tensile property specimens were fabricated using dog-bone type specimens for room temperature, and cylindrical specimens for higher than room temperature. The dimensions of the dog-bone type specimens and cylindrical specimens were 200 mmL × 25 mmW × 5 mmT, and 100 mmL × dia. 4 mmT, respectively. Tensile tests were conducted at room temperature (RT), 100 ◦ C, 200 ◦ C, 300 ◦ C, 400 ◦ C, 500 ◦ C and 550 ◦ C. Fig. 5 shows the temperature dependence of the tensile strength for welded materials. The tensile strength decreased as the temperature increased. The yield strength (YS) and ultimate tensile strength (UTS) were 553 MPa and 652 MPa, respectively. The elongation value and reduction of the area are shown in Fig. 6. The tensile specimens were fractured at the base metal as, shown in Fig. 7. It appeared that the tensile test would not show any noticeable property deterioration owing to the hardening in the HAZ and weld zone. Bend tests were conducted for the face and root bend tests without any failure (Fig. 7). 3.3. Microstructure observation The microstructures of ARAA in the as-welded and as-PWHTed conditions were investigated. As shown in Fig. 8, the as-welded BM exhibited a tempered martensitic structure with carbides formed along the grain boundaries. Such microstructure was little affected by the post welding heat treatment conducted at 730 ◦ C for 1 h,
Please cite this article in press as: J.S. Yoon, et al., Mechanical properties of ARAA steel after electron beam welding, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.046
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observed in the as-weld WM as the carbides were fully dissolved during the melting, and the subsequent air cooling suppressed the carbide formation. The PWHT of the WM led to the formation of a tempered martensite structure. The carbides, which are finer than those in the as-PWHTed BM, were also observed at the grain boundaries. There is no change in the size of grain before or after heat treatment. This is because the tempering temperature can change the size of the precipitates, but it is not enough to change the grain size. 4. Conclusion
Fig. 7. Results of mechanical property tests: (a) Charpy impact test specimens, (b) tensile test specimens of cylindrical type, (c) tensile test specimens of dog-bone type, and (d) bend test specimens.
E-beam welding technology was applied to many joints during the fabrication process of the HCCR TBM. To conduct mechanical property tests of an E-beam welded ARAA plate, 7-mm thick and 1350-mm long specimens were prepared for testing. The welded specimens were fabricated using the E-beam welding conditions for the ARAA materials used for TBM fabrication. Tensile, impact, and bend tests were performed after post-weld heat treatment, and hardness, microstructure characteristics were studied before and after post-weld heat treatment to evaluate the welded specimens under the determined welding conditions. The micro-hardness results indicate that the hardness value of the as-welded (before PWHT) point was much higher than that obtained after conducting the PWHT. The average hardness values before PWHT were 375 HV in the HAZ and 352 HV in the WM, and after PWHT, the values decreased to 242 HV in the HAZ and 263 HV in the WM. The maximum Charpy impact values in the HAZ and WM regions are 112 J at 0 ◦ C and 127 J at RT, respectively. The ductile brittle transition temperatures (DBTT) of all specimens were −42 ◦ C on the HAZ and −43 ◦ C on the WM. The DBTT of the weld metal was slightly less than the DBTT of HAZ, while the upper shelf energy (USE) was greater. The tensile property specimens were fabricated using dog-bone type specimens for room temperature, and cylindrical specimens for higher than room temperature. The tensile strength decreased as the temperature increased. The yield strength (YS) and ultimate tensile strength (UTS) were 553 MPa and 652 MPa, respectively. Bend tests were conducted for the face and root bend tests without any failure. Acknowledgments This work was supported by the R&D Program through National Fusion Research Institute (NFRI) funded by the Ministry of Education, Science and Technology of the Republic of Korea (NFRIIN1603). References
Fig. 8. Microstructure of E-beam welded ARAA plates (x400) (a) BM as welded, (b) BM after PWHT of 730 ◦ C, (c) WM as welded, (d) WM after PWHT of 730 ◦ C.
except for a slight coarsening of carbides formed throughout the microstructure. On the other hand, a martensite structure with much coarser blocks and packets developed in the as-welded WM, which can be attributed to the coarsened prior austenite grains formed during the solidification of the melt. Carbides were rarely
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