Martensitic steel for the Korean ITER–TBM

Journal of Nuclear Materials 417 (2011) 67–71

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TIG and HIP joining of Reduced Activation Ferrite/Martensitic steel for the Korean ITER–TBM Duck Young Ku a, Seungjin Oh b,⇑, Mu-Young Ahn a, In-Keun Yu a, Duck-Hoi Kim a, Seungyon Cho a, Im-Sub Choi c, Ki-Bum Kwon c a b c

ITER Korea, National Fusion Research Institute, Gwahangno 113, Yuseong-gu, Daejeon 305-333, Republic of Korea Nuclear Engineering and Technology Institute, KHNP, 508 Keumbyeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea Soles Co. LTD., 9-1 Horim-dong, Dalseo-gu, Daegu 704-249, Republic of Korea

a r t i c l e

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Article history: Available online 24 December 2010

a b s t r a c t Korea is developing a Helium Cooled Solid Breeder Test Blanket Module for ITER. The primary candidate structural material is a Reduced Activation Ferritic/Martensitic steel. The complex TBM structure requires developing joining technologies for successful fabrication. The characteristics of Tungsten Inert Gas welding and Hot Isostatic Pressing joining of RAFM steel were investigated. Metallurgical examinations showed that the steel grain size was increased after HIP joining and recovered by post joining heat treatment. Both TIG welding and HIP joining are found to be acceptable for ITER TBM based on mechanical tests and microstructure examination. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

Test Blanket Modules (TBM) that are like the modules needed for DEMO blankets will be tested in ITER to verify the capability of tritium breeding and recovery and the extraction of high grade heat suitable for the production of electricity. A Helium Cooled Solid Breeder (HCSB) TBM has been developed in Korea to accomplish these goals [1–7]. Because the TBM is operated at high temperatures (300–500 °C) under neutron irradiation, the structural material of the TBM must have good mechanical properties at high temperature, and high resistance to radiation-induced swelling and He embrittlement. Reduced Activation Ferritic/Martensitic (RAFM) steel is known for superior swelling resistance under irradiation, excellent thermal properties at high temperature, and good weldability [8]. Therefore, the RAFM steel has been chosen as the primary candidate structural material for Korean TBMs. Due to the complexity of the First wall (FW) and Side wall (SW), it is necessary to develop various joining technologies, such as Hot Isostatic Pressing (HIP), Electron Beam Welding (EBW) and Tungsten Inert Gas (TIG) welding, for the successful fabrication of TBM. In this study, TIG welding and HIP joining technologies for RAFM steel were investigated. Various mechanical tests of TIGwelded RAFM steel were performed to obtain the optimal TIG welding process for RAFM steel. The effect of degassing condition was investigated to enhance the quality of HIP joining.

2.1. Material

⇑ Corresponding author. Tel.: +82 42 719 1233; fax: +82 42 719 1209. E-mail address: [email protected] (S. Oh). 0022-3115/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.169

The material used in this study is F82H RAFM steel manufactured by JFE TECHNO-RESEARCH CORPORATION, with chemical composition in wt.% of Fe–0.09C–7.9Cr–1.81 W–0.2 V–0.03 Ta. This steel was normalized at 960 °C for 30 min and air cooled, then tempered at 750 °C for 90 min followed by air cooling. 2.2. TIG welding The edges of 10 mm thick plates were machined for single-V welding with the groove angle of 60° and 2 mm root opening, as shown Fig. 1. The TIG welding conditions are listed in Table 1. To control the grain size and to reduce the residual stress, a PostWelded Heat Treatment (PWHT) was applied to the plates by tempering at 720 °C for 90 min followed by air cooling [9]. Before and after PWHT, the TIG welded plates were examined using a Scanning Electron Microscope (SEM), and Vickers micro hardness was measured. Tensile and Charpy tests were used to evaluate the mechanical properties. Tensile specimens with the TIG weld metal located in the center of gauge length were fabricated in accordance with ASTM standards E8 [10] (see Fig. 1b). Tensile tests were performed at room temperature, 300, 450, 550 and 600 °C. Charpy test specimens were fabricated in accordance with ASTM standards E23 [11]. The size of Charpy V-notched specimens is 5 mm (W)  55 mm (L)  5 mm (t), with a 1 mm V-notch (see Fig. 1c). The location of the V-notch is in the weld metal, in the heat affected zone (HAZ), or in the base metal. The Charpy impact tests

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3. Results and discussion 3.1. Mechanical properties of TIG welded F82H The PWHT condition selected was 90 min at 720 °C for the recovery of hardening and toughness degradation. The microstructures of the TIG weld before and after PWHT are shown in Fig. 2. The grain size of weld metal was around ASTM 6, but it decreased to around 7 after PWHT, a size that is close to the grain size of base metal (8–9). Fig. 3 shows the hardness dependence measured using a Micro Vickers tester. In the as-welded condition, the weld zone shows the typical hardness of a martensitic structure. The maximum hardness in the HAZ was 400 Hv and decreased to 240 Hv after PWHT. The tensile properties of TIG welded F82H for various temperatures are shown in Fig. 4. The yield stress and the ultimate tensile strength of base metal and weld metal decrease as temperature increases, and the rate of decrease is greater above 450 °C. The total elongation of base metal and weld metal rapidly increase with increase of test temperature above 450 °C. The yield stress and tensile strength of weld metal are slightly higher than those of the base metal, while the total elongation of base metal is higher than that of weld metal. Fig. 5 shows the Charpy energy as a function of temperature. The upper shelf energy has the highest value in weld metal and the lowest value in base metal. On the other hand, all specimens show similar values of lower shelf energy. The DBTT of each material was calculated from the curves in Fig. 5 using the hyperbolic Fig. 1. Dimension (mm) of V-groove weld plate and test specimens.

Table 1 TIG welding conditions. Welding type Late thickness Groove Voltage Current Layer Post-welded heat treatment Wire chemical composition Base metal chemical composition

Manual 10 mm V-groove shown in Fig. 1 9–10 V 270 A (Pulse) 8 720 °C, 1.5 h Fe–0.08C–7.7Cr–2 W–0.2 V–0.02 Ta [8] Fe–0.09C–7.9Cr–1.81 W–0.2 V–0.03 Ta

were performed at 150–50 °C, and the Ductile–Brittle Transition Temperature (DBTT) was evaluated for the base metal, HAZ and weld metal. 2.3. HIP joining The blocks for HIP joining were 71.5 mm (W)  31.5 mm (L)  10 mm (t), and were mechanically polished to a surface roughness of less than 1 lm [12]. The blocks were degassed in a vacuum of 10 3 torr at 500, 600 and 700 °C for 2 h. The HIPping of the samples was performed at 1100 °C and 100 MPa for 2 h. After HIPping the samples were normalized and tempered at 960 °C for 30 min followed by air cooling and tempered at 750 °C for 90 min and air cooling, to control the grain size and reduce the residual stress. After this heat treatment the microstructure of the HIP interface was examined by optical microscopy and SEM. The specimens for the mechanical test of HIP joints were manufactured as shown in Fig. 1. The 7 mm diameter tensile specimens were fabricated and tested in accordance with ASTM standards E8 [10]. The HIP interface was located in the center of the gauge length. Tensile tests were at room temperature with strain rate 5  10 4 s 1. The Charpy specimens were as for the TIG material, with the root of the V-notch located at the HIP interface.

Fig. 2. SEM micrographs of TIG weld in the (a) base metal, (b) weld metal, and (c) HAZ zones.

D.Y. Ku et al. / Journal of Nuclear Materials 417 (2011) 67–71

Fig. 3. Vickers hardness profile across TIG weld zones before and after PWHT of 1 h at 720 °C.

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Fig. 5. Charpy properties of welded F82H.

3.2. HIP joint of F82H 3.2.1. Microstructure observation The HIPped samples were degassed in vacuum of 10 3 torr at 500, 600 and 700 °C for 2 h. The microstructures at HIP interfaces were examined using an optical microscope, and the results are shown in Fig. 6. The ASTM grain size number [13] of prior austenite grains (PAG) in the HIPped samples obtained from Fig. 6 was about 6 while that of as-received F82H is about 8–9. Tantalum carbide (TaC) is reported to retard the grain growth of PAG [14]. However, it was found in this study that TaC dissolved during the HIP process because the TaC dissolution temperature is below 1100 °C [15]. Therefore, it is thought that the coarsening of PAG is caused by TaC dissolving. The HIP interface was also examined using SEM– BSE (back scattered electron) mode which shows the differences of chemical components, shown in Fig. 7. The HIP interface consists of second phase particles like small islands. It is thought that these second phase particles are oxides formed from the residual oxygen not removed during the degassing process. However, the distribution of these particles is independent of degassing temperature, as shown in this figure, and Figs. 6 and 7 additional studies of the second phase particles in HIP interfaces are needed.

Fig. 4. Tensile properties of TIG welded F82H from room temperature to 600 °C.

tangent curve fitting method [4]. The DBTTs of base metal, HAZ and weld metal are 70, 90 and 110 °C, respectively.

3.2.2. Mechanical properties of HIP bonded F82H The effects of degassing temperature on the mechanical properties of HIP joining are shown in Figs. 8 and 9. The tensile strength and the yield stress of the HIP bonded F82H are reduced by about 5% from as-received material (see Fig. 8). As mentioned in the previous section, the HIP bonded F82H showed larger PAG size than for the as-received condition. The degassing temperature has not affected the microstructure of HIP bonded F82H. Similar to the behavior of the microstructure of HIP bonded F82H, the tensile properties are decreased due to the increase of PAG, and the tensile properties were independent of degassing temperatures. However, in contrast to the tensile properties, the Charpy impact properties are affected by degassing temperature. Fig. 9 shows the impact energy at room temperature for several degassing temperatures. The room temperature impact energy of HIP bonded material decreases with increasing degassing temperature, showing the highest value when the samples were degassed at 500 °C. However, the maximum value of impact energy was only 25% of the impact energy of as-received F82H.

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Fig. 6. Optical micrographs of HIP interfaces for degassing temperatures of (a) 500, (b) 600, and (c) 700 °C.

Fig. 7. SEM–BSE micrographs of HIP interfaces for degassing temperatures of (a) 500, (b) 600, and (c) 700 °C.

Fig. 9. Charpy impact energy of HIP bonded specimens at room temperature for degassing temperatures of 500, 600, and 700 °C.

4. Conclusions

Fig. 8. Tensile properties of HIPped F82H as a function of degassing temperature.

TIG welding and HIP joining of RAFM steel were evaluated for the successful fabrication of the Korean Helium Cooled Solid Breeder ITER TBM. Various mechanical tests of TIG-welded RAFM steel were performed to obtain the optimal TIG welding process. Tensile test results of TIG welds showed the yield stress and ultimate tensile stress increased significantly as test temperature increases, especially above 450 °C. The total elongation of both base metal and weld metal was found to increase when the test temperature is above 450 °C. For HIP joining, the effect of degassing temperature was investigated to enhance joining quality. The degassing temperature has a minimal impact on the microstructure of the HIP interface. The room temperature Charpy impact energy of HIP bonded F82H was only about 25% of the base metal value. Compared with as-received F82H, tensile strength, yield stress and total elongation of HIP bonded F82H were reduced by about

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5%. Additional R&D is in progress to enhance the joint quality and to have a better understanding of joining processes. Acknowledgements This research was supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology & Ministry of Knowledge Economy (2009-0081593). Authors also appreciate H. Tanigawa from Japan Atomic Energy Agency for providing F82H TIG welding rod under Korea-Japan collaboration program. References [1] Seungyon Cho, Mu-Young Ahn, Duck Young Ku, Duck Hoi Kim, In-Keun Yu, Seunghee Han, Dong-Ik Kim, Han-Ki Yoon, Sang-Jin Lee, Fusion Eng. Des. 56 (2009) 216–220. [2] Sunghwan Yun, Nam Zin Cho, Mu-Young Ahn, Seungyon Cho, Fusion Sci. Technol. 56 (2009) 232–239. [3] Mu-young Ahn, Seungyon Cho, Duck Young Ku, Hyung-Seok Kim, Jae-seung Suh, Fusion Eng. Des. 84 (2009) 380–384.

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