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Influence of heat input on the microstructure and mechanical properties of CLAM steel multilayer butt-welded joints
Fusion Engineering and Design 152 (2020) 111413
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Influence of heat input on the microstructure and mechanical properties of CLAM steel multilayer butt-welded joints
T
Junyu Zhang, Kaixuan Cui, Bo Huang*, Xiaodong Mao, Mingjie Zheng* Key Laboratory of Neutronics and Radiation Safety, Institute of Nuclear Energy Safety Technology, Chinese Academy of Sciences, Hefei, Anhui 230031, China
A R T I C LE I N FO
A B S T R A C T
Keywords: CLAM Heat input Microstructure Mechanical properties
Tungsten inert gas (TIG) welding was considered as one of the candidate technologies to achieve the box structure of test blanket module (TBM). In this paper, the effects of heat input on the microstructure and mechanical properties of multilayer TIG butt-welded China Low Activation Martensitic (CLAM) steel were investigated by microstructure observations, Vickers hardness, tensile and Charpy impact tests. The results showed that the microstructure of the weld metal (WM) were coarse quenched martensite and residual delta-ferrite. When the heat input value increased from 1.73 kJ/mm to 2.41 kJ/mm, the width of martensite laths increased slightly and the delta-ferrite content decreased firstly then increased. Moreover, the increasing heat input resulted in remarkable tempering effect on the previous layer of the WM by the following deposited layers. The transverse hardness distribution of the joints showed that softening occurred in the heat-affected zone (HAZ), and the hardness of WM decreased slightly as the heat input increased. The ultimate tensile strength values of the joints with various heat input were all higher than 630 MPa, and the rupture position located at the HAZ. Charpy impact toughness of the as-welded joints met the toughness criteria for the welded joint (≥27 J) of creep resistant 9Cr steel according to the standard of ISO-3580.
1. Introduction China is developing tritium breeder blanket concepts for China Fusion Experimental Test Reactor (CFETR), i.e. the He-cooled ceramic blanket, the He-cooled LiPb blanket, and the water-cooled ceramic blanket [1–5]. China low activation martensitic (CLAM) steel, one kind of reduced activation ferritic/martensitic (RAFM) steels, has been considered as a structural material for the PbLi blankets and CFETR blankets, and China test blanket module (TBM) of International Thermonuclear Experimental Reactor (ITER) [6–9]. During the past one and half decades, great efforts have been devoted on CLAM steel development by Institute of Nuclear Energy Safety Technology (INEST), Chinese Academy of Sciences (CAS) under wide collaboration with many institutes and universities [10–12]. Compared with other RAFM steels, adjustment on chemical compositions of CLAM steel was made to achieve better properties [13]. The Tungsten (W) content of 1.5 wt.% could decrease the possibility of Laves phase precipitation and effectively improve the strength at on-service temperature. The Tantalum (Ta) content of 0.15 wt.% is much higher than that of other RAFM steels to refine the grain size and improve creep resistance at elevated temperature [14]. For its multifunctional purposes, TBM was designed with a ⁎
complicated box structure, consisting of different sub-components like first wall, breeder cassettes, cap plates, inlet/outlet pipes for coolant, etc. In the fabrication of these sub-components and final assembly of the TBM, several kinds of welding technologies, such as electron beam welding (EBW), tungsten inert gas (TIG) welding, laser-TIG hybrid welding, were employed for CLAM steel welding [15–19]. With good sealing gap allowance and the advantages of all-position welding, TIG welding was chosen as an important candidate welding technology for TBM assembly, and the study on weldability of CLAM steel by TIG welding has been conducted. Jiang et al. [20] investigated the microstructure and mechanical properties of the TIG welded joints of the CLAM steel with a heat input value of about 1.75 kJ/mm per layer, showing that the microstructure of the weld metal (WM) was coarse lath martensite with residual delta-ferrite and the properties of the joints could be improved by a reasonable post-welding heat treatment (PWHT). Zhu et al. [21] performed a pilot study on the TIG welding joints with the heat input of ∼0.416 kJ/mm in each weld pass, pointing out that residual delta ferrite in the WM would be the primary crack initiation sites. Lei et al. [22] studied the tensile properties and fracture behavior of TIG welded joints of CLAM steel at high temperatures and found that the precipitates and ferrite in the WM was the origin of micro-crack.
Corresponding authors. E-mail addresses: [email protected] (B. Huang), [email protected] (M. Zheng).
https://doi.org/10.1016/j.fusengdes.2019.111413 Received 27 August 2019; Accepted 15 November 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 152 (2020) 111413
J. Zhang, et al.
Table 1 Chemical compositions of CLAM plates and filler wires (wt.%).
Table 2 The main welding parameters.
Element
C
Cr
W
V
Ta
Mn
P
S
Parameters
Sample No.1
Sample No.2
Sample No.3
CLAM plates Filler wires
0.11 0.14
8.76 8.15
1.40 1.41
0.22 0.19
0.18 0.13
0.43 0.58
0.011 0.0064
0.006 0.0033
Pre-heating (°C) Welding current (A) Arc voltage (V) Welding speed (mm/min) Filler wire feed rate (m/min) Nominal heat input (kJ/mm) Ar gas flowing rate (L/min) Filler wire diameter (mm) Number of passes
250 150-160 10-12 60 0.5-0.6 1.73 11 1.2 7
250 190-200 10-13 60 0.6-0.8 2.26 11 1.2 7
250 220-225 10-12 60 0.8-1.0 2.41 11 1.2 7
The residual ferrite in WM was found to be detrimental to the toughness of the joint and its formation was mainly determined by the chemical composition of WM, cooling rate during welding [23]. Furthermore, the size of the weld pool could also affect the content of residual ferrite in the WM [24]. In previous work, a new filler wire for CLAM TIG welding was designed and proposed based on the chemical compositions of the base metal according to empirical equation and experimental analysis, expecting to lower the tendency towards the residual of delta-ferrite in the WM [25]. However, the cooling rate and the size of the weld pool were also affected by the heat input which would play a key role in determining the content of the residual ferrite with a given filler material. Moreover, the multi-pass welding would have a significant tempering effect on the microstructure and mechanical properties of the welded joints. Therefore, the study on effect of heat input in the CLAM multi-pass welding is a key issue to obtain joints with good comprehensive performance. In this study, multilayer butt-welded CLAM steel joints with various heat input values were prepared using a welding robot which could control the heat input value precisely. Metallurgical observations, hardness measurements, tensile and impact tests were conducted to investigate the influence of heat input on the microstructure and mechanical properties of the TIG multi-pass welded joints. This study can provide a reference for the application of TIG welding in the TBM assembly.
welded plate, then polished and etched for microstructure observations. Area fraction of delta-ferrite (AFD) in the welded joint was estimated by MATLAB software based on the statistical average value of at least 10 microphotographs for each welded joint. Vickers hardness test was carried out along the red dashed lines in Fig. 1 with the distance between adjacent indentations of 0.25 mm to show the transverse and vertical hardness distribution of the welded joints. The applied load of the test was 300 gf and the holding time was 10 s. Uni-axial tensile specimens and impact specimens were machined from the welded pads with the WM in the center of the specimens. The smooth cylindrical tensile specimens with gauge of Ø 5 mm × 25 (length) mm were subjected to quasi-static testing in an Instron 3369 type electronic universal material testing machine with the strain rate of 1 × 10−2 s−1 at RT. The standard impact samples with dimensions of 10 × 10 × 55 mm3 were tested by instrumented Charpy impact tests (ICIT) with the V-notch located in the WM. At least three specimens were used in tensile and impact tests, and the mechanical property data in this paper were the average value.
2. Experimental procedure
3. Results and discussion
The dimensions of the plates to be butt-welded in this paper are 150 mm × 60 mm × 12 mm. Before welding, the plates were normalized at 980 °C for 30 min followed by air cooling, and tempered at 760 °C for 90 min followed by air cooling to room temperature. The filler wires with modified compositions employed in this research were manufactured using cold drawing to final diameter of 1.2 mm. The chemical compositions of the CLAM plates (base metal) and filler wires were listed in Table 1. The main mechanical properties of the CLAM plates were given as follows: yield strength is 575 MPa, ultimate tensile strength is 704 MPa, total elongation is 21 %, and impact absorbed energy at room temperature (RT) is 184 J. Previous optimization on welding parameters was conducted to get a good appearance for the weld beads. The sequence of the weld passes are shown in Fig. 1 and the main welding parameters are listed in Table 2, respectively. After welding, specimens containing WM, heataffected zone (HAZ) and base metal (BM) were cut from each butt-
3.1. Microstructure The optical microstructures of the WM subject to different heat input values were the quenched martensite and delta-ferrite with a wide variation in morphology and size, as shown in Fig. 2. The deltaferrite content at the interface of the top two layers was higher than that at the center of WM for all three samples. Among the three listed heat input in Table 2, the sample with a heat input of 2.26 kJ/mm exhibited the lowest AFD values at both the interface area of the top two layers and the center of the WM, as shown in Fig. 2(c) and (d), respectively. The main AFD values at the interface and at the center of the weld bead were shown in Fig. 3 as mentioned in Ref. [6]. The residual delta-ferrite was obviously depressed to lower than 1.4 % with heat input of 2.26 kJ/mm. The main contributing factor for different AFD values under different heat input values was the time spent by the weld metal from δ +
Fig. 1. Dimensional details of the groove and sequence of the weld passes (the location of hardness indentations labeled with red dashed lines) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 2
Fusion Engineering and Design 152 (2020) 111413
J. Zhang, et al.
Fig. 2. The OM in the interface area of the top two layers (a), (c), (e) and the center of the WM (b), (d), (f) with the heat input of 1.73 kJ/mm (a), (b); 2.26 kJ/mm (c), (d); 2.41 kJ/mm (e), (f).
Fig. 3. Area fraction of delta-ferrite of the joints with different heat input values.
γ to γ phase fields (from point 1–2 as shown in Fig. 4) [22]. When the time was not enough for delta-ferrite to transform completely to austenite during solidification, the residual of delta-ferrite in the WM was observed. The more fraction of delta-ferrite retained with the higher cooling rate. The cooling rate was determined by many factors, such as heat input, pre-heating temperature, thickness of plate, etc. Since Rosenthal proposed analytical solution to predict cooling rate during welding, which based on an assumption that the heat source was a moving point and the heat transfer from the surface to the thickness direction was neglected, great efforts were devoted to establish an approach for predicting the cooling rate under actual welding process [26]. As for the determination relationship between cooling rate and heat input, there was still no consensus especially with an extended range of heat input values, which might due to more complicated phenomenon such as segregation occurred in the larger weld pool under higher heat input condition [27]. To further clarify the influence of heat input on the microstructure of CLAM joints, morphologies of martensite laths in the center of the WM was observed by TEM in Fig. 5. The martensite laths could be clearly observed according to Fig. 5(a) and (b), and the width of
Fig. 4. Equilibrium phase diagram of Fe-xC-9Cr alloy.
martensite laths at the heat input of 1.73 kJ/mm and 2.26 kJ/mm was 0.23 μm and 0.29 μm respectively on basis of at least 20 microphotographs analysis. The tempering effect at the heat input of 2.41 kJ/ mm was more remarkable, as the boundaries of martensite laths was not clear as shown in Fig. 5(c). 3.2. Hardness distribution The Vickers hardness distributions in the vertical and transverse directions of the cross section of the joints were given in Figs. 6 and 7, respectively. As shown in Fig. 6(a), the vertical hardness distribution of the three welded joints with different heat input values shared the same tendency, which decreased at a certain distance from the top surface of the weld pads. It could account for the self-tempering effect of the multilayer welding, the quenched martensite in the previous deposited layers of the WM were tempered by the heat input of following layers. Moreover, the abnormal hardness fluctuation of the joint would be induced by the delta ferrite in the WM, as the Fig. 6(b) shown. 3
Fusion Engineering and Design 152 (2020) 111413
J. Zhang, et al.
Fig. 5. Morphologies of martensite laths in the center of the WM with different heat input values of: (a) 1.73 kJ/mm, (b) 2.26 kJ/mm; (c) 2.41 kJ/mm.
Fig. 6. The hardness vertical distribution of the joints: (a) the hardness distribution diagram; (b) the enlarged view of specific hardness indentation near delta ferrite.
Fig. 7. The hardness transverse distributions at the (a) top and (b) bottom of the joints.
input conditions. The hardness value near the interface of the WM and the HAZ was higher than that in the center of the WM, owing to the coarse quenching martensite formed since the high cooling rate near the interface. As shown from the hardness distribution profiles, the weld pool was enlarged gradually with the increasing heat input value, which might cause segregation phenomenon. The hardness transverse distribution along the bottom line in Fig. 7(b) displayed a similar tendency of that along the top line. However, the maximum value was less than 285 HV much lower than the top one indicating the self-tempering effect of the multilayer welding again. Softening occurred in HAZ was also observed from hardness distribution along both along top and bottom line.
Table 3 Tensile properties of the joints. Tensile properties
Sample No.1
Sample No.2
Sample No.3
UTS (MPa) YS (MPa) % Elongation % Reduction in Area
631 476 15.67 70.8
640 492 16.17 71.4
645 499 15.07 68.2
The hardness transverse distributions along the top and bottom line as labeled in Fig. 1 were shown in Fig. 7(a) and (b), respectively. According to Fig. 7(a), a sharp shift of hardness at the interface of the WM and HAZ and the similar width of the HAZ were found in the three heat 4
Fusion Engineering and Design 152 (2020) 111413
J. Zhang, et al.
Fig. 8. The fractographs of tensile test specimens (a) 1.73 kJ/mm, (b) 2.26 kJ/mm; (c) 2.41 kJ/mm.
Fig. 9. Force-displacement diagram (a) and absorbed energy (b) of V-notched impact specimens with different heat input values, and the typical fractographs of the impact specimens in initiation stage (c) and propagation stage (d).
showed relative high impact absorbed energy because of the remarkable tempering effect. The typical fractographs of the specimens in initiation and propagation stage were shown in Fig. 9(c) and (d) respectively. The fractograph in initiation stage exhibited quasi-cleavage crack consisting of massive tear ridges, while that in propagation stage revealed typical cleavage crack characteristics. Toughness of the as welded joints with heat input of 2.26 kJ/mm and 2.41 kJ/mm met the toughness criteria for the welded joint (≥27 J) of creep resistant 9Cr steel according to ISO 3580.
3.3. Tensile properties The main results of tensile tests of the joints were listed in Table 3. The ultimate tensile strength (UTS) of the specimens were all above 630 MPa, which was more than 90 % that of the base metal. The rupture positions located at the HAZ of the welded joints, where the softening occurred as shown in hardness distribution (Fig. 6). The SEM fractographs of the tensile specimens were shown in Fig. 8. As shown, although the fracture mode was ductile, the fracture surfaces of the tested specimens revealed a slight difference on the size and density of dimples. The dimples in Fig. 8(b) and (c) were larger and deeper representing comparable better ductility.
4. Conclusions Multi-pass butt welded joints of CLAM steel were prepared with various heat input values, and the microstructure and mechanical properties of the joints were investigated. The main results are listed as follows:
3.4. Impact toughness Based on the ICIT data acquisition system, force-displacement diagram of the impact specimens was obtained and shown in Fig. 9(a). The total absorbed energy could be divided into the energy for crack initiation and propagation [28], and the crack initiation energy for all three conditions was almost identical according to Fig. 9(b). The crack propagation energy of specimens with heat input of 1.73 kJ/mm was relatively lower, which would be related to the high delta-ferrite content. However, the specimens with the heat input of 2.41 kJ/mm
1) The microstructures of the WM under different heat input conditions were the quenched martensite and delta-ferrite. The delta-ferrite content at the interface area of the WM and HAZ was higher than that in the center of the WM. Relative lower AFD was achieved at the heat input of 2.26 kJ/mm. 5
Fusion Engineering and Design 152 (2020) 111413
J. Zhang, et al.
2) With the increase of the heat input, the width of martensite laths increased slightly, and the tempering effect on the previous deposited layers of the WM by the following layers was remarkable. 3) The ultimate tensile strength of all the specimens reached more than 90 % that of BM. The rupture positions located at the HAZ of the joints where the softening occurred. 4) The crack initiation energy for the three heat input conditions were almost identical. However, the crack propagation energy of the samples with heat input of 2.26 kJ/mm and 2.41 kJ/mm were higher, and the total impact absorbed energy met the toughness criteria for welded joint (≥27 J) of creep resistant 9Cr steel based on the standard of ISO 3580.
cooled breeder blanket for CFETR, Fusion Eng. Des. 89 (2014) 1380–1385. [6] Q. Huang, Status and improvement of CLAM for nuclear application, Nucl. Fusion 57 (2017) 086042. [7] Q. Huang, N. Baluc, Y. Dai, S. Jitsukawa, A. Kimura, J. Konys, et al., Recent progress of R&D activities on reduced activation ferritic/martensitic steels, J. Nucl. Mater. 442 (2013) S2–S8. [8] Y. Wu, Design status and development strategy of China liquid lithium-lead blankets and related material technology, J. Nucl. Mater. 367 (2007) 1410–1415. [9] Q. Huang, S. Gao, Z. Zhu, M. Zhang, Y. Song, C. Li, et al., Progress in compatibility experiments on lithium–lead with candidate structural materials for fusion in China, Fusion Eng. Des. 84 (2009) 242–246. [10] Q. Huang, C. Li, Q. Wu, S. Liu, S. Gao, Z. Guo, et al., Progress in development of CLAM steel and fabrication of small TBM in China, J. Nucl. Mater. 417 (2011) 85–88. [11] J. Yu, Q. Huang, F. Wan, Research and development on the China low activation martensitic steel (CLAM), J. Nucl. Mater. 367 (2007) 97–101. [12] Q. Huang, Development status of CLAM steel for fusion application, J. Nucl. Mater. 455 (2014) 649–654. [13] Q. Huang, C. Li, Y. Li, M. Chen, M. Zhang, L. Peng, et al., Progress in development of china low activation martensitic steel for fusion application, J. Nucl. Mater. 367 (2007) 142–146. [14] B. Zhong, B. Huang, C. Li, S. Liu, G. Xu, Y. Zhao, et al., Creep deformation and rupture behavior of CLAM steel at 823K and 873K, J. Nucl. Mater. 455 (2014) 640–644. [15] Y. Zhao, C. Li, B. Huang, S. Liu, Q. Huang, Verification of the effect of surface preparation on Hot Isostatic pressing diffusion bonding joints of CLAM steel, J. Nucl. Mater. 455 (2014) 486–490. [16] J. Zhang, B. Huang, Y. Zhai, S. Liu, Q. Wu, Q. Huang, et al., Overview on the welding technologies of CLAM steel and the DFLL TBM fabrication, Nucl. Mater. Energy 9 (2016) 317–323. [17] X. Li, J. Chen, P. Hua, K. Chen, W. Kong, H. Chu, et al., Effect of post weld heat treatment on the microstructure and properties of Laser-TIG hybrid welded joints for CLAM steel, Fusion Eng. Des. 128 (2018) 175–181. [18] C. Li, Q. Huang, P. Zhang, Effect of surface preparation on CLAM/CLAM hot isostatic pressing diffusion bonding joints, J. Nucl. Mater. 386–88 (2009) 550–552. [19] Q. Wu, S. Zheng, S. Liu, C. Li, Q. Huang, Effect of post-weld heat treatment on the mechanical properties of electron beam welded joints for CLAM steel, J. Nucl. Mater. 442 (2013) 512–517. [20] Z. Jiang, L. Ren, J. Huang, X. Ju, H. Wu, Q. Huang, et al., Microstructure and mechanical properties of the TIG welded joints of fusion CLAM steel, Fusion Eng. Des. 85 (2010) 1903–1908. [21] Q. Zhu, Y. Lei, X. Chen, W. Ren, X. Ju, Y. Ye, Microstructure and mechanical properties in TIG welding of CLAM steel, Fusion Eng. Des. 86 (2011) 407–411. [22] Y. Lei, C. Xiao, X. Wang, J. Yue, Q. Zhu, Tensile properties and fracturing behavior of weld joints in the CLAM at high temperatures, Fusion Eng. Des. 95 (2015) 27–33. [23] B. Ganesh, S. Raju, A. Rai, E. Mohandas, M. Vijayalakshmi, K. Rao, B. Raj, Differential scanning calorimetry study of diffusional and martensite phase transformation in some 9wt. Cr low carbon ferritic steels, Mater. Sci. Tech. 27 (2011) 500–512. [24] S. Sam, C. Das, V. Ramasubbu, S. Albert, A. Bhaduri, T. Jayakumar, et al., Delta ferrite in the weld metal of reduced activation ferritic martensitic steel, J. Nucl. Mater. 455 (2014) 343–348. [25] B. Huang, J. Zhang, Q. Wu, Microstructure and mechanical properties of China low activation martensitic steel joint by TIG multi-pass welding with a new filler wire, J. Nucl. Mater. 490 (2017) 115–124. [26] K. Poorhaydari, B. Patchett, D. Ivey, Estimation of cooling rate in the welding of plates with intermediate thickness, Weld. J. 84 (2005) 149s–155s. [27] B. Arivazhagan, G. Srinivasan, S. Albert, A. Bhaduri, A study on influence of heat input variation on microstructure of reduced activation ferritic martensitic steel weld metal produced by GTAW process, Fusion Eng. Des. 86 (2011) 192–197. [28] T. Kobayashi, On the information about fracture characteristics obtained from instrumented impact test of A533 steel for reactor pressure vessel, Eng. Fract. Mech. 19 (1984) 67–79.
Based on the residual delta ferrite content in the WM, softening effect in the HAZ and the strength and toughness testing results, the joints with relative high comprehensive performance were obtained when the heat input was 2.26 kJ/mm in this research. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported and funded by the National Magnetic Confinement Fusion Science Program of China with Grant No. 2018YFE0306102, the National Key Research and Development Plan of China with the Grant No. 2017YFE0300601, the National Natural Science Foundation of China with Grant Nos. 11632001 and 51601190, the Provincial Natural Science Foundation of Anhui with Grant No. 1808085QE163, and Youth Innovation Promotion Association of the Chinese Academy of Sciences with grant No. 2017486. The authors give thanks to Prof. Yican Wu, Prof. Qunying Huang and Dr. Shaojun Liu for the guidance on this work and show great appreciation to other members in FDS team for their support and contribution to the research. References [1] Y. Wu, Conceptual design activities of FDS series fusion power plants in China, Fusion Eng. Des. 81 (2006) 2713–2718. [2] Y. Wu, F. Team, Conceptual design of the China fusion power plant FDS-II, Fusion Eng. Des. 83 (2008) 1683–1689. [3] Y. Wu, J. Jiang, M. Wang, M. Jin, A fusion-driven subcritical system concept based on viable technologies, Nucl. Fusion 51 (2011) 103036. [4] M. Ni, C. Lian, S. Zhang, B. Nie, J. Jiang, Structural design and preliminary analysis of liquid lead–lithium blanket for China Fusion Engineering Test Reactor, Fusion Eng. Des. 94 (2015) 61–66. [5] S. Liu, Y. Pu, X. Cheng, J. Li, C. Peng, X. Ma, et al., Conceptual design of a water
6
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Influence of heat input on the microstructure and mechanical properties of CLAM steel multilayer butt-welded joints
T
Junyu Zhang, Kaixuan Cui, Bo Huang*, Xiaodong Mao, Mingjie Zheng* Key Laboratory of Neutronics and Radiation Safety, Institute of Nuclear Energy Safety Technology, Chinese Academy of Sciences, Hefei, Anhui 230031, China
A R T I C LE I N FO
A B S T R A C T
Keywords: CLAM Heat input Microstructure Mechanical properties
Tungsten inert gas (TIG) welding was considered as one of the candidate technologies to achieve the box structure of test blanket module (TBM). In this paper, the effects of heat input on the microstructure and mechanical properties of multilayer TIG butt-welded China Low Activation Martensitic (CLAM) steel were investigated by microstructure observations, Vickers hardness, tensile and Charpy impact tests. The results showed that the microstructure of the weld metal (WM) were coarse quenched martensite and residual delta-ferrite. When the heat input value increased from 1.73 kJ/mm to 2.41 kJ/mm, the width of martensite laths increased slightly and the delta-ferrite content decreased firstly then increased. Moreover, the increasing heat input resulted in remarkable tempering effect on the previous layer of the WM by the following deposited layers. The transverse hardness distribution of the joints showed that softening occurred in the heat-affected zone (HAZ), and the hardness of WM decreased slightly as the heat input increased. The ultimate tensile strength values of the joints with various heat input were all higher than 630 MPa, and the rupture position located at the HAZ. Charpy impact toughness of the as-welded joints met the toughness criteria for the welded joint (≥27 J) of creep resistant 9Cr steel according to the standard of ISO-3580.
1. Introduction China is developing tritium breeder blanket concepts for China Fusion Experimental Test Reactor (CFETR), i.e. the He-cooled ceramic blanket, the He-cooled LiPb blanket, and the water-cooled ceramic blanket [1–5]. China low activation martensitic (CLAM) steel, one kind of reduced activation ferritic/martensitic (RAFM) steels, has been considered as a structural material for the PbLi blankets and CFETR blankets, and China test blanket module (TBM) of International Thermonuclear Experimental Reactor (ITER) [6–9]. During the past one and half decades, great efforts have been devoted on CLAM steel development by Institute of Nuclear Energy Safety Technology (INEST), Chinese Academy of Sciences (CAS) under wide collaboration with many institutes and universities [10–12]. Compared with other RAFM steels, adjustment on chemical compositions of CLAM steel was made to achieve better properties [13]. The Tungsten (W) content of 1.5 wt.% could decrease the possibility of Laves phase precipitation and effectively improve the strength at on-service temperature. The Tantalum (Ta) content of 0.15 wt.% is much higher than that of other RAFM steels to refine the grain size and improve creep resistance at elevated temperature [14]. For its multifunctional purposes, TBM was designed with a ⁎
complicated box structure, consisting of different sub-components like first wall, breeder cassettes, cap plates, inlet/outlet pipes for coolant, etc. In the fabrication of these sub-components and final assembly of the TBM, several kinds of welding technologies, such as electron beam welding (EBW), tungsten inert gas (TIG) welding, laser-TIG hybrid welding, were employed for CLAM steel welding [15–19]. With good sealing gap allowance and the advantages of all-position welding, TIG welding was chosen as an important candidate welding technology for TBM assembly, and the study on weldability of CLAM steel by TIG welding has been conducted. Jiang et al. [20] investigated the microstructure and mechanical properties of the TIG welded joints of the CLAM steel with a heat input value of about 1.75 kJ/mm per layer, showing that the microstructure of the weld metal (WM) was coarse lath martensite with residual delta-ferrite and the properties of the joints could be improved by a reasonable post-welding heat treatment (PWHT). Zhu et al. [21] performed a pilot study on the TIG welding joints with the heat input of ∼0.416 kJ/mm in each weld pass, pointing out that residual delta ferrite in the WM would be the primary crack initiation sites. Lei et al. [22] studied the tensile properties and fracture behavior of TIG welded joints of CLAM steel at high temperatures and found that the precipitates and ferrite in the WM was the origin of micro-crack.
Corresponding authors. E-mail addresses: [email protected] (B. Huang), [email protected] (M. Zheng).
https://doi.org/10.1016/j.fusengdes.2019.111413 Received 27 August 2019; Accepted 15 November 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 152 (2020) 111413
J. Zhang, et al.
Table 1 Chemical compositions of CLAM plates and filler wires (wt.%).
Table 2 The main welding parameters.
Element
C
Cr
W
V
Ta
Mn
P
S
Parameters
Sample No.1
Sample No.2
Sample No.3
CLAM plates Filler wires
0.11 0.14
8.76 8.15
1.40 1.41
0.22 0.19
0.18 0.13
0.43 0.58
0.011 0.0064
0.006 0.0033
Pre-heating (°C) Welding current (A) Arc voltage (V) Welding speed (mm/min) Filler wire feed rate (m/min) Nominal heat input (kJ/mm) Ar gas flowing rate (L/min) Filler wire diameter (mm) Number of passes
250 150-160 10-12 60 0.5-0.6 1.73 11 1.2 7
250 190-200 10-13 60 0.6-0.8 2.26 11 1.2 7
250 220-225 10-12 60 0.8-1.0 2.41 11 1.2 7
The residual ferrite in WM was found to be detrimental to the toughness of the joint and its formation was mainly determined by the chemical composition of WM, cooling rate during welding [23]. Furthermore, the size of the weld pool could also affect the content of residual ferrite in the WM [24]. In previous work, a new filler wire for CLAM TIG welding was designed and proposed based on the chemical compositions of the base metal according to empirical equation and experimental analysis, expecting to lower the tendency towards the residual of delta-ferrite in the WM [25]. However, the cooling rate and the size of the weld pool were also affected by the heat input which would play a key role in determining the content of the residual ferrite with a given filler material. Moreover, the multi-pass welding would have a significant tempering effect on the microstructure and mechanical properties of the welded joints. Therefore, the study on effect of heat input in the CLAM multi-pass welding is a key issue to obtain joints with good comprehensive performance. In this study, multilayer butt-welded CLAM steel joints with various heat input values were prepared using a welding robot which could control the heat input value precisely. Metallurgical observations, hardness measurements, tensile and impact tests were conducted to investigate the influence of heat input on the microstructure and mechanical properties of the TIG multi-pass welded joints. This study can provide a reference for the application of TIG welding in the TBM assembly.
welded plate, then polished and etched for microstructure observations. Area fraction of delta-ferrite (AFD) in the welded joint was estimated by MATLAB software based on the statistical average value of at least 10 microphotographs for each welded joint. Vickers hardness test was carried out along the red dashed lines in Fig. 1 with the distance between adjacent indentations of 0.25 mm to show the transverse and vertical hardness distribution of the welded joints. The applied load of the test was 300 gf and the holding time was 10 s. Uni-axial tensile specimens and impact specimens were machined from the welded pads with the WM in the center of the specimens. The smooth cylindrical tensile specimens with gauge of Ø 5 mm × 25 (length) mm were subjected to quasi-static testing in an Instron 3369 type electronic universal material testing machine with the strain rate of 1 × 10−2 s−1 at RT. The standard impact samples with dimensions of 10 × 10 × 55 mm3 were tested by instrumented Charpy impact tests (ICIT) with the V-notch located in the WM. At least three specimens were used in tensile and impact tests, and the mechanical property data in this paper were the average value.
2. Experimental procedure
3. Results and discussion
The dimensions of the plates to be butt-welded in this paper are 150 mm × 60 mm × 12 mm. Before welding, the plates were normalized at 980 °C for 30 min followed by air cooling, and tempered at 760 °C for 90 min followed by air cooling to room temperature. The filler wires with modified compositions employed in this research were manufactured using cold drawing to final diameter of 1.2 mm. The chemical compositions of the CLAM plates (base metal) and filler wires were listed in Table 1. The main mechanical properties of the CLAM plates were given as follows: yield strength is 575 MPa, ultimate tensile strength is 704 MPa, total elongation is 21 %, and impact absorbed energy at room temperature (RT) is 184 J. Previous optimization on welding parameters was conducted to get a good appearance for the weld beads. The sequence of the weld passes are shown in Fig. 1 and the main welding parameters are listed in Table 2, respectively. After welding, specimens containing WM, heataffected zone (HAZ) and base metal (BM) were cut from each butt-
3.1. Microstructure The optical microstructures of the WM subject to different heat input values were the quenched martensite and delta-ferrite with a wide variation in morphology and size, as shown in Fig. 2. The deltaferrite content at the interface of the top two layers was higher than that at the center of WM for all three samples. Among the three listed heat input in Table 2, the sample with a heat input of 2.26 kJ/mm exhibited the lowest AFD values at both the interface area of the top two layers and the center of the WM, as shown in Fig. 2(c) and (d), respectively. The main AFD values at the interface and at the center of the weld bead were shown in Fig. 3 as mentioned in Ref. [6]. The residual delta-ferrite was obviously depressed to lower than 1.4 % with heat input of 2.26 kJ/mm. The main contributing factor for different AFD values under different heat input values was the time spent by the weld metal from δ +
Fig. 1. Dimensional details of the groove and sequence of the weld passes (the location of hardness indentations labeled with red dashed lines) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 2
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Fig. 2. The OM in the interface area of the top two layers (a), (c), (e) and the center of the WM (b), (d), (f) with the heat input of 1.73 kJ/mm (a), (b); 2.26 kJ/mm (c), (d); 2.41 kJ/mm (e), (f).
Fig. 3. Area fraction of delta-ferrite of the joints with different heat input values.
γ to γ phase fields (from point 1–2 as shown in Fig. 4) [22]. When the time was not enough for delta-ferrite to transform completely to austenite during solidification, the residual of delta-ferrite in the WM was observed. The more fraction of delta-ferrite retained with the higher cooling rate. The cooling rate was determined by many factors, such as heat input, pre-heating temperature, thickness of plate, etc. Since Rosenthal proposed analytical solution to predict cooling rate during welding, which based on an assumption that the heat source was a moving point and the heat transfer from the surface to the thickness direction was neglected, great efforts were devoted to establish an approach for predicting the cooling rate under actual welding process [26]. As for the determination relationship between cooling rate and heat input, there was still no consensus especially with an extended range of heat input values, which might due to more complicated phenomenon such as segregation occurred in the larger weld pool under higher heat input condition [27]. To further clarify the influence of heat input on the microstructure of CLAM joints, morphologies of martensite laths in the center of the WM was observed by TEM in Fig. 5. The martensite laths could be clearly observed according to Fig. 5(a) and (b), and the width of
Fig. 4. Equilibrium phase diagram of Fe-xC-9Cr alloy.
martensite laths at the heat input of 1.73 kJ/mm and 2.26 kJ/mm was 0.23 μm and 0.29 μm respectively on basis of at least 20 microphotographs analysis. The tempering effect at the heat input of 2.41 kJ/ mm was more remarkable, as the boundaries of martensite laths was not clear as shown in Fig. 5(c). 3.2. Hardness distribution The Vickers hardness distributions in the vertical and transverse directions of the cross section of the joints were given in Figs. 6 and 7, respectively. As shown in Fig. 6(a), the vertical hardness distribution of the three welded joints with different heat input values shared the same tendency, which decreased at a certain distance from the top surface of the weld pads. It could account for the self-tempering effect of the multilayer welding, the quenched martensite in the previous deposited layers of the WM were tempered by the heat input of following layers. Moreover, the abnormal hardness fluctuation of the joint would be induced by the delta ferrite in the WM, as the Fig. 6(b) shown. 3
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Fig. 5. Morphologies of martensite laths in the center of the WM with different heat input values of: (a) 1.73 kJ/mm, (b) 2.26 kJ/mm; (c) 2.41 kJ/mm.
Fig. 6. The hardness vertical distribution of the joints: (a) the hardness distribution diagram; (b) the enlarged view of specific hardness indentation near delta ferrite.
Fig. 7. The hardness transverse distributions at the (a) top and (b) bottom of the joints.
input conditions. The hardness value near the interface of the WM and the HAZ was higher than that in the center of the WM, owing to the coarse quenching martensite formed since the high cooling rate near the interface. As shown from the hardness distribution profiles, the weld pool was enlarged gradually with the increasing heat input value, which might cause segregation phenomenon. The hardness transverse distribution along the bottom line in Fig. 7(b) displayed a similar tendency of that along the top line. However, the maximum value was less than 285 HV much lower than the top one indicating the self-tempering effect of the multilayer welding again. Softening occurred in HAZ was also observed from hardness distribution along both along top and bottom line.
Table 3 Tensile properties of the joints. Tensile properties
Sample No.1
Sample No.2
Sample No.3
UTS (MPa) YS (MPa) % Elongation % Reduction in Area
631 476 15.67 70.8
640 492 16.17 71.4
645 499 15.07 68.2
The hardness transverse distributions along the top and bottom line as labeled in Fig. 1 were shown in Fig. 7(a) and (b), respectively. According to Fig. 7(a), a sharp shift of hardness at the interface of the WM and HAZ and the similar width of the HAZ were found in the three heat 4
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Fig. 8. The fractographs of tensile test specimens (a) 1.73 kJ/mm, (b) 2.26 kJ/mm; (c) 2.41 kJ/mm.
Fig. 9. Force-displacement diagram (a) and absorbed energy (b) of V-notched impact specimens with different heat input values, and the typical fractographs of the impact specimens in initiation stage (c) and propagation stage (d).
showed relative high impact absorbed energy because of the remarkable tempering effect. The typical fractographs of the specimens in initiation and propagation stage were shown in Fig. 9(c) and (d) respectively. The fractograph in initiation stage exhibited quasi-cleavage crack consisting of massive tear ridges, while that in propagation stage revealed typical cleavage crack characteristics. Toughness of the as welded joints with heat input of 2.26 kJ/mm and 2.41 kJ/mm met the toughness criteria for the welded joint (≥27 J) of creep resistant 9Cr steel according to ISO 3580.
3.3. Tensile properties The main results of tensile tests of the joints were listed in Table 3. The ultimate tensile strength (UTS) of the specimens were all above 630 MPa, which was more than 90 % that of the base metal. The rupture positions located at the HAZ of the welded joints, where the softening occurred as shown in hardness distribution (Fig. 6). The SEM fractographs of the tensile specimens were shown in Fig. 8. As shown, although the fracture mode was ductile, the fracture surfaces of the tested specimens revealed a slight difference on the size and density of dimples. The dimples in Fig. 8(b) and (c) were larger and deeper representing comparable better ductility.
4. Conclusions Multi-pass butt welded joints of CLAM steel were prepared with various heat input values, and the microstructure and mechanical properties of the joints were investigated. The main results are listed as follows:
3.4. Impact toughness Based on the ICIT data acquisition system, force-displacement diagram of the impact specimens was obtained and shown in Fig. 9(a). The total absorbed energy could be divided into the energy for crack initiation and propagation [28], and the crack initiation energy for all three conditions was almost identical according to Fig. 9(b). The crack propagation energy of specimens with heat input of 1.73 kJ/mm was relatively lower, which would be related to the high delta-ferrite content. However, the specimens with the heat input of 2.41 kJ/mm
1) The microstructures of the WM under different heat input conditions were the quenched martensite and delta-ferrite. The delta-ferrite content at the interface area of the WM and HAZ was higher than that in the center of the WM. Relative lower AFD was achieved at the heat input of 2.26 kJ/mm. 5
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2) With the increase of the heat input, the width of martensite laths increased slightly, and the tempering effect on the previous deposited layers of the WM by the following layers was remarkable. 3) The ultimate tensile strength of all the specimens reached more than 90 % that of BM. The rupture positions located at the HAZ of the joints where the softening occurred. 4) The crack initiation energy for the three heat input conditions were almost identical. However, the crack propagation energy of the samples with heat input of 2.26 kJ/mm and 2.41 kJ/mm were higher, and the total impact absorbed energy met the toughness criteria for welded joint (≥27 J) of creep resistant 9Cr steel based on the standard of ISO 3580.
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Based on the residual delta ferrite content in the WM, softening effect in the HAZ and the strength and toughness testing results, the joints with relative high comprehensive performance were obtained when the heat input was 2.26 kJ/mm in this research. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported and funded by the National Magnetic Confinement Fusion Science Program of China with Grant No. 2018YFE0306102, the National Key Research and Development Plan of China with the Grant No. 2017YFE0300601, the National Natural Science Foundation of China with Grant Nos. 11632001 and 51601190, the Provincial Natural Science Foundation of Anhui with Grant No. 1808085QE163, and Youth Innovation Promotion Association of the Chinese Academy of Sciences with grant No. 2017486. The authors give thanks to Prof. Yican Wu, Prof. Qunying Huang and Dr. Shaojun Liu for the guidance on this work and show great appreciation to other members in FDS team for their support and contribution to the research. References [1] Y. Wu, Conceptual design activities of FDS series fusion power plants in China, Fusion Eng. Des. 81 (2006) 2713–2718. [2] Y. Wu, F. Team, Conceptual design of the China fusion power plant FDS-II, Fusion Eng. Des. 83 (2008) 1683–1689. [3] Y. Wu, J. Jiang, M. Wang, M. Jin, A fusion-driven subcritical system concept based on viable technologies, Nucl. Fusion 51 (2011) 103036. [4] M. Ni, C. Lian, S. Zhang, B. Nie, J. Jiang, Structural design and preliminary analysis of liquid lead–lithium blanket for China Fusion Engineering Test Reactor, Fusion Eng. Des. 94 (2015) 61–66. [5] S. Liu, Y. Pu, X. Cheng, J. Li, C. Peng, X. Ma, et al., Conceptual design of a water
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