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Microstructure and mechanical properties of the TIG welded joints of fusion CLAM steel
Fusion Engineering and Design 85 (2010) 1903–1908
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Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Microstructure and mechanical properties of the TIG welded joints of fusion CLAM steel Zhizhong Jiang a,∗ , Litian Ren a , Jihua Huang a , Xin Ju a , Huibin Wu a , Qunying Huang b , Yican Wu b a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Xueyuan Road, Beijing 100083, China Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China
a r t i c l e
i n f o
Article history: Available online 15 July 2010 Keywords: CLAM steel Tungsten inert gas arc welding Microstructure Mechanical properties
a b s t r a c t The CLAM steel plates were butt-welded through manual tungsten inert gas welding (TIG) process, and the following post-welding heat treatment (PWHT) at 740 ◦ C for 1 h. The microstructure and mechanical properties of the welded joints were measured. The results show that both hardening and softening occur in the weld joints before PWHT, but the hardening is not removed completely in the weld metal and the fusion zone after PWHT. In as-welded condition, the microstructure of the weld metal is coarse lath martensite, and softened zone in heat-affected zone (HAZ) consists of a mixture of tempered martensite and ferrite. After PWHT, a lot of carbides precipitate at all zones in weld joints. The microstructure of softened zone transforms to tempered sorbite. Tensile strength of the weld metal is higher than that of HAZ and base metal regardless of PWHT. However, the weld metal has poor toughness without PWHT. The impact energy of the weld metal after PWHT reaches almost the same level as the base metal. So it is concluded that microstructure and mechanical properties of the CLAM steel welded joints can be improved by a reasonable PWHT. © 2010 Elsevier B.V. All rights reserved.
1. Introduction With good irradiation swelling resistance, thermo-physical and thermo-mechanical properties, reduced activation ferritic/martensitic (RAFM) steels are recognized as the leading candidate structural materials for test blanket module (TBM) in ITER and first wall and tritium breeding blankets in DEMO reactor [1]. The composition designs of the China low activation martensitic (CLAM) steel had been started since 2001. Some physical and mechanical properties have been tested in recent years, which are similar to JLF-1 and Eurofer 97 [2,3]. At present, the development of CLAM steel is at stage of large scale smelting and appraisals for its weldability [4]. Fusion welding methods, such as tungsten inert gas (TIG) welding, electron beam welding (EBW) and laser beam welding (LBW), are considered to be essential manufacturing technologies for TBM constructions. The relevant research institutes in Europe and Japan have made extensive attempts for fabrication and components assembly of small-sized TBM [5,6]. According to the structure features of TBM conceptions for ITER in China, TIG welding will be applied to produce sealing, such as for headers of the first wall, side wall, and
∗ Corresponding author. Tel.: +86 10 62334859. E-mail address: [email protected] (Z. Jiang). 0920-3796/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2010.06.019
back wall, and TIG welding was chosen for sealing, because it has the largest gap allowance compared to the other welding methods. In order to accumulate practical experiences for the fabrication of TBM and establish database on properties of CLAM steel, the research on weldability for CLAM steel is greatly needed. In this work, the TIG welding experiments of CLAM steel have been carried out, then the microstructure and mechanical properties of the welded joints are investigated before and after PWHT.
2. Materials and experimental procedures CLAM steel plates with 13 mm thick for welding experiments were supplied by the National Engineering Research Center of Advanced Rolling in USTB. The plates after rolling were quenched at 980 ◦ C for 30 min and followed by water-cooling, then tempered at 760 ◦ C for 90 min and followed by air-cooling. The actual measured compositions and mechanical properties of the paltes are listed in Tables 1 and 2, respectively. Two plates were butt-welded by manual TIG process. Double-Y groove was machined to control welding deformation as shown schematically in Fig. 1. The filler wires with cross-section of 2 mm × 2 mm were extracted from CLAM plates. The details of welding conditions are listed in Table 3. Specimens used for microstructure examination and hardness tests were transversally sectioned, mechanically grounded and polished, then etched in a mixed solution of 95 ml C2 H5 OH + 5 ml
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Z. Jiang et al. / Fusion Engineering and Design 85 (2010) 1903–1908
Table 1 The actual measured compositions of CLAM steel (wt%). C
Cr
W
V
Mn
Ta
S
Nb
Ni
Mo
P
N
Fe
0.093
9.15
1.49
0.20
0.44
0.097
< 0.003
< 0.01
< 0.015
< 0.005
0.0051
< 0.0047
Bal.
Fig. 1. Schematic diagram of the double-Y groove.
Table 2 Mechanical properties of CLAM steel plates. Temperature (◦ C)
25 600
Strength (MPa) YS
UTS
567 392
686 407
Elongation (%)
Reduction of area (%)
26 14
74.7 87
Table 3 TIG welding conditions. Welding current Welding voltage Welding speed Preheating temperature Interlayer temperature Number of passes PWHT condition
220–240 A 10.8 V 80–90 mm/min <200 ◦ C <200 ◦ C 8 740 ◦ C/1 h (vacuum furnace)
HCl + 1 g C6 H3 O7 N3 . The microstructure was finally examined by optical microscope. Hardness distribution at various positions on the welded joints was measured using a Vickers hardness tester, with testing load of 1 kg holding for 15 s. Standard V-notch Charpy specimens of 10 mm × 10 mm × 55 mm were machined from welded joints with their notched roots located in the base metal, HAZ and weld metal, respectively, the axis of V-notch for each impact specimen was perpendicular to the surface of welded plates. After impact testing, fracture surfaces of the impact specimens were examined carefully under a scanning electron microscope (SEM). Plate type tensile specimens were used to measure the strength of the weld joints. The gauge size of the tensile specimens is 11T mm × 25W mm × 77L mm. All the tests were carried out at room temperature (25 ◦ C). 3. Results and discussion 3.1. Hardness tests and microstructure observation No obvious defects were found by means of the nondestructive examinations, X-rays and ultra-sound testing. The macrograph and Vickers hardness distribution of the welded joints are presented in Fig. 2. The weld metal, HAZ and base metal can be clearly distinguished in the welded joints (see Fig. 2(a)). The trends of the hardness variation are presented in Fig. 2(b) and (c). The locations for measurements are also depicted in Fig. 2(a): the top line locates at 2 mm from the top surface of the welded plate. The central line corresponds to the horizontal center of the welded plate. As shown in Fig. 2(b) and (c), in as-welded condition, the hardness values at different positions along the top line are higher than those along the central line, especially for the weld metal and fusion zone. The hard-
Fig. 2. Vickers hardness distribution cross-welded joint: (a) macrograph of the welded joint, (b) hardness distribution measured along top line, (c) hardness distribution measured along central line.
ening phenomenon emerges at weld metal and fusion zone in HAZ. The maximum hardness is in the fusion zone, which is up to 376 HV (top line) and 306 HV (central line), respectively. The softening was also detected outside of fusion zone in HAZ, the lowest hardness is 186 HV (top line) and 173 HV (central line), respectively. After PWHT, the hardness of the weld metal and fusion zone decreases remarkably, but the hardening does not disappear completely, and
Z. Jiang et al. / Fusion Engineering and Design 85 (2010) 1903–1908
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Fig. 3. Optical microstructure of weld metal, HAZ and base metal in welded joint (left column: before PWHT, right column: after PWHT).
the maximum hardness in fusion zone decreases to 249 HV (top line) and 260 HV (central line), respectively. The hardness of the base metal and softened zone shows no obvious change, which is slightly softer than that before PWHT. The microstructure of the weld metal, softened zone in HAZ and base metal was observed before and after PWHT. The results are shown in Fig. 3. In as-welded condition, as illustrated in Fig. 3(a), (c) and (e), the weld metal is entirely composed of coarse lath martensite. The softened zone consists of a mixture of ferrite and lath martensite. The base metal is composed of lath martensite and well-tempered martensite. After PWHT, as revealed in Fig. 3(b), a lot of carbide precipitates are observed at the weld metal, and also observed at prior austenite boundaries, which make those boundaries clearly. It can be found from Fig. 3(d) that the microstructure of softened zone transforms to tempered sorbite, which consists of carbide precipitates and ferrite. It is the original lath martensite observed before PWHT that decomposes into ferrite and carbides, and the carbides are dispersed in ferrite matrix. As shown in Fig. 3(f), the microstructure morphology of base metal has no obvious change, but the amount of precipitates show a tendency to increase. The characteristics of hardness distribution are essentially determined by the microstructure at the various zones, owing to their different welding thermal cycles during welding. The CLAM steel is so quench-hardenable that the cooling of weld metal and
fusion zone in HAZ where the temperature is above Ac3 , which easily produces a fully lath martensitic structure. It is well known that the lath martensite has the high dislocation density and large lattice distortion, which leads to the hardening behavior occurring in those zones. The softened zone where the temperature is ranged from Ac3 to Ac1 , has different proportions of ferrite and tempered martensite, which makes this zone softer than other zones in the welded joints. In addition, the PWHT at 740 ◦ C/1 h has significant effects on the hardness distribution and microstructure of the welded joints. Carbides precipitation and reduction of dislocation density of the martensite laths decrease the hardness of the weld metal and fusion zone due to the tempering of PWHT. The hardness of the softened zone is decreased in a little degree after PWHT, since the tempered sorbite maintains relative high hardness for the dispersion strengthening of carbides in ferrite matrix. 3.2. Tensile tests The tensile strength of the welded joints is up to 730 MPa before PWHT, and it decreases to 687 MPa after PWHT, which is nearly equal to the quenched-and-tempered condition. Fig. 4 shows the macrograph of the rupture regions in the tensile specimens tested at room temperature. Although the softened zone with strength reduction is found in HAZ from the hardness tests, it can be seen
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Fig. 4. Macrograph of rupture regions in tensile specimens tested at room temperature.
clearly from the figure that all the tensile specimens fractured at the base metal instead of the HAZ or the weld metal. It is concluded that the tensile strength of the weld metal is higher than the base metal and HAZ before and after PWHT. This result indicates that the welding conditions and the PWHT employed in this experiment are quite reasonable. It is well known that the plastic deformation of the softened zone is constrained by the surrounding zones with high hardness,
Fig. 5. Charpy impact energy of Charpy impact specimens notched the weld metal, HAZ and base metal tested at room temperature.
which leads to the strain strengthening during tensile tests, and finally increases its strength [7]. Furthermore, it should be mentioned that strain strengthening happens only in the case when softened zone is narrow and the degree of softening is relative low. Otherwise, the wide softened zone as well as low hardness will result in the decreasing of the tensile strength of the welded joints [8]. Therefore, the width and softening degree of softened zone in welded joints of CLAM steel should be minimized as soon as possible through the welding heat input and PWHT.
Fig. 6. Fracture surface of Charpy impact specimens notched at the weld metal, HAZ and base metal tested at room temperature (left column: before PWHT, right column: after PWHT).
Z. Jiang et al. / Fusion Engineering and Design 85 (2010) 1903–1908 Table 4 The content of the main alloying elements in weld metal (wt%). C
Cr
W
Mn
V
Ta
0.061
9.23
0.18
0.49
0.20
0.010
3.3. Charpy impact tests Charpy impact specimens were tested at room temperature and the results are given in Fig. 5. In the as-welded condition, the impact energies of weld metal, HAZ and base metal are 40 J, 210 J and 246 J, respectively. Obviously, the weld metal exhibits relatively lower toughness level compared with the base metal due to its fresh martensite with coarse laths. After PWHT, the impact toughness of various zones in welded joints has been improved remarkably. Especially for the weld metal, the impact energy is increased to 249 J as result of PWHT, which reaches almost the same level as the base metal. In addition, the impact energy of HAZ and base metal slightly increases to 237 J and 257 J due to the PWHT, respectively. It is concluded that the improvement in toughness of welded joint of CLAM steel was realized by the proper PWHT condition. This improvement is attributed to reducing the quench-hardening tendency of fresh martensite in the weld metal and HAZ. Fig. 6 shows the SEM images of fracture surface for the impactfractured specimens tested at room temperature. In as-welded condition, Fig. 6(a) shows that the fracture appearance of weld metal reveals typical quasi-cleavage fracture with fine tear ridges. Fig. 6(b) shows that the weld metal is ductile fracture with smallsized dimples on the fracture surface after PWHT. It is obvious that the impact fracture mode of weld metal changes from the brittle into the ductile due to the PWHT. All the tested specimens with notches in the base metal and the HAZ reveal dimpled fracture surfaces regardless of PWHT (see Fig. 6(c–f)), which means that the impact fracture mode is predominantly ductile in the HAZ and base metal. Furthermore, the size of dimples becomes larger and deeper than that in the as-welded specimens. 3.4. Chemical analysis of weld metal The contents of main alloying elements for CLAM steel, such as C, Cr, W, V, Mn and Ta, were controlled strictly in order to enhance the mechanical properties and anti-radiation performance. It is necessary to evaluate the burning loss of alloying elements in weld metal. The contents of main alloying elements in weld metal were analyzed and the result is listed in Table 4. Comparing Table 4 with Table 1, the chemical compositions difference of weld metal and base metal can be easily identified. The C, W and Ta contents in the weld metal are markedly decreased, which are only about 0.061%, 0.18% and 0.010%, respectively. It indicates that C, W and Ta are liable to form oxides in the process of welding. However, the Cr, Mn and V in the weld metal are stable elements, which remain nearly the same as that in the base metal. It is very important for RAFMs to have good toughness properties and high creep strength for fusion applications, and it is also important to have the good aging resistance. C, W and Ta are the key elements for these properties. It is well known that the content of C is chosen as 0.10% in order to obtain low carbon martensite with higher strength and toughness after PWHT. Ta is used in RAFMs as it expected to perform just same as Nb and contribute to have the good toughness and high creep property. On the other hand, based on the optimal designs for F82H wires, the content of C is designed as 0.08% in order to reduce harden quenching tendency of the weld metal. Ta is decreased to 0.02% for reducing the hot cracking susceptibility [9]. This suggests that proper reduction of C and Ta will increase weldability. The increasing W in RAFMs will achieve higher creep property but loss toughness and vice versa [2]. Therefore, these elements should be maintained at a certain
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level or within a certain range in the weld metal. But in practice, the contents of these elements are difficult to control exactly due to oxidation without effective gas-shielding measures. Only on the basis of solving this problem, the optimization designs for CLAM wires and related high-temperature properties can be performed in the future. It is suggested that the high-temperature weld metal should be controlled by using strict gas-shielding measures such as welding in a glove-box fulfilled with argon. 4. Conclusions Butt-welded joints of CLAM steel plates were prepared with the manual TIG welding process. The PWHT condition was selected to be 740 ◦ C/1 h in vacuum furnace in order to achieve the recovery from hardening and toughness degradation. Tests were carried out in order to evaluate the microstructure and mechanical properties of the butt-welded joints before and after PWHT at room temperature. The main results are listed as follows: 1. In the as-welded condition, the hardening and softening behaviors were detected in the welded joints. The maximum hardness is located in fusion zone of HAZ. After PWHT, the hardness of the weld metal and the fusion zone decreased significantly. Furthermore, the hardness of the base metal and the soften zone in HAZ was slightly lower than it is in the as-welded condition. 2. The microstructures of all the zones except for the softened zone are lath martensite before and after PWHT. In as-welded condition, the soften zone in HAZ consist of a mixture of tempered martensite and ferrite. After PWHT, a lot of carbides precipitated at all zones in weld joints. The microstructure of soften zone transformed to the tempered sorbite. 3. The tensile strength of the weld metal is found to be higher than the base metal and HAZ regardless of PWHT. However, it has relatively poor toughness before PWHT. The measurements of mechanical properties show that the excellent combined properties of weld joint have been achieved after PWHT. 4. The contents of C, W and Ta in the weld metal are markedly decreased during welding, however the contents of Cr, Mn and V remain nearly the same as that of the base metal. Acknowledgements This work was supported by the National Basic Research Program of China with a Grant No. 2008CB717802. The authors would like to acknowledge Drs Haitao Wang and Jinhui Xiong for fruitful discussions. We also thank Dr Huajie Li for providing with CLAM steel to be investigated. References [1] B. van der Schaaf, D.S. Gelles, S. Jitsukawa, S. Jitsukawa, A. Kimura, R.L. Klueh, A. MoÈslang, G.R. Odette, Progress and critical issues of reduced activation ferritic/martensitic steel development, J. Nucl. Mater. 283–287 (2000) 52–59. [2] J. Yu, Q. Huang, F Wan, Research and development on the China low activation martensitic steel (CLAM), J. Nucl. Mater. 367–370 (2007) 97–101. [3] Y. Li, Q. Huang, Y. Wu, T. Nagasaka, T. Muroga, Mechanical properties and microstructures of China low activation martensitic steel compared with JLF-1, J. Nucl. Mater. 117–121 (2007) 367–370. [4] C. Li, Q. Huang, Q. Wu, S. Liua, Y. Lei, T. Muroga, Welding techniques development of CLAM steel for test blanket module, Fusion Eng. Des. 84 (2009) 1184–1187. [5] H. Tanigawa, T. Hirose, K. Shiba, R. Kasada, E. Wakai, H. Serizawa, Y. Kawahito, S. Jitsukawa, et al., Technical issues of reduced activation ferritic/martensitic steels for fabrication of ITER test blanket modules, Fusion Eng. Des. 83 (2008) 1471–1476. [6] A. Cardella, E. Rigal, L. Bedel, P. Bucci, J. Fiek, L. Forest, L.V. Boccaccini, The manufacturing technologies of the European breeding blankets, J. Nucl. Mater. 329–333 (2004) 133–140. [7] K.S. Bang, W.Y. Kim, Estimation and prediction of HAZ softening in thermomechanically controlled-rolled and accelerated-cooled steel, Welding J. 81 (2002) 174–179.
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[8] T. Mohandas, G. Madhusudan Reddy, B. Satish Kumar, Heat-affected zone softening in high-strength low-alloy steels, J. Mater. Process. Technol. 88 (1999) 284–294.
[9] H. Hayakama, A. Yoshitake, M. Tamura, S. Natsume, A. Gotoh, A. Hishnuma, Mechanical properties of the reduced-activation ferritic steel: 8%Cr–2%W–0.2%V–0.04%Ta–Fe, J. Nucl. Mater. 179–181 (1991) 693–696.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Microstructure and mechanical properties of the TIG welded joints of fusion CLAM steel Zhizhong Jiang a,∗ , Litian Ren a , Jihua Huang a , Xin Ju a , Huibin Wu a , Qunying Huang b , Yican Wu b a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Xueyuan Road, Beijing 100083, China Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China
a r t i c l e
i n f o
Article history: Available online 15 July 2010 Keywords: CLAM steel Tungsten inert gas arc welding Microstructure Mechanical properties
a b s t r a c t The CLAM steel plates were butt-welded through manual tungsten inert gas welding (TIG) process, and the following post-welding heat treatment (PWHT) at 740 ◦ C for 1 h. The microstructure and mechanical properties of the welded joints were measured. The results show that both hardening and softening occur in the weld joints before PWHT, but the hardening is not removed completely in the weld metal and the fusion zone after PWHT. In as-welded condition, the microstructure of the weld metal is coarse lath martensite, and softened zone in heat-affected zone (HAZ) consists of a mixture of tempered martensite and ferrite. After PWHT, a lot of carbides precipitate at all zones in weld joints. The microstructure of softened zone transforms to tempered sorbite. Tensile strength of the weld metal is higher than that of HAZ and base metal regardless of PWHT. However, the weld metal has poor toughness without PWHT. The impact energy of the weld metal after PWHT reaches almost the same level as the base metal. So it is concluded that microstructure and mechanical properties of the CLAM steel welded joints can be improved by a reasonable PWHT. © 2010 Elsevier B.V. All rights reserved.
1. Introduction With good irradiation swelling resistance, thermo-physical and thermo-mechanical properties, reduced activation ferritic/martensitic (RAFM) steels are recognized as the leading candidate structural materials for test blanket module (TBM) in ITER and first wall and tritium breeding blankets in DEMO reactor [1]. The composition designs of the China low activation martensitic (CLAM) steel had been started since 2001. Some physical and mechanical properties have been tested in recent years, which are similar to JLF-1 and Eurofer 97 [2,3]. At present, the development of CLAM steel is at stage of large scale smelting and appraisals for its weldability [4]. Fusion welding methods, such as tungsten inert gas (TIG) welding, electron beam welding (EBW) and laser beam welding (LBW), are considered to be essential manufacturing technologies for TBM constructions. The relevant research institutes in Europe and Japan have made extensive attempts for fabrication and components assembly of small-sized TBM [5,6]. According to the structure features of TBM conceptions for ITER in China, TIG welding will be applied to produce sealing, such as for headers of the first wall, side wall, and
∗ Corresponding author. Tel.: +86 10 62334859. E-mail address: [email protected] (Z. Jiang). 0920-3796/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2010.06.019
back wall, and TIG welding was chosen for sealing, because it has the largest gap allowance compared to the other welding methods. In order to accumulate practical experiences for the fabrication of TBM and establish database on properties of CLAM steel, the research on weldability for CLAM steel is greatly needed. In this work, the TIG welding experiments of CLAM steel have been carried out, then the microstructure and mechanical properties of the welded joints are investigated before and after PWHT.
2. Materials and experimental procedures CLAM steel plates with 13 mm thick for welding experiments were supplied by the National Engineering Research Center of Advanced Rolling in USTB. The plates after rolling were quenched at 980 ◦ C for 30 min and followed by water-cooling, then tempered at 760 ◦ C for 90 min and followed by air-cooling. The actual measured compositions and mechanical properties of the paltes are listed in Tables 1 and 2, respectively. Two plates were butt-welded by manual TIG process. Double-Y groove was machined to control welding deformation as shown schematically in Fig. 1. The filler wires with cross-section of 2 mm × 2 mm were extracted from CLAM plates. The details of welding conditions are listed in Table 3. Specimens used for microstructure examination and hardness tests were transversally sectioned, mechanically grounded and polished, then etched in a mixed solution of 95 ml C2 H5 OH + 5 ml
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Z. Jiang et al. / Fusion Engineering and Design 85 (2010) 1903–1908
Table 1 The actual measured compositions of CLAM steel (wt%). C
Cr
W
V
Mn
Ta
S
Nb
Ni
Mo
P
N
Fe
0.093
9.15
1.49
0.20
0.44
0.097
< 0.003
< 0.01
< 0.015
< 0.005
0.0051
< 0.0047
Bal.
Fig. 1. Schematic diagram of the double-Y groove.
Table 2 Mechanical properties of CLAM steel plates. Temperature (◦ C)
25 600
Strength (MPa) YS
UTS
567 392
686 407
Elongation (%)
Reduction of area (%)
26 14
74.7 87
Table 3 TIG welding conditions. Welding current Welding voltage Welding speed Preheating temperature Interlayer temperature Number of passes PWHT condition
220–240 A 10.8 V 80–90 mm/min <200 ◦ C <200 ◦ C 8 740 ◦ C/1 h (vacuum furnace)
HCl + 1 g C6 H3 O7 N3 . The microstructure was finally examined by optical microscope. Hardness distribution at various positions on the welded joints was measured using a Vickers hardness tester, with testing load of 1 kg holding for 15 s. Standard V-notch Charpy specimens of 10 mm × 10 mm × 55 mm were machined from welded joints with their notched roots located in the base metal, HAZ and weld metal, respectively, the axis of V-notch for each impact specimen was perpendicular to the surface of welded plates. After impact testing, fracture surfaces of the impact specimens were examined carefully under a scanning electron microscope (SEM). Plate type tensile specimens were used to measure the strength of the weld joints. The gauge size of the tensile specimens is 11T mm × 25W mm × 77L mm. All the tests were carried out at room temperature (25 ◦ C). 3. Results and discussion 3.1. Hardness tests and microstructure observation No obvious defects were found by means of the nondestructive examinations, X-rays and ultra-sound testing. The macrograph and Vickers hardness distribution of the welded joints are presented in Fig. 2. The weld metal, HAZ and base metal can be clearly distinguished in the welded joints (see Fig. 2(a)). The trends of the hardness variation are presented in Fig. 2(b) and (c). The locations for measurements are also depicted in Fig. 2(a): the top line locates at 2 mm from the top surface of the welded plate. The central line corresponds to the horizontal center of the welded plate. As shown in Fig. 2(b) and (c), in as-welded condition, the hardness values at different positions along the top line are higher than those along the central line, especially for the weld metal and fusion zone. The hard-
Fig. 2. Vickers hardness distribution cross-welded joint: (a) macrograph of the welded joint, (b) hardness distribution measured along top line, (c) hardness distribution measured along central line.
ening phenomenon emerges at weld metal and fusion zone in HAZ. The maximum hardness is in the fusion zone, which is up to 376 HV (top line) and 306 HV (central line), respectively. The softening was also detected outside of fusion zone in HAZ, the lowest hardness is 186 HV (top line) and 173 HV (central line), respectively. After PWHT, the hardness of the weld metal and fusion zone decreases remarkably, but the hardening does not disappear completely, and
Z. Jiang et al. / Fusion Engineering and Design 85 (2010) 1903–1908
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Fig. 3. Optical microstructure of weld metal, HAZ and base metal in welded joint (left column: before PWHT, right column: after PWHT).
the maximum hardness in fusion zone decreases to 249 HV (top line) and 260 HV (central line), respectively. The hardness of the base metal and softened zone shows no obvious change, which is slightly softer than that before PWHT. The microstructure of the weld metal, softened zone in HAZ and base metal was observed before and after PWHT. The results are shown in Fig. 3. In as-welded condition, as illustrated in Fig. 3(a), (c) and (e), the weld metal is entirely composed of coarse lath martensite. The softened zone consists of a mixture of ferrite and lath martensite. The base metal is composed of lath martensite and well-tempered martensite. After PWHT, as revealed in Fig. 3(b), a lot of carbide precipitates are observed at the weld metal, and also observed at prior austenite boundaries, which make those boundaries clearly. It can be found from Fig. 3(d) that the microstructure of softened zone transforms to tempered sorbite, which consists of carbide precipitates and ferrite. It is the original lath martensite observed before PWHT that decomposes into ferrite and carbides, and the carbides are dispersed in ferrite matrix. As shown in Fig. 3(f), the microstructure morphology of base metal has no obvious change, but the amount of precipitates show a tendency to increase. The characteristics of hardness distribution are essentially determined by the microstructure at the various zones, owing to their different welding thermal cycles during welding. The CLAM steel is so quench-hardenable that the cooling of weld metal and
fusion zone in HAZ where the temperature is above Ac3 , which easily produces a fully lath martensitic structure. It is well known that the lath martensite has the high dislocation density and large lattice distortion, which leads to the hardening behavior occurring in those zones. The softened zone where the temperature is ranged from Ac3 to Ac1 , has different proportions of ferrite and tempered martensite, which makes this zone softer than other zones in the welded joints. In addition, the PWHT at 740 ◦ C/1 h has significant effects on the hardness distribution and microstructure of the welded joints. Carbides precipitation and reduction of dislocation density of the martensite laths decrease the hardness of the weld metal and fusion zone due to the tempering of PWHT. The hardness of the softened zone is decreased in a little degree after PWHT, since the tempered sorbite maintains relative high hardness for the dispersion strengthening of carbides in ferrite matrix. 3.2. Tensile tests The tensile strength of the welded joints is up to 730 MPa before PWHT, and it decreases to 687 MPa after PWHT, which is nearly equal to the quenched-and-tempered condition. Fig. 4 shows the macrograph of the rupture regions in the tensile specimens tested at room temperature. Although the softened zone with strength reduction is found in HAZ from the hardness tests, it can be seen
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Z. Jiang et al. / Fusion Engineering and Design 85 (2010) 1903–1908
Fig. 4. Macrograph of rupture regions in tensile specimens tested at room temperature.
clearly from the figure that all the tensile specimens fractured at the base metal instead of the HAZ or the weld metal. It is concluded that the tensile strength of the weld metal is higher than the base metal and HAZ before and after PWHT. This result indicates that the welding conditions and the PWHT employed in this experiment are quite reasonable. It is well known that the plastic deformation of the softened zone is constrained by the surrounding zones with high hardness,
Fig. 5. Charpy impact energy of Charpy impact specimens notched the weld metal, HAZ and base metal tested at room temperature.
which leads to the strain strengthening during tensile tests, and finally increases its strength [7]. Furthermore, it should be mentioned that strain strengthening happens only in the case when softened zone is narrow and the degree of softening is relative low. Otherwise, the wide softened zone as well as low hardness will result in the decreasing of the tensile strength of the welded joints [8]. Therefore, the width and softening degree of softened zone in welded joints of CLAM steel should be minimized as soon as possible through the welding heat input and PWHT.
Fig. 6. Fracture surface of Charpy impact specimens notched at the weld metal, HAZ and base metal tested at room temperature (left column: before PWHT, right column: after PWHT).
Z. Jiang et al. / Fusion Engineering and Design 85 (2010) 1903–1908 Table 4 The content of the main alloying elements in weld metal (wt%). C
Cr
W
Mn
V
Ta
0.061
9.23
0.18
0.49
0.20
0.010
3.3. Charpy impact tests Charpy impact specimens were tested at room temperature and the results are given in Fig. 5. In the as-welded condition, the impact energies of weld metal, HAZ and base metal are 40 J, 210 J and 246 J, respectively. Obviously, the weld metal exhibits relatively lower toughness level compared with the base metal due to its fresh martensite with coarse laths. After PWHT, the impact toughness of various zones in welded joints has been improved remarkably. Especially for the weld metal, the impact energy is increased to 249 J as result of PWHT, which reaches almost the same level as the base metal. In addition, the impact energy of HAZ and base metal slightly increases to 237 J and 257 J due to the PWHT, respectively. It is concluded that the improvement in toughness of welded joint of CLAM steel was realized by the proper PWHT condition. This improvement is attributed to reducing the quench-hardening tendency of fresh martensite in the weld metal and HAZ. Fig. 6 shows the SEM images of fracture surface for the impactfractured specimens tested at room temperature. In as-welded condition, Fig. 6(a) shows that the fracture appearance of weld metal reveals typical quasi-cleavage fracture with fine tear ridges. Fig. 6(b) shows that the weld metal is ductile fracture with smallsized dimples on the fracture surface after PWHT. It is obvious that the impact fracture mode of weld metal changes from the brittle into the ductile due to the PWHT. All the tested specimens with notches in the base metal and the HAZ reveal dimpled fracture surfaces regardless of PWHT (see Fig. 6(c–f)), which means that the impact fracture mode is predominantly ductile in the HAZ and base metal. Furthermore, the size of dimples becomes larger and deeper than that in the as-welded specimens. 3.4. Chemical analysis of weld metal The contents of main alloying elements for CLAM steel, such as C, Cr, W, V, Mn and Ta, were controlled strictly in order to enhance the mechanical properties and anti-radiation performance. It is necessary to evaluate the burning loss of alloying elements in weld metal. The contents of main alloying elements in weld metal were analyzed and the result is listed in Table 4. Comparing Table 4 with Table 1, the chemical compositions difference of weld metal and base metal can be easily identified. The C, W and Ta contents in the weld metal are markedly decreased, which are only about 0.061%, 0.18% and 0.010%, respectively. It indicates that C, W and Ta are liable to form oxides in the process of welding. However, the Cr, Mn and V in the weld metal are stable elements, which remain nearly the same as that in the base metal. It is very important for RAFMs to have good toughness properties and high creep strength for fusion applications, and it is also important to have the good aging resistance. C, W and Ta are the key elements for these properties. It is well known that the content of C is chosen as 0.10% in order to obtain low carbon martensite with higher strength and toughness after PWHT. Ta is used in RAFMs as it expected to perform just same as Nb and contribute to have the good toughness and high creep property. On the other hand, based on the optimal designs for F82H wires, the content of C is designed as 0.08% in order to reduce harden quenching tendency of the weld metal. Ta is decreased to 0.02% for reducing the hot cracking susceptibility [9]. This suggests that proper reduction of C and Ta will increase weldability. The increasing W in RAFMs will achieve higher creep property but loss toughness and vice versa [2]. Therefore, these elements should be maintained at a certain
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level or within a certain range in the weld metal. But in practice, the contents of these elements are difficult to control exactly due to oxidation without effective gas-shielding measures. Only on the basis of solving this problem, the optimization designs for CLAM wires and related high-temperature properties can be performed in the future. It is suggested that the high-temperature weld metal should be controlled by using strict gas-shielding measures such as welding in a glove-box fulfilled with argon. 4. Conclusions Butt-welded joints of CLAM steel plates were prepared with the manual TIG welding process. The PWHT condition was selected to be 740 ◦ C/1 h in vacuum furnace in order to achieve the recovery from hardening and toughness degradation. Tests were carried out in order to evaluate the microstructure and mechanical properties of the butt-welded joints before and after PWHT at room temperature. The main results are listed as follows: 1. In the as-welded condition, the hardening and softening behaviors were detected in the welded joints. The maximum hardness is located in fusion zone of HAZ. After PWHT, the hardness of the weld metal and the fusion zone decreased significantly. Furthermore, the hardness of the base metal and the soften zone in HAZ was slightly lower than it is in the as-welded condition. 2. The microstructures of all the zones except for the softened zone are lath martensite before and after PWHT. In as-welded condition, the soften zone in HAZ consist of a mixture of tempered martensite and ferrite. After PWHT, a lot of carbides precipitated at all zones in weld joints. The microstructure of soften zone transformed to the tempered sorbite. 3. The tensile strength of the weld metal is found to be higher than the base metal and HAZ regardless of PWHT. However, it has relatively poor toughness before PWHT. The measurements of mechanical properties show that the excellent combined properties of weld joint have been achieved after PWHT. 4. The contents of C, W and Ta in the weld metal are markedly decreased during welding, however the contents of Cr, Mn and V remain nearly the same as that of the base metal. Acknowledgements This work was supported by the National Basic Research Program of China with a Grant No. 2008CB717802. The authors would like to acknowledge Drs Haitao Wang and Jinhui Xiong for fruitful discussions. We also thank Dr Huajie Li for providing with CLAM steel to be investigated. References [1] B. van der Schaaf, D.S. Gelles, S. Jitsukawa, S. Jitsukawa, A. Kimura, R.L. Klueh, A. MoÈslang, G.R. Odette, Progress and critical issues of reduced activation ferritic/martensitic steel development, J. Nucl. Mater. 283–287 (2000) 52–59. [2] J. Yu, Q. Huang, F Wan, Research and development on the China low activation martensitic steel (CLAM), J. Nucl. Mater. 367–370 (2007) 97–101. [3] Y. Li, Q. Huang, Y. Wu, T. Nagasaka, T. Muroga, Mechanical properties and microstructures of China low activation martensitic steel compared with JLF-1, J. Nucl. Mater. 117–121 (2007) 367–370. [4] C. Li, Q. Huang, Q. Wu, S. Liua, Y. Lei, T. Muroga, Welding techniques development of CLAM steel for test blanket module, Fusion Eng. Des. 84 (2009) 1184–1187. [5] H. Tanigawa, T. Hirose, K. Shiba, R. Kasada, E. Wakai, H. Serizawa, Y. Kawahito, S. Jitsukawa, et al., Technical issues of reduced activation ferritic/martensitic steels for fabrication of ITER test blanket modules, Fusion Eng. Des. 83 (2008) 1471–1476. [6] A. Cardella, E. Rigal, L. Bedel, P. Bucci, J. Fiek, L. Forest, L.V. Boccaccini, The manufacturing technologies of the European breeding blankets, J. Nucl. Mater. 329–333 (2004) 133–140. [7] K.S. Bang, W.Y. Kim, Estimation and prediction of HAZ softening in thermomechanically controlled-rolled and accelerated-cooled steel, Welding J. 81 (2002) 174–179.
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[8] T. Mohandas, G. Madhusudan Reddy, B. Satish Kumar, Heat-affected zone softening in high-strength low-alloy steels, J. Mater. Process. Technol. 88 (1999) 284–294.
[9] H. Hayakama, A. Yoshitake, M. Tamura, S. Natsume, A. Gotoh, A. Hishnuma, Mechanical properties of the reduced-activation ferritic steel: 8%Cr–2%W–0.2%V–0.04%Ta–Fe, J. Nucl. Mater. 179–181 (1991) 693–696.