- Email: [email protected]
Eurofer dissimilar electron beam welds
Journal of Nuclear Materials xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Characterization of ODS (Oxide Dispersion Strengthened) Eurofer/Eurofer dissimilar electron beam welds L. Commin a,b,⇑, M. Rieth a, V. Widak a, B. Dafferner a, S. Heger a, H. Zimmermann a, E. Materna-Morris a, R. Lindau a a b
Karlsruhe Institute for Technology, IAM-AWP, Karlsruhe, Germany Institute for Research on Magnetic Fusion, CEA Cadarache, France
a r t i c l e
i n f o
Article history: Available online xxxx
a b s t r a c t Breeding blanket fabrication will need the assembly of reduced activation ferritic martensitic steels and their ODS alloyed grades. Dissimilar electron beam welds were performed using 6 mm thick Y2O3 ODS Eurofer and Eurofer 97/2 samples. Thereafter, an austenitization step and an annealing post-weld heat treatment were applied. The joining process was qualified using the microstructural and mechanical properties characterization of as-welded and post-weld heat treated samples. The results show that electron beam welding is efficient for joining dissimilar ODS Eurofer/Eurofer materials and that a further PWHT (post-weld heat treatment) is required. The weld microstructure presents large oxide particles that are detrimental to mechanical properties. However, the mechanical properties are improved compared to similar Eurofer joints and they are approaching Eurofer behavior. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Materials and methods
The concepts of breeding blankets that are developed in Europe (helium cooled liquid lithium and helium cooled pebble bed) will operate in the 523–823 K range for conservative approaches using reduced activation ferritic martensitic (RAFM) steels, and in the range of 523–923 K for more advanced versions based on ODS steels [1,2]. Indeed, ODS steels exhibit a similar creep strength at a 100 K higher temperature [1]. In ITER, breeding blankets mocks-up (called test blanket modules (TBMs)) will be tested using the same structural materials [3,4]. TBM fabrication involves joining procedures and the use of suitable post-weld heat treatments (PWHTs). Joining technologies and procedures for RAFM steels have already been widely studied [5–8] and some trials have been carried out using ODS RAFM steels [9]. In this study, dissimilar electron beam welding was performed between samples of ODS Eurofer and Eurofer 97/2 base materials [1]. The resulting microstructures were analyzed and their impact on the mechanical properties was investigated. This allowed for an assessment of the effectiveness of beam welding components for nuclear fusion applications.
The base materials used for the dissimilar welds are 6 mm thick plates of Eurofer 97/2 and ODS Eurofer. Electron beam welds were processed using 60 kV accelerating voltage, 27 mA beam current at 10 mm/s welding speed. The welding direction is parallel to the rolling direction. Two-step post-weld heat treatment (PWHT) was applied at 1323 K for 1 h followed by 1043 K for 2 h. The 1step post-weld heat treatment (1043 K for 2 h) was also studied. The PWHT influence was studied in [5] with the following conclusions: 2-step heat treatment modifies the grain weld microstructure and leads to lower hardness levels; single-step heat treatments do not change the microstructure but soften the weld. Optical microscopy was performed on etched samples (with 4 g picric acid, 400 ml ethanol, 5 ml hydrochloric acid, and 5 ml nitric acid) to assess the weld quality and characterize the grain microstructure. Beraha etching (1 g potassium bisulfite, 100 ml stock solution (24 g ammonium difluoride, 1000 ml distilled water, 200 ml hydrochloric acid)) was used to characterize the martensitic structure. Philips XL30 ESEM was used to observe the precipitation evolution and for fractography analysis. Transmission electron microscopy (TEM) samples were made from the 6 mm thick plates by grinding down samples of one square centimeter to 100 lm thickness by using different silicon carbide polishing papers. They were then polished electrochemically in a Struers Tenupol-3 jet polisher. Microhardness tests (10 N, 30 s) were carried out using a Zwick 3212 facility. Charpy tests were performed using a Zwick Roell
⇑ Corresponding author. Address: Lorelei Commin, KIT, IAM-AWP, P.O. Box 3640, 76021 Karlsruhe, Germany. E-mail address: [email protected] (L. Commin). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.11.019
Please cite this article in press as: L. Commin et al., J. Nucl. Mater. (2012), http://dx.doi.org/10.1016/j.jnucmat.2012.11.019
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L. Commin et al. / Journal of Nuclear Materials xxx (2012) xxx–xxx
Fig. 1. Sketch of (a) the creep samples, and (b) the Charpy samples.
Fig. 4. TEM Bright Field image of the ODS base metal.
facility. The Charpy sample notch was located in the fusion zone (FZ). Creep tests were performed from 773 K to 973 K using conventional lever arms and weights. Fig. 1 illustrates the geometry of the Charpy and creep samples. 3. Results and discussion 3.1. As-welded condition Fig. 2. Optical micrography of the weld cross-section.
The weld cross-section micrography did not present any cracks or any porosity (Fig. 2). Fig. 2 exhibits the five different zones of the EB weld: the FZ, the heat affected zones (HAZs) on the Eurofer side and on the ODS Eurofer side and the base metals (BMs).
Fig. 3. SEM Microstructure of (a) Eurofer 97/2 base metal, (b) ODS Eurofer base metal, (c) FZ as-welded and (d) FZ after PWHT.
Please cite this article in press as: L. Commin et al., J. Nucl. Mater. (2012), http://dx.doi.org/10.1016/j.jnucmat.2012.11.019
L. Commin et al. / Journal of Nuclear Materials xxx (2012) xxx–xxx
3
Fig. 5. Optical micrographs of each zone of the as-welded sample using Beraha etchant.
Eurofer base metal is characterized by a 10 lm grain size microstructure, with 100 nm large M23C6 carbide precipitates located at the grain boundaries (Fig. 3a). ODS Eurofer base metal is characterized by a 3 lm grain size microstructure (Fig. 3b) with 10–20 nm diameter intragranular ODS particles (Fig. 4), as already fully described in previous papers [10,11]. After EB welding, the FZ presents a large increase in its grain size compared to the BM, carbide dissolution and a huge coarsening of the ODS particles (Figs. 3c and 4). In Fig. 5, Beraha etching was used so that the dark areas highlight the martensitic phase. Therefore, it can be seen that the martensitic structure is refined in both HAZs. The carbide morphology remained similar in the HAZ.
3.2. Microstructure after PWHT After PWHT (1323 K for 1 h + 1043 K for 2 h), a significant increase in the grain size is observed in the FZ and in the Eurofer side HAZ (Fig. 6). The grain size increases as well in the boundary between the FZ and the ODS Eurofer HAZ. The grain size remained similar in the ODS Eurofer side due to the stabilizing effect of the oxide particles. Carbide re-precipitation was observed along the edges of the martensite laths in the FZ and they grew with the thermal treatment in the other zones (Fig. 3d). The large ODS particles in the FZ did not act as carbide nucleation sites. When only applying the 1043 K for 2 h tempering treatment the precipitation evolution observed was the same as after the 2-step PWHT, whereas the grain size was similar to the as-welded sample. Therefore, the grain size is influenced solely by the solution treatment whereas the carbide precipitation is driven by the tempering treatment. 3.3. Microhardness profiles The microhardness profile across the weld and its evolution after PWHT is shown in Fig. 7. The microhardness is higher on the ODS Eurofer side due to the strengthening effect of the ODS particles. In the as-welded sample, hardening is observed in both HAZs due to the finer martensitic structure, and then a slight softening occurs in the FZ due to a larger grain size, larger martensite laths, and the loss of the ODS strengthening effect. After PWHT, general softening occurs caused by the tempering of the martensite structure and the carbide precipitation. The loss of the strengthening effect of the ODS particles due to their coarsening is obvious since the hardness values in the FZ are similar to the Eurofer base metal values. 3.4. Charpy testing
Fig. 6. Grain size comparison of as-welded and PWHT samples, measurements taken at the mid thickness.
Charpy tests were carried out on as-welded samples and on samples after PWHT (Fig. 8). The results are compared to those
Fig. 7. HV profile across the weld and its change due to the PWHT.
Please cite this article in press as: L. Commin et al., J. Nucl. Mater. (2012), http://dx.doi.org/10.1016/j.jnucmat.2012.11.019
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L. Commin et al. / Journal of Nuclear Materials xxx (2012) xxx–xxx
Fig. 8. Charpy results and typical fracture surface of a Charpy sample exhibiting an ODS particle.
Fig. 9. Comparison of Larson–Miller diagrams of ODS Eurofer Eurofer 97/2 EB welds, Eurofer 97/2 EB welds, ODS Eurofer base metal and Eurofer 97/2 base metal.
of Eurofer BM, ODS Eurofer BM, Eurofer similar EB welds, and ODS Eurofer similar EB welds taken from previous work [9]. After dissimilar EB welding, the fracture is located in the FZ for most of the samples, apart from as-welded brittle fractures that occurred in the ODS Eurofer HAZ. After PWHT, the impact energy is higher and the DBTT decreases. The Charpy properties are therefore much better after PWHT due to the tempered microstructure. The DBTT obtained after PWHT is lower than that of the ODS Eurofer samples (BM and welds); and it is close to the DBTT of the Eurofer welds. Fig. 8 shows a typical fracture surface. The large ODS particles that were seen in the FZ within the microstructural investigations can be seen quite clearly embedded in the dimples. This phenomenon had already been observed in similar ODS EB welds [9] and is generally responsible for the loss in ductility of EB welds containing ODS particles.
Fig. 10. (a) Optical micrograph of the typical fracture of ODS Eurofer Eurofer 97/2 EB welds until 823 K, (b) optical and (c) SEM images of the typical fracture of ODS Eurofer Eurofer 97/2 EB welds from 873 K.
Please cite this article in press as: L. Commin et al., J. Nucl. Mater. (2012), http://dx.doi.org/10.1016/j.jnucmat.2012.11.019
L. Commin et al. / Journal of Nuclear Materials xxx (2012) xxx–xxx
3.5. Creep tests The Larson–Miller diagram of the dissimilar welds was plotted and compared to the BM and similar Eurofer 97/2 EB weld results taken from previous works (Fig. 9) [1,5]. For temperature below 823 K, the fracture occurred in the Eurofer 97/2 base metal zone with a characteristic radial symmetrical necking (Fig. 10a). At higher temperatures (from 873 K), the creep strength is reduced and the fracture occurred in the FZ without section reduction (Fig. 10b). Large ODS particles, resulting from the coarsening occurring during the welding process, were observed on the fracture surface (Fig. 10c). With increasing temperature, at lower stresses, creep inter-granular fracture is observed due to the presence of the large oxide particles which act as stress concentrators, causing cavity nucleation. ODS Eurofer base metal presents the higher creep strength values as the fine dispersed ODS particles inhibit dislocation motion. The dissimilar EB weld creep behavior is close to that of the Eurofer 97/2 base metal. Indeed, due to their coarsening, the strengthening effect and creep deformation resistance of the ODS particles is lost. The dissimilar EB weld shows higher values than the similar EB weld. Eurofer similar EB welds showed gliding deformation above 773 K [5]. Fracture occurred in the fine grained HAZ that remains after a single-step 973 K/2 h PWHT. In the dissimilar weld, an austenitization PWHT was applied which modifies the grain structure in the weld zone. That would explain the higher creep properties obtained for the dissimilar welds. 4. Conclusions This study shows the effectiveness of joining ODS Eurofer to Eurofer 97 by EB welding and the requirement of PWHT. Higher creep strength and lower DBTT can be achieved compared to both similar EB welds. The use of PWHT is required, resulting in:
5
– Grain structure modification in the weld zone and therefore, improved creep behavior. – Carbide re-precipitation in the FZ and therefore, softening of the weld zone. The formation of large oxide particles in the weld zone leads to mechanical property reduction compared to the base metal. Acknowledgements The authors wish to thank S. Fischer, for the electron beam welding and T. Fabri, for fabrication samples. References [1] R. Lindau, A. Möslang, M. Rieth, M. Klimiankou, E. Materna-Morris, A. Alamo, A.-A.F. Tavassoli, C. Cayron, A.-M. Lancha, P. Fernandez, N. Baluc, R. Schäublin, E. Diegele, G. Filacchioni, J.W. Rensman, B.v.d. Schaaf, E. Lucon, W. Dietz, Fusion Eng. Des. 75–79 (2005) 989–996. [2] R. Lässer, N. Baluc, J.-L. Boutard, E. Diegele, S. Dudarev, M. Gasparotto, A. Möslang, R. Pippan, B. Riccardi, B. van der Schaaf, Fusion Eng. Des. 82 (2007) 511–520. [3] Y. Poitevin, L.V. Boccaccini, A. Cardella, L. Giancarli, R. Meyder, E. Diegele, R. Laesser, G. Benamati, Fusion Eng. Des. 75–79 (2005) 741–749. [4] L.V. Boccaccini, J.-F. Salavy, O. Bede, H. Neuberger, I. Ricapito, P. Sardain, L. Sedano, K. Splichal, Fusion Eng. Des. 84 (2009) 333–337. [5] M. Rieth, J. Rey, J. Nucl. Mater. 386–388 (2009) 471–474. [6] P. Aubert, F. Tavassoli, M. Rieth, E. Diegele, Y. Poitevin, J. Nucl. Mater. 409 (2011) 156–162. [7] M. Rieth, Fusion Eng. Des. 84 (2009) 1602–1605. [8] A. Cardella, E. Rigal, L. Bedel, Ph. Bucci, J. Fiek, L. Forest, L.V. Boccaccini, E. Diegele, L. Giancarli, S. Hermsmeyer, G. Janeschitz, R. Lässer, A. Li Puma, J.D. Lulewicz, A. Möslang, Y. Poitevin, E. Rabaglino, J. Nucl. Mater. 329–333 (2004) 133–140. [9] R. Lindau, M. Klimenkov, U. Jäntsch, A. Möslang, L. Commin, J. Nucl. Mater. 416 (2011) 22–29. [10] M. Klimiankou, R. Lindau, A. Möslang, J. Nucl. Mater. 329–333 (2004) 347–351. [11] M. Klimiankou, R. Lindau, A. Möslang, J. Nucl. Mater. 367–370 (2007) 173–178.
Please cite this article in press as: L. Commin et al., J. Nucl. Mater. (2012), http://dx.doi.org/10.1016/j.jnucmat.2012.11.019
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Characterization of ODS (Oxide Dispersion Strengthened) Eurofer/Eurofer dissimilar electron beam welds L. Commin a,b,⇑, M. Rieth a, V. Widak a, B. Dafferner a, S. Heger a, H. Zimmermann a, E. Materna-Morris a, R. Lindau a a b
Karlsruhe Institute for Technology, IAM-AWP, Karlsruhe, Germany Institute for Research on Magnetic Fusion, CEA Cadarache, France
a r t i c l e
i n f o
Article history: Available online xxxx
a b s t r a c t Breeding blanket fabrication will need the assembly of reduced activation ferritic martensitic steels and their ODS alloyed grades. Dissimilar electron beam welds were performed using 6 mm thick Y2O3 ODS Eurofer and Eurofer 97/2 samples. Thereafter, an austenitization step and an annealing post-weld heat treatment were applied. The joining process was qualified using the microstructural and mechanical properties characterization of as-welded and post-weld heat treated samples. The results show that electron beam welding is efficient for joining dissimilar ODS Eurofer/Eurofer materials and that a further PWHT (post-weld heat treatment) is required. The weld microstructure presents large oxide particles that are detrimental to mechanical properties. However, the mechanical properties are improved compared to similar Eurofer joints and they are approaching Eurofer behavior. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Materials and methods
The concepts of breeding blankets that are developed in Europe (helium cooled liquid lithium and helium cooled pebble bed) will operate in the 523–823 K range for conservative approaches using reduced activation ferritic martensitic (RAFM) steels, and in the range of 523–923 K for more advanced versions based on ODS steels [1,2]. Indeed, ODS steels exhibit a similar creep strength at a 100 K higher temperature [1]. In ITER, breeding blankets mocks-up (called test blanket modules (TBMs)) will be tested using the same structural materials [3,4]. TBM fabrication involves joining procedures and the use of suitable post-weld heat treatments (PWHTs). Joining technologies and procedures for RAFM steels have already been widely studied [5–8] and some trials have been carried out using ODS RAFM steels [9]. In this study, dissimilar electron beam welding was performed between samples of ODS Eurofer and Eurofer 97/2 base materials [1]. The resulting microstructures were analyzed and their impact on the mechanical properties was investigated. This allowed for an assessment of the effectiveness of beam welding components for nuclear fusion applications.
The base materials used for the dissimilar welds are 6 mm thick plates of Eurofer 97/2 and ODS Eurofer. Electron beam welds were processed using 60 kV accelerating voltage, 27 mA beam current at 10 mm/s welding speed. The welding direction is parallel to the rolling direction. Two-step post-weld heat treatment (PWHT) was applied at 1323 K for 1 h followed by 1043 K for 2 h. The 1step post-weld heat treatment (1043 K for 2 h) was also studied. The PWHT influence was studied in [5] with the following conclusions: 2-step heat treatment modifies the grain weld microstructure and leads to lower hardness levels; single-step heat treatments do not change the microstructure but soften the weld. Optical microscopy was performed on etched samples (with 4 g picric acid, 400 ml ethanol, 5 ml hydrochloric acid, and 5 ml nitric acid) to assess the weld quality and characterize the grain microstructure. Beraha etching (1 g potassium bisulfite, 100 ml stock solution (24 g ammonium difluoride, 1000 ml distilled water, 200 ml hydrochloric acid)) was used to characterize the martensitic structure. Philips XL30 ESEM was used to observe the precipitation evolution and for fractography analysis. Transmission electron microscopy (TEM) samples were made from the 6 mm thick plates by grinding down samples of one square centimeter to 100 lm thickness by using different silicon carbide polishing papers. They were then polished electrochemically in a Struers Tenupol-3 jet polisher. Microhardness tests (10 N, 30 s) were carried out using a Zwick 3212 facility. Charpy tests were performed using a Zwick Roell
⇑ Corresponding author. Address: Lorelei Commin, KIT, IAM-AWP, P.O. Box 3640, 76021 Karlsruhe, Germany. E-mail address: [email protected] (L. Commin). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.11.019
Please cite this article in press as: L. Commin et al., J. Nucl. Mater. (2012), http://dx.doi.org/10.1016/j.jnucmat.2012.11.019
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L. Commin et al. / Journal of Nuclear Materials xxx (2012) xxx–xxx
Fig. 1. Sketch of (a) the creep samples, and (b) the Charpy samples.
Fig. 4. TEM Bright Field image of the ODS base metal.
facility. The Charpy sample notch was located in the fusion zone (FZ). Creep tests were performed from 773 K to 973 K using conventional lever arms and weights. Fig. 1 illustrates the geometry of the Charpy and creep samples. 3. Results and discussion 3.1. As-welded condition Fig. 2. Optical micrography of the weld cross-section.
The weld cross-section micrography did not present any cracks or any porosity (Fig. 2). Fig. 2 exhibits the five different zones of the EB weld: the FZ, the heat affected zones (HAZs) on the Eurofer side and on the ODS Eurofer side and the base metals (BMs).
Fig. 3. SEM Microstructure of (a) Eurofer 97/2 base metal, (b) ODS Eurofer base metal, (c) FZ as-welded and (d) FZ after PWHT.
Please cite this article in press as: L. Commin et al., J. Nucl. Mater. (2012), http://dx.doi.org/10.1016/j.jnucmat.2012.11.019
L. Commin et al. / Journal of Nuclear Materials xxx (2012) xxx–xxx
3
Fig. 5. Optical micrographs of each zone of the as-welded sample using Beraha etchant.
Eurofer base metal is characterized by a 10 lm grain size microstructure, with 100 nm large M23C6 carbide precipitates located at the grain boundaries (Fig. 3a). ODS Eurofer base metal is characterized by a 3 lm grain size microstructure (Fig. 3b) with 10–20 nm diameter intragranular ODS particles (Fig. 4), as already fully described in previous papers [10,11]. After EB welding, the FZ presents a large increase in its grain size compared to the BM, carbide dissolution and a huge coarsening of the ODS particles (Figs. 3c and 4). In Fig. 5, Beraha etching was used so that the dark areas highlight the martensitic phase. Therefore, it can be seen that the martensitic structure is refined in both HAZs. The carbide morphology remained similar in the HAZ.
3.2. Microstructure after PWHT After PWHT (1323 K for 1 h + 1043 K for 2 h), a significant increase in the grain size is observed in the FZ and in the Eurofer side HAZ (Fig. 6). The grain size increases as well in the boundary between the FZ and the ODS Eurofer HAZ. The grain size remained similar in the ODS Eurofer side due to the stabilizing effect of the oxide particles. Carbide re-precipitation was observed along the edges of the martensite laths in the FZ and they grew with the thermal treatment in the other zones (Fig. 3d). The large ODS particles in the FZ did not act as carbide nucleation sites. When only applying the 1043 K for 2 h tempering treatment the precipitation evolution observed was the same as after the 2-step PWHT, whereas the grain size was similar to the as-welded sample. Therefore, the grain size is influenced solely by the solution treatment whereas the carbide precipitation is driven by the tempering treatment. 3.3. Microhardness profiles The microhardness profile across the weld and its evolution after PWHT is shown in Fig. 7. The microhardness is higher on the ODS Eurofer side due to the strengthening effect of the ODS particles. In the as-welded sample, hardening is observed in both HAZs due to the finer martensitic structure, and then a slight softening occurs in the FZ due to a larger grain size, larger martensite laths, and the loss of the ODS strengthening effect. After PWHT, general softening occurs caused by the tempering of the martensite structure and the carbide precipitation. The loss of the strengthening effect of the ODS particles due to their coarsening is obvious since the hardness values in the FZ are similar to the Eurofer base metal values. 3.4. Charpy testing
Fig. 6. Grain size comparison of as-welded and PWHT samples, measurements taken at the mid thickness.
Charpy tests were carried out on as-welded samples and on samples after PWHT (Fig. 8). The results are compared to those
Fig. 7. HV profile across the weld and its change due to the PWHT.
Please cite this article in press as: L. Commin et al., J. Nucl. Mater. (2012), http://dx.doi.org/10.1016/j.jnucmat.2012.11.019
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Fig. 8. Charpy results and typical fracture surface of a Charpy sample exhibiting an ODS particle.
Fig. 9. Comparison of Larson–Miller diagrams of ODS Eurofer Eurofer 97/2 EB welds, Eurofer 97/2 EB welds, ODS Eurofer base metal and Eurofer 97/2 base metal.
of Eurofer BM, ODS Eurofer BM, Eurofer similar EB welds, and ODS Eurofer similar EB welds taken from previous work [9]. After dissimilar EB welding, the fracture is located in the FZ for most of the samples, apart from as-welded brittle fractures that occurred in the ODS Eurofer HAZ. After PWHT, the impact energy is higher and the DBTT decreases. The Charpy properties are therefore much better after PWHT due to the tempered microstructure. The DBTT obtained after PWHT is lower than that of the ODS Eurofer samples (BM and welds); and it is close to the DBTT of the Eurofer welds. Fig. 8 shows a typical fracture surface. The large ODS particles that were seen in the FZ within the microstructural investigations can be seen quite clearly embedded in the dimples. This phenomenon had already been observed in similar ODS EB welds [9] and is generally responsible for the loss in ductility of EB welds containing ODS particles.
Fig. 10. (a) Optical micrograph of the typical fracture of ODS Eurofer Eurofer 97/2 EB welds until 823 K, (b) optical and (c) SEM images of the typical fracture of ODS Eurofer Eurofer 97/2 EB welds from 873 K.
Please cite this article in press as: L. Commin et al., J. Nucl. Mater. (2012), http://dx.doi.org/10.1016/j.jnucmat.2012.11.019
L. Commin et al. / Journal of Nuclear Materials xxx (2012) xxx–xxx
3.5. Creep tests The Larson–Miller diagram of the dissimilar welds was plotted and compared to the BM and similar Eurofer 97/2 EB weld results taken from previous works (Fig. 9) [1,5]. For temperature below 823 K, the fracture occurred in the Eurofer 97/2 base metal zone with a characteristic radial symmetrical necking (Fig. 10a). At higher temperatures (from 873 K), the creep strength is reduced and the fracture occurred in the FZ without section reduction (Fig. 10b). Large ODS particles, resulting from the coarsening occurring during the welding process, were observed on the fracture surface (Fig. 10c). With increasing temperature, at lower stresses, creep inter-granular fracture is observed due to the presence of the large oxide particles which act as stress concentrators, causing cavity nucleation. ODS Eurofer base metal presents the higher creep strength values as the fine dispersed ODS particles inhibit dislocation motion. The dissimilar EB weld creep behavior is close to that of the Eurofer 97/2 base metal. Indeed, due to their coarsening, the strengthening effect and creep deformation resistance of the ODS particles is lost. The dissimilar EB weld shows higher values than the similar EB weld. Eurofer similar EB welds showed gliding deformation above 773 K [5]. Fracture occurred in the fine grained HAZ that remains after a single-step 973 K/2 h PWHT. In the dissimilar weld, an austenitization PWHT was applied which modifies the grain structure in the weld zone. That would explain the higher creep properties obtained for the dissimilar welds. 4. Conclusions This study shows the effectiveness of joining ODS Eurofer to Eurofer 97 by EB welding and the requirement of PWHT. Higher creep strength and lower DBTT can be achieved compared to both similar EB welds. The use of PWHT is required, resulting in:
5
– Grain structure modification in the weld zone and therefore, improved creep behavior. – Carbide re-precipitation in the FZ and therefore, softening of the weld zone. The formation of large oxide particles in the weld zone leads to mechanical property reduction compared to the base metal. Acknowledgements The authors wish to thank S. Fischer, for the electron beam welding and T. Fabri, for fabrication samples. References [1] R. Lindau, A. Möslang, M. Rieth, M. Klimiankou, E. Materna-Morris, A. Alamo, A.-A.F. Tavassoli, C. Cayron, A.-M. Lancha, P. Fernandez, N. Baluc, R. Schäublin, E. Diegele, G. Filacchioni, J.W. Rensman, B.v.d. Schaaf, E. Lucon, W. Dietz, Fusion Eng. Des. 75–79 (2005) 989–996. [2] R. Lässer, N. Baluc, J.-L. Boutard, E. Diegele, S. Dudarev, M. Gasparotto, A. Möslang, R. Pippan, B. Riccardi, B. van der Schaaf, Fusion Eng. Des. 82 (2007) 511–520. [3] Y. Poitevin, L.V. Boccaccini, A. Cardella, L. Giancarli, R. Meyder, E. Diegele, R. Laesser, G. Benamati, Fusion Eng. Des. 75–79 (2005) 741–749. [4] L.V. Boccaccini, J.-F. Salavy, O. Bede, H. Neuberger, I. Ricapito, P. Sardain, L. Sedano, K. Splichal, Fusion Eng. Des. 84 (2009) 333–337. [5] M. Rieth, J. Rey, J. Nucl. Mater. 386–388 (2009) 471–474. [6] P. Aubert, F. Tavassoli, M. Rieth, E. Diegele, Y. Poitevin, J. Nucl. Mater. 409 (2011) 156–162. [7] M. Rieth, Fusion Eng. Des. 84 (2009) 1602–1605. [8] A. Cardella, E. Rigal, L. Bedel, Ph. Bucci, J. Fiek, L. Forest, L.V. Boccaccini, E. Diegele, L. Giancarli, S. Hermsmeyer, G. Janeschitz, R. Lässer, A. Li Puma, J.D. Lulewicz, A. Möslang, Y. Poitevin, E. Rabaglino, J. Nucl. Mater. 329–333 (2004) 133–140. [9] R. Lindau, M. Klimenkov, U. Jäntsch, A. Möslang, L. Commin, J. Nucl. Mater. 416 (2011) 22–29. [10] M. Klimiankou, R. Lindau, A. Möslang, J. Nucl. Mater. 329–333 (2004) 347–351. [11] M. Klimiankou, R. Lindau, A. Möslang, J. Nucl. Mater. 367–370 (2007) 173–178.
Please cite this article in press as: L. Commin et al., J. Nucl. Mater. (2012), http://dx.doi.org/10.1016/j.jnucmat.2012.11.019