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Low activation steels welding with PWHT and coating for ITER test blanket modules and DEMO
Journal of Nuclear Materials 409 (2011) 156–162
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Low activation steels welding with PWHT and coating for ITER test blanket modules and DEMO P. Aubert a,⇑, F. Tavassoli b, M. Rieth c, E. Diegele d, Y. Poitevin d a
CEA Saclay, DEN/DM2S, F-91191 Gif sur Yvette, France CEA Saclay, DEN/DMN, F-91191 Gif sur Yvette, France c KIT, IMF I, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany d Fusion for Energy (F4E), C/Josep Pla 2 – Ed. B3, 08019 Barcelona, Spain b
a r t i c l e
i n f o
Article history: Available online 1 October 2010
a b s t r a c t EUROFER weldability is investigated in support of the European material properties database and TBM manufacturing. Electron Beam, Hybrid, laser and narrow gap TIG processes have been carried out on the EUROFER-97 steel (thickness up to 40 mm), a reduced activation ferritic–martensitic steel developed in Europe. These welding processes produce similar welding results with high joint coefficients and are well adapted for minimizing residual distortions. The fusion zones are typically composed of martensite laths, with small grain sizes. In the heat-affected zones, martensite grains contain carbide precipitates. High hardness values are measured in all these zones that if not tempered would degrade toughness and creep resistance. PWHT developments have driven to a one-step PWHT (750 °C/3 h), successfully applied to joints restoring good material performances. It will produce less distortion levels than a full austenitization PWHT process, not really applicable to a complex welded structure such as the TBM. Different tungsten coatings have been successfully processed on EUROFER material. It has shown no really effect on the EUROFER base material microstructure. Ó 2011 Published by Elsevier B.V.
1. Introduction
2. Welding developments
Two references Tritium Breeder Blanket concepts for a DEMO fusion reactor [1] are developed in EU: a Helium-Cooled Lithium–Lead (HCLL) concept which uses PbLi as breeder and neutron multiplier, and a Helium-Cooled Pebble-Bed (HCPB) concept containing lithiated ceramic pebbles as breeder and beryllium pebbles as neutron multiplier. Both concepts use EUROFER steel, reduced activation ferritic–martensitic steel developed in Europe [2]. This paper will focus on welding aspects of the EUROFER steel. TBM modules inherently have a very complex assembly sequence [3–6] and need a sophisticated joining strategy for mastering residual stresses (Fig. 1). PWHT developments have been driven on the former welds for restoring good material performances, compatible to a pressure structure such TBM. Different tungsten coatings have been processed on EUROFER material for protecting the EUROFER steel against corrosion by liquid PbLi metal. The coatings effects on the EUROFER metallurgical behaviour have been considered, related to joints metallurgical transformations.
The design of TBM’s structures imposes intrinsic fabrication difficulties like rectangular box shape, internal grid structure, meandering channels embedded within plates for pressurized helium coolant circulation as well as limited access during assembly. These require the implementation and optimization of several welding processes like laser, TIG and diffusion welding processes often developed beyond the state-of-the-art. It shows what the limitation of conventional arc or beam welding processes are for an application on EUROFER material and for TBM design requirements. The achievements obtained in more advanced processes are presented in particular the ones making use of the high power YAG laser technology. The strategy for the future qualification of optimized welding procedures against nuclear codes and standards must be established. Welding joints can be classified in two categories.
⇑ Corresponding author. Tel.: +33 16908 3799; fax: +33 16908 6642. E-mail address: [email protected] (P. Aubert). 0022-3115/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.jnucmat.2010.09.009
2.1. Low thickness welding (below 11 mm) These welds are related to cooling plates manufacturing (range of 1–2 mm) and stiffening grid assembly (range of 8–11 mm). TIG, laser, and Electron Beam (EB) processes have been investigated with joint geometry and thicknesses coming from the TBMs
P. Aubert et al. / Journal of Nuclear Materials 409 (2011) 156–162
157
EB access to the joint must be carefully managed for TBM manufacturing. This process can be taken in account for SG welding.
Fig. 1. Welds to perform in TBM box measuring 1655 (poloidal) 484 (toroidal) 575 (radial) mm3.
designs. In the specific case of stiffening grid (SG) joining, the welding developments have been driven by metallurgical concerns and geometrical constraints, like 8 mm distance between the weld axe and the SG wall. This configuration requires using dedicated welding tools, particularly to meet the size constraint (14 mm diameter). 2.1.1. TIG welding Narrow gap TIG welding (NGTIG) experiments have been conducted for an 8 mm plate with two roots passes (one for each side) and four filling passes [7]. Distortion level in the joint was controlled through conventional methods of decreasing the welding energy and the weld width (Fig. 2). No cracks or inclusions were observed in the weld but distortion levels were inacceptable. Obviously, the same behaviour is observed on lower thickness TIG welds such cooling channel manufacturing in the two step HIP welding process of the cooling plates [3,8,9]. Weld fusion zone (FZ) presents equiaxed martensite lath structure. Grain size is quite irregular and in the range of 40 lm to 100 lm, 2–4 times larger than the base metal (BM) grain size (20 lm). The Heat-Affected Zone (HAZ) has a fine martensite grain size, smaller by about a factor of 2 in comparison with the BM, and extends over 5.5 mm. In HAZ, fine M23C6 carbide precipitates are observed, driven by a?b transformation fully martensitic. The hardness profile average level for FZ is very high, around 436 HV0.5, which is 205 HV0.5 higher than the BM. The slight decrease in hardness in the HAZ is due to carbide formation. 2.1.2. Electron Beam EB welding can be successfully used for thin plates welding, up to about 15 mm, for SG and Capping Plate (CP) joining [10–12]. The weldability of thin sections has been fully demonstrated, as shown in Fig. 3. Welds metallurgical behaviour is the same as laser process. Low porosity levels are achieved with enough weld widths. Vacuum atmosphere provides shiny welds without any oxidation. However,
2.1.3. Laser welding The first work, using a 4 kW laser on 8 mm thickness EUROFER samples, was to develop a set of laser welding parameters that would avoid hot cracking, observed in the initial welding trials [7]. Twin laser spot welding gave a wider FZ, by increasing the surface of the laser beam deposition, and thus decreased strongly the cooling rate at the solidification phase. Welds cross-section showed quite good weld aspects, with no problem in the overlapping of the two weld passes. On 1 mm thickness samples, sound welds have been performed with no metallurgical defects. Those welds are dedicated to the cooling channel manufacturing of the cooling plates, before hipping. Laser process induces less welding distortions than TIG welding, due to better energy input in the material. Until 2006, available industrial laser power sources were less than 6 kW, which required using two opposite welding passes when welding thick sections. In using the new 8 kW laser of CEA, single pass on 8 and even 11 mm with the increasing of the welding travel speed have been processed to decrease residual stresses and thus distortions [9]. All the samples were welded using EUROFER 8 mm thickness plates, in butt configuration with the following welding parameters: 1 pass welding at 8 kW average power, gas shield: argon, laser spot size: 150 lm, welding travel speed: 0.5 m/min. For 11 mm samples, an impressive increase in the welding travel speed from 0.3 mm (8 kW laser) to 2.5 m/min (10 kW laser) on 11 mm samples has been carried out. Sound joints were produced confirming earlier assessment that travel speeds greater than 1 m/min are required. Subsequent examinations showed that FZs are parallel and narrow. The seams obtained are narrow and look like EB welds, which is quite good for performing low distortions level. Full penetration and low pore numbers have been achieved, with low top and bottom weld undercuts. Hardness profiles across sections (Fig. 4) showed no effect on the cooling channels, and no pores were observed. Micro structural observations show the same metallurgical aspects as those described above for 4 kW welding. The enhanced weld shape did not produce any change in the FZ or the HAZ. Fig. 4 presents microstructures observed in the joint from FZ towards BM. In the HAZ, exposed to high temperatures during welding operation, partial austenitization occurs. In the subsequent cooling phase, austenitic grains are transformed into martensite. Thus, in this area, microstructure is composed of tempered martensite with fine carbide precipitations and transformed martensite. In moving towards the FZ, temperature increases, as the amount of martensite. Above Ac3 temperature, the austenitization is fully achieved and the microstructure is fully
TIG Fusion Zone
TIG welds Heat Affected Zone Fig. 2. TIG welding configuration for stiffening grid and multi-pass weld microstructure in FZ and HAZ.
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Fig. 3. Macrographs of (a) 5 mm (b) and 40 mm (c) EB welds.
FZ
I
Full Austenitization I Partial Austenitization I
Base Material
Fig. 4. Microstructures observed in laser welded joint, from FZ to BM (11 mm thick EUROFER steel welded with EDM channels).
martensitic. Near the FZ, temperatures become very high, and induce a secondary recrystallisation (overheating), increasing the grain size. In this area, no carbide precipitation is observed. In the FZ, solidification mode is columnar dendritic. This solidification mode, resulting from high welding travel speeds used, pushes the impurities to the weld centreline, which can be critical. These areas are richer in chromium and tungsten, confirmed by EDS examina-
tions. Sulphur enrichment was also observed and hot cracking can occur. The FZ structure is fully martensitic with high hardness. The laser welding parameters need to be refined in the future work, by reducing the cooling rate and/or increasing the laser spot size. The quite good welding results obtained have permitted since development of a repair operation. It consists to apply a second weld pass, with the same laser nominal welding parameters, on
P. Aubert et al. / Journal of Nuclear Materials 409 (2011) 156–162
the first pass. The results obtained are quite encouraging. No differences in metallurgical aspects are observed, except a small increase (10%) in the width of the FZ and the HAZ. No hardness increase is observed in the FZ. If a PWHT is applied, this effect will be smoothed or fully annealed out. A real welding breakthrough is achieved. 2.2. High depth welding (range 13–40 mm) It consists of welding the CP to the FW. The considered thicknesses to be welded are the following: 30 mm for the first wall and 42 mm for box covers. TBM Back Plates thicknesses are in the same range. Two welding strategies are possible: – Single pass welding with a huge molten pool and high cooling rate by EB process. – Multi-pass welding; root pass and filling passes with low penetration depth, depending on welding process: NGTIG or Hybrid process. 2.2.1. Electron Beam The EB process has been investigated for thick EUROFER plate welding application [10–12]. There, a 40 mm thick EUROFER sample was butt-welded. The width of the EB weld was less than
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2 mm. Micro structural observations show no evidence of defects inside the weld and some pores in the welds. However, delta-ferrite is found in the FZ. This can be avoided by pre-heating, but then the formation of pores is enhanced. In principle, the unfavorable outcome of EB welding can be suppressed by using a weld underlay (e.g. temporary backing strip), but this can work for a weld depth of about 20 mm. Therefore, for welding thicknesses greater than 20 mm, additional welding developments are needed.
2.2.2. Hybrid welding In several ITER tasks [13–15], this technology has been developed. It combines YAG continuous laser and MAG welding processes in the same molten pool [7,9]. This multi-pass welding process has been fully applied to EUROFER 25 mm thickness plates. Hybrid process is a high speed, allowing a travel speed in the range of 1.3 m/min, compared to the NGTIG: 0.1 m/min, with reduced top groove width: 8 mm (NGTIG: 14 mm). Sidewall penetration thickness is about 1 mm, in the same depth as in NGTIG reference process. Fourteen filling passes were necessary for filling the joint grooves, with the chamfer design of Fig. 5. Each welding pass performs 2.5 mm penetration depth thickness (1.2 mm maximum in NGTIG). Deposited mass rate is 10 times higher in the Hybrid process, and distortions are thus markedly reduced. Good external aspect without any oxidation is observed and welds present a very
Fig. 5. Hybrid welding configuration and application examples in TBM design.
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Table 1 Welds microstructures analysis. Weld penetration depth (mm)
Process
Base material 40 20 8 8
– EB Hybrid TIG Laser
Grain size (lm) Fusion zone
HAZ
12–15 30 20 40–100 10–20
12–15 15–20 8 8 10–15
D-Ferrite
Pores (mm)
No Yes No No No
0 0.3 0.3 0.26 0.32
good compacity. They show no defects and only a few pores (0.3 mm diameter). The HAZ microstructure is composed of fine martensite grains with fine M23C6 carbide precipitates. Maximum hardness level is observed in the FZ: between 410 and 490 HV1, depending on the EUROFER filler metal used. The FZ hardness fluctuations are explained by the multi-pass welding procedure. The influence of 200 °C pre- and post-heating is too low for reducing significantly the FZ hardness fluctuations. In-house work performed by Hybrid process on 9 Cr steels (same steel family) has shown that 400 °C pre- and post-heating treatment are more effective and produce 350 HV1 in FZ, but have no effect on the carbide precipitation in the HAZ. 2.3. Metallurgical considerations Ferritic–martensitic steels like EUROFER suffer hardening and embrittlement due to uncontrolled martensite formation in the weld and softening effect in HAZ. Welding behaviour is summarized in Table 1. Whatever the welding process, the joint penetration depths and welding strategy (single or multipass), the same welding behaviour is observed, with attenuation or enhanced effects, depending on cooling rates and weld penetration. Strong hardness increasing in FZ and smoothing effects in HAZ are observed for each welding configuration. The FZ are composed of martensite laths, with low grain size. In HAZ, martensite grains are observed with M23C6 carbide precipitation. Ferrite d has been observed only on Electron Beam welds, due to the very high cooling rate in solidification phase, related to strong enhanced weld shape. To restore good weld properties, post weld heat treatment (PWHT) is necessary. 3. Post weld heat treatment The high hardness level observed in the FZ and the hardness decrease in HAZ show clearly the necessity to perform a PWHT. It shall help reducing the coarse grain region of the FZ and the carbides precipitation of the HAZ. Two approaches (Fig. 6) were tested on 8/11 mm thick butt welded samples [9]: – Tempering at 750 °C ± 10 °C, 3 h, then cooling down to 20 °C in 3–5 h.
Fusion zone
HAZ
Width (mm)
Hardness level HV1
Width (mm)
Hardness level HV
No 3 9 7.6 6.8
220–230 420–460 410–490 400–460 430–480
No 1.5–2 1.5–2 2–3 0.8
230 200–210 210–220 200–210 210–220
– Full re-austenitization at 1050 °C, aimed at restoring a uniformly distributed fine grain size, followed by tempering (750 °C, see above). In samples tempered at 750 °C, the FZ structure is a fully tempered martensitic (confirmed by hardness profiles) with fine carbides precipitation. In the HAZ a progressive increase of the granular structure size is observed. The precipitation type and morphology is different to the one of the FZ. The base material has undergone an over-tempering (before and after welding). Hardness profiles indicated a softening of the FZ and HAZ at 240 HV. On the other hand, the two-steps PWHT lead to a full re-austenitization aimed at eliminating the structure gradients induced by welding thermal cycles. The resulting structure is fully tempered martensite which explains a homogeneous hardness profile all along the joint. However, a temperature of 1050 °C did not allow providing a homogenous granular structure in the whole joint: in the FZ, the grain size of former austenite grains remains higher than in other joint areas. Hardness profiles indicate a homogenous hardness level of 210 HV, close to the one of the base material. In conclusion, it does not seem necessary to perform a full reaustenitization PWHT. A standard tempering at 750 °C appears sufficient to restore joints performances sufficiently close to the base material. This result is a favorable outcome with respect to the assembly sequence and potentially induced distortions issues.
4. Filler wire optimization EUROFER welding behaviour is quite close to 9% Chromium steels, for which a filler wire chemical composition has been recommended, NGTIG process [16–19]. On second hand, it has been selected two filler wires optimized for 12 Cr steel welding. Previous work done for optimizing the chemical composition of the EUROFER filler welding wire has shown large flexibility (Table 2). EUROFER weldability can be concluded as not critical for the TBM development. Whatever the filler Wire chemical composition, even for wires out the standard selection window for 9 Cr families (12 Cr steels), no hot and cold cracks are observed in EUROFER TIG welding (Fig. 7). No defects and very good compacity have been processed. No internal defects such as porosity or cracks have been
Fig. 6. Definition of the 2 PWHT processes.
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P. Aubert et al. / Journal of Nuclear Materials 409 (2011) 156–162 Table 2 Chemical compositions of the EUROFER steel and its recommended filler metal. Filler wire chemical composition Weight (%)
EUROFER base material
Supercore F92 FCW
Thermanit MTS 616
C Mn P S Si Cu Ni Cr Mo Al Nb V N2 O2 W Ti Co B Ta
0.105 0.550 0.003 0.001 0.030 0.005 0.030 8.950 0.005 0.006 0.004 0.202 0.038 0.001 1.040 0.001 0.009 0.001 0.140
0.110 0.800 0.017 0.010 0.300 0.050 0.500 9.000 0.450 0.010 0.040 0.200 0.040
0.110 0.630
0.180 0.600
0.220
0.300
0.690 8.800 0.530
0.600 11.000 1.000
0.050 0.190 0.050
0.300
1.700
1.520 0.007
Thermanit MTS 4 Si
0.500
0.003
EUROFER filler wire 0.105 0.550 0.003 0.001 0.030 0.005 0.030 8.950 0.005 0.006 0.004 0.202 0.038 0.001 1.040 0.001 0.009 0.001 0.140
Fig. 7. EUROFER welds with different filler wires.
Fig. 8. Example of tungsten coating on EUROFER base material (LAAPS process: laser combined to plasma).
revealed. The hardness measurements show differences in fusion zone, comprised between 400 and 500 HV1 according to the filler metal. This difference depends on the composition of the welding consumables which implies differences in chemical composition of the fusion zone. It has been clearly shown on those welds the
benefit of an optimized PWHT which decrease strongly to a rate of 330 HV1 in fusion zone by applying by example 750 °C – 2 or 3 h. EUROFER weldability can be concluded as not critical. It has been demonstrated a wide chemical composition window which allow to concentrate the filler wire composition on low activation
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and creep strength adjustments.
P. Aubert et al. / Journal of Nuclear Materials 409 (2011) 156–162
increasing,
by
filler
wire
compounds
5. Coatings Parts of EUROFER steel should be protected by anti-corrosion coatings. On one hand the coating material has to meet the selection criteria for materials used in radioactive environments. On the other hand the coating has to satisfy high quality standards concerning density and adhesion on the substrate surface. Several materials and coating technologies have been investigated [20], to find an appropriate tungsten coating solution: Atmospheric Plasma Spray (APS), Laser cladding, Laser Assisted Atmospheric Plasma Spraying (LAAPS), High Velocity Oxy-Fuel Spraying (HVOF), Cold Spray, Screen Printing, Detonation Spraying. They have produced interesting results: dense coatings, no lack of adhesion on the substrate, no overheating of the substrate (example given in Fig. 8). It is not necessary to use intermediate bond layer between tungsten and EUROFER steel, but direct coating is achievable. It has been shown the great interest of laser remelting on each coating process, procuring enhanced coating performances in terms of density, grain size and diffusion effect on the EUROFER substrate. It can be pointed the low level of overheating, produced by the coating process, below the tempering temperature of 750 °C with acceptable cooling rate. No damage effect will be driven inside EUROFER base material and welds. So, high temperature coatings such tungsten layers procure not damage troubles on EUROFER material (base material and welds). It remains to answer to the following question: PWHT process before or after coating carrying out. It seems to be considered yet both situations available, depending on coating process selection. 6. Conclusion With regard to weldability of EUROFER steel, it can be concluded: – EUROFER weldability is excellent with a large tolerance for filler metal composition. – Several welding processes can be used for welding first wall, cap plates and horizontal and vertical stiffening grids of TBM. The most promising are EB, high power laser and Hybrid welding depending on section size and design. – Procedures for application of a single post weld heat treatment, 750 °C for 3 h, have been established. – Coating processes will not procure any metallurgical trouble on EUROFER base material and welds. For the grid assembly by welding, promising results have been obtained with a 10 kW YAG laser process up to the fabrication of a one-cell mock-up where deformation remained limited, which is a favorable sign for selecting this process for building the entire grid. The conventional multi-passes TIG welding process induces more distortions but it could eventually still be considered when excessive gaps between plates are appearing along the grid assembly se-
quence. The assembly of thicker box subcomponents like the first wall and the covers will necessitate a multi-passes welding process. Hybrid MIG/laser or NGTIG could be considered and shall be discriminated in term of residual deformation and potential damage to neighboring cooling channels. The EB welding is not retained because of apparition of delta-ferrite in the fusion zone. PWHT processing is preferable. Acknowledgment This work, supported by the Euratom Communities under the contracts of Association between EURATOM-CEA and EURATOMFZK, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] Y. Poitevin, L.V. Boccaccini, A. Cardella, L. Giancarli, R. Meyder, E. Diegele, et al., Fus. Eng. Des. 75–79 (2005) 741–749. [2] R. Lindau, A. Moeslang, M. Rieth, M. Klimiankou, E. Materna-Morris, A. Alamo, A.-A.F. Tavassoli, et al., Fus. Eng. Des. 75–79 (2005) 989–996. [3] Y. Poitevin, Ph. Aubert, G. de Dinechin, E. Rigal, A. von der Weth, E. Diegele, M. Rieth, L.V. Boccaccini, J.-L. Boutard, F. Tavassoli, Breeder Blanket Manufacturing Technologies, ICFMC-14 Conference, in press. [4] T. Muroga, M. Gasparotto, S.J. Zinkle, Fus. Eng. Des. 61–62 (2002) 13–25. [5] Y. Poitevin et al., Fus. Eng. Des. 75–79 (2005) 741–749. [6] J.-F. Salavy, G. Aiello, Ph. Aubert, L.V. Boccaccini, M. Daichendt, G. de Dinechin, E. Diegele, L. M. Giancarli, R. Lässer, H. Neuberger, Y. Poitevin, Y. Stephan, G. Rampal, E. Rigal. Ferritic–martensitic steel test blanket modules: status and future needs for design criteria requirements and fabrication validation. In: 13th International Conference on Fusion Reactor Material, ICFRM 13, Nice, 10– 14 December 2007. [7] P. Aubert, F. Janin, E. Diegele, Y. Poitevin. Welding state of art for EUROFER 97 application to test blanket module for ITER reactor. In: 13th International Conference on Fusion Reactor Material, ICFRM 13, Nice, 10–14 December 2007. [8] E. Rigal, G. de Dinechin, G. Rampal, G. Laffont, J.-F. Salavy, Y. Poitevin, Fus. Eng. Des. 83 (2008) 1268–1272. [9] Ph. Aubert, F. Tavassoli, M. Rieth, E. Diegele, Y. Poitevin. Review of candidate welding processes for RAFM steels for test blanket modules for ITER and DEMO breeder blankets. In: 14th International Conference on Fusion Reactor Material, ICFRM 14, Sapporo, Japan, 6–11 September 2009. [10] Michael Rieth, Specific welds for test blanket modules, in: 13th International Conference on Fusion Reactor Material, ICFRM 13, Nice, 10–14 December 2007. [11] M. Rieth, J. Rey, J. Nucl. Mater. 386 (2009) 789–792. [12] M. Rieth, Fus. Eng. Des. 84 (2009) 1602. [13] L.P. Jones, P. Aubert, V. Avilov, W.F. Daenner, T. Jokinen, K.R. Nightingale, M. Wykes, Fus. Eng. Des. 69 (2003) 215–220. [14] F. Coste, Ph. Aubert, L. Jones, Nd: YAG laser welding of 60 mm thickness 316L parts using multiple passes, in: Proceedings of ICALEO’2001. Conference Jacksonville, USA. [15] Ph. Aubert, G. de Dinechin, F. Janin, Soudage hybride, pour des applications de soudage en chanfrein étroit, Matériaux 2006 congrès, Dijon (F), 14 November 2006. [16] F. Tavassoli, J. Nucl. Mater. 302 (2000) 73. [17] B. van der Schaaf, F. Tavassoli, C. Fazio, E. Rigal, E. Diegele, R. Lindau, Fus. Eng. Des. 69 (2003) 197–203. [18] Private Discussions with Pierre-Alexandre LEGAIT, Technical Assistance Engineer, Böhler Thyssen Welding. [19] A. Strang, Vodárek, Precipitation processes in martensitic 12 CrMoVNb steels during high temperature creep, in: A. Strang (Ed.), Micro Structural Development and Stability in High Chromium Ferritic Power Plant Steels, The Institute of Materials, 1997, pp. 31–51. [20] Ph. Aubert, Coating Qualification. TW2-TTBC003-D05 EFDA Task, 2007 Fusion Annual report.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Low activation steels welding with PWHT and coating for ITER test blanket modules and DEMO P. Aubert a,⇑, F. Tavassoli b, M. Rieth c, E. Diegele d, Y. Poitevin d a
CEA Saclay, DEN/DM2S, F-91191 Gif sur Yvette, France CEA Saclay, DEN/DMN, F-91191 Gif sur Yvette, France c KIT, IMF I, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany d Fusion for Energy (F4E), C/Josep Pla 2 – Ed. B3, 08019 Barcelona, Spain b
a r t i c l e
i n f o
Article history: Available online 1 October 2010
a b s t r a c t EUROFER weldability is investigated in support of the European material properties database and TBM manufacturing. Electron Beam, Hybrid, laser and narrow gap TIG processes have been carried out on the EUROFER-97 steel (thickness up to 40 mm), a reduced activation ferritic–martensitic steel developed in Europe. These welding processes produce similar welding results with high joint coefficients and are well adapted for minimizing residual distortions. The fusion zones are typically composed of martensite laths, with small grain sizes. In the heat-affected zones, martensite grains contain carbide precipitates. High hardness values are measured in all these zones that if not tempered would degrade toughness and creep resistance. PWHT developments have driven to a one-step PWHT (750 °C/3 h), successfully applied to joints restoring good material performances. It will produce less distortion levels than a full austenitization PWHT process, not really applicable to a complex welded structure such as the TBM. Different tungsten coatings have been successfully processed on EUROFER material. It has shown no really effect on the EUROFER base material microstructure. Ó 2011 Published by Elsevier B.V.
1. Introduction
2. Welding developments
Two references Tritium Breeder Blanket concepts for a DEMO fusion reactor [1] are developed in EU: a Helium-Cooled Lithium–Lead (HCLL) concept which uses PbLi as breeder and neutron multiplier, and a Helium-Cooled Pebble-Bed (HCPB) concept containing lithiated ceramic pebbles as breeder and beryllium pebbles as neutron multiplier. Both concepts use EUROFER steel, reduced activation ferritic–martensitic steel developed in Europe [2]. This paper will focus on welding aspects of the EUROFER steel. TBM modules inherently have a very complex assembly sequence [3–6] and need a sophisticated joining strategy for mastering residual stresses (Fig. 1). PWHT developments have been driven on the former welds for restoring good material performances, compatible to a pressure structure such TBM. Different tungsten coatings have been processed on EUROFER material for protecting the EUROFER steel against corrosion by liquid PbLi metal. The coatings effects on the EUROFER metallurgical behaviour have been considered, related to joints metallurgical transformations.
The design of TBM’s structures imposes intrinsic fabrication difficulties like rectangular box shape, internal grid structure, meandering channels embedded within plates for pressurized helium coolant circulation as well as limited access during assembly. These require the implementation and optimization of several welding processes like laser, TIG and diffusion welding processes often developed beyond the state-of-the-art. It shows what the limitation of conventional arc or beam welding processes are for an application on EUROFER material and for TBM design requirements. The achievements obtained in more advanced processes are presented in particular the ones making use of the high power YAG laser technology. The strategy for the future qualification of optimized welding procedures against nuclear codes and standards must be established. Welding joints can be classified in two categories.
⇑ Corresponding author. Tel.: +33 16908 3799; fax: +33 16908 6642. E-mail address: [email protected] (P. Aubert). 0022-3115/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.jnucmat.2010.09.009
2.1. Low thickness welding (below 11 mm) These welds are related to cooling plates manufacturing (range of 1–2 mm) and stiffening grid assembly (range of 8–11 mm). TIG, laser, and Electron Beam (EB) processes have been investigated with joint geometry and thicknesses coming from the TBMs
P. Aubert et al. / Journal of Nuclear Materials 409 (2011) 156–162
157
EB access to the joint must be carefully managed for TBM manufacturing. This process can be taken in account for SG welding.
Fig. 1. Welds to perform in TBM box measuring 1655 (poloidal) 484 (toroidal) 575 (radial) mm3.
designs. In the specific case of stiffening grid (SG) joining, the welding developments have been driven by metallurgical concerns and geometrical constraints, like 8 mm distance between the weld axe and the SG wall. This configuration requires using dedicated welding tools, particularly to meet the size constraint (14 mm diameter). 2.1.1. TIG welding Narrow gap TIG welding (NGTIG) experiments have been conducted for an 8 mm plate with two roots passes (one for each side) and four filling passes [7]. Distortion level in the joint was controlled through conventional methods of decreasing the welding energy and the weld width (Fig. 2). No cracks or inclusions were observed in the weld but distortion levels were inacceptable. Obviously, the same behaviour is observed on lower thickness TIG welds such cooling channel manufacturing in the two step HIP welding process of the cooling plates [3,8,9]. Weld fusion zone (FZ) presents equiaxed martensite lath structure. Grain size is quite irregular and in the range of 40 lm to 100 lm, 2–4 times larger than the base metal (BM) grain size (20 lm). The Heat-Affected Zone (HAZ) has a fine martensite grain size, smaller by about a factor of 2 in comparison with the BM, and extends over 5.5 mm. In HAZ, fine M23C6 carbide precipitates are observed, driven by a?b transformation fully martensitic. The hardness profile average level for FZ is very high, around 436 HV0.5, which is 205 HV0.5 higher than the BM. The slight decrease in hardness in the HAZ is due to carbide formation. 2.1.2. Electron Beam EB welding can be successfully used for thin plates welding, up to about 15 mm, for SG and Capping Plate (CP) joining [10–12]. The weldability of thin sections has been fully demonstrated, as shown in Fig. 3. Welds metallurgical behaviour is the same as laser process. Low porosity levels are achieved with enough weld widths. Vacuum atmosphere provides shiny welds without any oxidation. However,
2.1.3. Laser welding The first work, using a 4 kW laser on 8 mm thickness EUROFER samples, was to develop a set of laser welding parameters that would avoid hot cracking, observed in the initial welding trials [7]. Twin laser spot welding gave a wider FZ, by increasing the surface of the laser beam deposition, and thus decreased strongly the cooling rate at the solidification phase. Welds cross-section showed quite good weld aspects, with no problem in the overlapping of the two weld passes. On 1 mm thickness samples, sound welds have been performed with no metallurgical defects. Those welds are dedicated to the cooling channel manufacturing of the cooling plates, before hipping. Laser process induces less welding distortions than TIG welding, due to better energy input in the material. Until 2006, available industrial laser power sources were less than 6 kW, which required using two opposite welding passes when welding thick sections. In using the new 8 kW laser of CEA, single pass on 8 and even 11 mm with the increasing of the welding travel speed have been processed to decrease residual stresses and thus distortions [9]. All the samples were welded using EUROFER 8 mm thickness plates, in butt configuration with the following welding parameters: 1 pass welding at 8 kW average power, gas shield: argon, laser spot size: 150 lm, welding travel speed: 0.5 m/min. For 11 mm samples, an impressive increase in the welding travel speed from 0.3 mm (8 kW laser) to 2.5 m/min (10 kW laser) on 11 mm samples has been carried out. Sound joints were produced confirming earlier assessment that travel speeds greater than 1 m/min are required. Subsequent examinations showed that FZs are parallel and narrow. The seams obtained are narrow and look like EB welds, which is quite good for performing low distortions level. Full penetration and low pore numbers have been achieved, with low top and bottom weld undercuts. Hardness profiles across sections (Fig. 4) showed no effect on the cooling channels, and no pores were observed. Micro structural observations show the same metallurgical aspects as those described above for 4 kW welding. The enhanced weld shape did not produce any change in the FZ or the HAZ. Fig. 4 presents microstructures observed in the joint from FZ towards BM. In the HAZ, exposed to high temperatures during welding operation, partial austenitization occurs. In the subsequent cooling phase, austenitic grains are transformed into martensite. Thus, in this area, microstructure is composed of tempered martensite with fine carbide precipitations and transformed martensite. In moving towards the FZ, temperature increases, as the amount of martensite. Above Ac3 temperature, the austenitization is fully achieved and the microstructure is fully
TIG Fusion Zone
TIG welds Heat Affected Zone Fig. 2. TIG welding configuration for stiffening grid and multi-pass weld microstructure in FZ and HAZ.
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Fig. 3. Macrographs of (a) 5 mm (b) and 40 mm (c) EB welds.
FZ
I
Full Austenitization I Partial Austenitization I
Base Material
Fig. 4. Microstructures observed in laser welded joint, from FZ to BM (11 mm thick EUROFER steel welded with EDM channels).
martensitic. Near the FZ, temperatures become very high, and induce a secondary recrystallisation (overheating), increasing the grain size. In this area, no carbide precipitation is observed. In the FZ, solidification mode is columnar dendritic. This solidification mode, resulting from high welding travel speeds used, pushes the impurities to the weld centreline, which can be critical. These areas are richer in chromium and tungsten, confirmed by EDS examina-
tions. Sulphur enrichment was also observed and hot cracking can occur. The FZ structure is fully martensitic with high hardness. The laser welding parameters need to be refined in the future work, by reducing the cooling rate and/or increasing the laser spot size. The quite good welding results obtained have permitted since development of a repair operation. It consists to apply a second weld pass, with the same laser nominal welding parameters, on
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the first pass. The results obtained are quite encouraging. No differences in metallurgical aspects are observed, except a small increase (10%) in the width of the FZ and the HAZ. No hardness increase is observed in the FZ. If a PWHT is applied, this effect will be smoothed or fully annealed out. A real welding breakthrough is achieved. 2.2. High depth welding (range 13–40 mm) It consists of welding the CP to the FW. The considered thicknesses to be welded are the following: 30 mm for the first wall and 42 mm for box covers. TBM Back Plates thicknesses are in the same range. Two welding strategies are possible: – Single pass welding with a huge molten pool and high cooling rate by EB process. – Multi-pass welding; root pass and filling passes with low penetration depth, depending on welding process: NGTIG or Hybrid process. 2.2.1. Electron Beam The EB process has been investigated for thick EUROFER plate welding application [10–12]. There, a 40 mm thick EUROFER sample was butt-welded. The width of the EB weld was less than
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2 mm. Micro structural observations show no evidence of defects inside the weld and some pores in the welds. However, delta-ferrite is found in the FZ. This can be avoided by pre-heating, but then the formation of pores is enhanced. In principle, the unfavorable outcome of EB welding can be suppressed by using a weld underlay (e.g. temporary backing strip), but this can work for a weld depth of about 20 mm. Therefore, for welding thicknesses greater than 20 mm, additional welding developments are needed.
2.2.2. Hybrid welding In several ITER tasks [13–15], this technology has been developed. It combines YAG continuous laser and MAG welding processes in the same molten pool [7,9]. This multi-pass welding process has been fully applied to EUROFER 25 mm thickness plates. Hybrid process is a high speed, allowing a travel speed in the range of 1.3 m/min, compared to the NGTIG: 0.1 m/min, with reduced top groove width: 8 mm (NGTIG: 14 mm). Sidewall penetration thickness is about 1 mm, in the same depth as in NGTIG reference process. Fourteen filling passes were necessary for filling the joint grooves, with the chamfer design of Fig. 5. Each welding pass performs 2.5 mm penetration depth thickness (1.2 mm maximum in NGTIG). Deposited mass rate is 10 times higher in the Hybrid process, and distortions are thus markedly reduced. Good external aspect without any oxidation is observed and welds present a very
Fig. 5. Hybrid welding configuration and application examples in TBM design.
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Table 1 Welds microstructures analysis. Weld penetration depth (mm)
Process
Base material 40 20 8 8
– EB Hybrid TIG Laser
Grain size (lm) Fusion zone
HAZ
12–15 30 20 40–100 10–20
12–15 15–20 8 8 10–15
D-Ferrite
Pores (mm)
No Yes No No No
0 0.3 0.3 0.26 0.32
good compacity. They show no defects and only a few pores (0.3 mm diameter). The HAZ microstructure is composed of fine martensite grains with fine M23C6 carbide precipitates. Maximum hardness level is observed in the FZ: between 410 and 490 HV1, depending on the EUROFER filler metal used. The FZ hardness fluctuations are explained by the multi-pass welding procedure. The influence of 200 °C pre- and post-heating is too low for reducing significantly the FZ hardness fluctuations. In-house work performed by Hybrid process on 9 Cr steels (same steel family) has shown that 400 °C pre- and post-heating treatment are more effective and produce 350 HV1 in FZ, but have no effect on the carbide precipitation in the HAZ. 2.3. Metallurgical considerations Ferritic–martensitic steels like EUROFER suffer hardening and embrittlement due to uncontrolled martensite formation in the weld and softening effect in HAZ. Welding behaviour is summarized in Table 1. Whatever the welding process, the joint penetration depths and welding strategy (single or multipass), the same welding behaviour is observed, with attenuation or enhanced effects, depending on cooling rates and weld penetration. Strong hardness increasing in FZ and smoothing effects in HAZ are observed for each welding configuration. The FZ are composed of martensite laths, with low grain size. In HAZ, martensite grains are observed with M23C6 carbide precipitation. Ferrite d has been observed only on Electron Beam welds, due to the very high cooling rate in solidification phase, related to strong enhanced weld shape. To restore good weld properties, post weld heat treatment (PWHT) is necessary. 3. Post weld heat treatment The high hardness level observed in the FZ and the hardness decrease in HAZ show clearly the necessity to perform a PWHT. It shall help reducing the coarse grain region of the FZ and the carbides precipitation of the HAZ. Two approaches (Fig. 6) were tested on 8/11 mm thick butt welded samples [9]: – Tempering at 750 °C ± 10 °C, 3 h, then cooling down to 20 °C in 3–5 h.
Fusion zone
HAZ
Width (mm)
Hardness level HV1
Width (mm)
Hardness level HV
No 3 9 7.6 6.8
220–230 420–460 410–490 400–460 430–480
No 1.5–2 1.5–2 2–3 0.8
230 200–210 210–220 200–210 210–220
– Full re-austenitization at 1050 °C, aimed at restoring a uniformly distributed fine grain size, followed by tempering (750 °C, see above). In samples tempered at 750 °C, the FZ structure is a fully tempered martensitic (confirmed by hardness profiles) with fine carbides precipitation. In the HAZ a progressive increase of the granular structure size is observed. The precipitation type and morphology is different to the one of the FZ. The base material has undergone an over-tempering (before and after welding). Hardness profiles indicated a softening of the FZ and HAZ at 240 HV. On the other hand, the two-steps PWHT lead to a full re-austenitization aimed at eliminating the structure gradients induced by welding thermal cycles. The resulting structure is fully tempered martensite which explains a homogeneous hardness profile all along the joint. However, a temperature of 1050 °C did not allow providing a homogenous granular structure in the whole joint: in the FZ, the grain size of former austenite grains remains higher than in other joint areas. Hardness profiles indicate a homogenous hardness level of 210 HV, close to the one of the base material. In conclusion, it does not seem necessary to perform a full reaustenitization PWHT. A standard tempering at 750 °C appears sufficient to restore joints performances sufficiently close to the base material. This result is a favorable outcome with respect to the assembly sequence and potentially induced distortions issues.
4. Filler wire optimization EUROFER welding behaviour is quite close to 9% Chromium steels, for which a filler wire chemical composition has been recommended, NGTIG process [16–19]. On second hand, it has been selected two filler wires optimized for 12 Cr steel welding. Previous work done for optimizing the chemical composition of the EUROFER filler welding wire has shown large flexibility (Table 2). EUROFER weldability can be concluded as not critical for the TBM development. Whatever the filler Wire chemical composition, even for wires out the standard selection window for 9 Cr families (12 Cr steels), no hot and cold cracks are observed in EUROFER TIG welding (Fig. 7). No defects and very good compacity have been processed. No internal defects such as porosity or cracks have been
Fig. 6. Definition of the 2 PWHT processes.
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P. Aubert et al. / Journal of Nuclear Materials 409 (2011) 156–162 Table 2 Chemical compositions of the EUROFER steel and its recommended filler metal. Filler wire chemical composition Weight (%)
EUROFER base material
Supercore F92 FCW
Thermanit MTS 616
C Mn P S Si Cu Ni Cr Mo Al Nb V N2 O2 W Ti Co B Ta
0.105 0.550 0.003 0.001 0.030 0.005 0.030 8.950 0.005 0.006 0.004 0.202 0.038 0.001 1.040 0.001 0.009 0.001 0.140
0.110 0.800 0.017 0.010 0.300 0.050 0.500 9.000 0.450 0.010 0.040 0.200 0.040
0.110 0.630
0.180 0.600
0.220
0.300
0.690 8.800 0.530
0.600 11.000 1.000
0.050 0.190 0.050
0.300
1.700
1.520 0.007
Thermanit MTS 4 Si
0.500
0.003
EUROFER filler wire 0.105 0.550 0.003 0.001 0.030 0.005 0.030 8.950 0.005 0.006 0.004 0.202 0.038 0.001 1.040 0.001 0.009 0.001 0.140
Fig. 7. EUROFER welds with different filler wires.
Fig. 8. Example of tungsten coating on EUROFER base material (LAAPS process: laser combined to plasma).
revealed. The hardness measurements show differences in fusion zone, comprised between 400 and 500 HV1 according to the filler metal. This difference depends on the composition of the welding consumables which implies differences in chemical composition of the fusion zone. It has been clearly shown on those welds the
benefit of an optimized PWHT which decrease strongly to a rate of 330 HV1 in fusion zone by applying by example 750 °C – 2 or 3 h. EUROFER weldability can be concluded as not critical. It has been demonstrated a wide chemical composition window which allow to concentrate the filler wire composition on low activation
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increasing,
by
filler
wire
compounds
5. Coatings Parts of EUROFER steel should be protected by anti-corrosion coatings. On one hand the coating material has to meet the selection criteria for materials used in radioactive environments. On the other hand the coating has to satisfy high quality standards concerning density and adhesion on the substrate surface. Several materials and coating technologies have been investigated [20], to find an appropriate tungsten coating solution: Atmospheric Plasma Spray (APS), Laser cladding, Laser Assisted Atmospheric Plasma Spraying (LAAPS), High Velocity Oxy-Fuel Spraying (HVOF), Cold Spray, Screen Printing, Detonation Spraying. They have produced interesting results: dense coatings, no lack of adhesion on the substrate, no overheating of the substrate (example given in Fig. 8). It is not necessary to use intermediate bond layer between tungsten and EUROFER steel, but direct coating is achievable. It has been shown the great interest of laser remelting on each coating process, procuring enhanced coating performances in terms of density, grain size and diffusion effect on the EUROFER substrate. It can be pointed the low level of overheating, produced by the coating process, below the tempering temperature of 750 °C with acceptable cooling rate. No damage effect will be driven inside EUROFER base material and welds. So, high temperature coatings such tungsten layers procure not damage troubles on EUROFER material (base material and welds). It remains to answer to the following question: PWHT process before or after coating carrying out. It seems to be considered yet both situations available, depending on coating process selection. 6. Conclusion With regard to weldability of EUROFER steel, it can be concluded: – EUROFER weldability is excellent with a large tolerance for filler metal composition. – Several welding processes can be used for welding first wall, cap plates and horizontal and vertical stiffening grids of TBM. The most promising are EB, high power laser and Hybrid welding depending on section size and design. – Procedures for application of a single post weld heat treatment, 750 °C for 3 h, have been established. – Coating processes will not procure any metallurgical trouble on EUROFER base material and welds. For the grid assembly by welding, promising results have been obtained with a 10 kW YAG laser process up to the fabrication of a one-cell mock-up where deformation remained limited, which is a favorable sign for selecting this process for building the entire grid. The conventional multi-passes TIG welding process induces more distortions but it could eventually still be considered when excessive gaps between plates are appearing along the grid assembly se-
quence. The assembly of thicker box subcomponents like the first wall and the covers will necessitate a multi-passes welding process. Hybrid MIG/laser or NGTIG could be considered and shall be discriminated in term of residual deformation and potential damage to neighboring cooling channels. The EB welding is not retained because of apparition of delta-ferrite in the fusion zone. PWHT processing is preferable. Acknowledgment This work, supported by the Euratom Communities under the contracts of Association between EURATOM-CEA and EURATOMFZK, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] Y. Poitevin, L.V. Boccaccini, A. Cardella, L. Giancarli, R. Meyder, E. Diegele, et al., Fus. Eng. Des. 75–79 (2005) 741–749. [2] R. Lindau, A. Moeslang, M. Rieth, M. Klimiankou, E. Materna-Morris, A. Alamo, A.-A.F. Tavassoli, et al., Fus. Eng. Des. 75–79 (2005) 989–996. [3] Y. Poitevin, Ph. Aubert, G. de Dinechin, E. Rigal, A. von der Weth, E. Diegele, M. Rieth, L.V. Boccaccini, J.-L. Boutard, F. Tavassoli, Breeder Blanket Manufacturing Technologies, ICFMC-14 Conference, in press. [4] T. Muroga, M. Gasparotto, S.J. Zinkle, Fus. Eng. Des. 61–62 (2002) 13–25. [5] Y. Poitevin et al., Fus. Eng. Des. 75–79 (2005) 741–749. [6] J.-F. Salavy, G. Aiello, Ph. Aubert, L.V. Boccaccini, M. Daichendt, G. de Dinechin, E. Diegele, L. M. Giancarli, R. Lässer, H. Neuberger, Y. Poitevin, Y. Stephan, G. Rampal, E. Rigal. Ferritic–martensitic steel test blanket modules: status and future needs for design criteria requirements and fabrication validation. In: 13th International Conference on Fusion Reactor Material, ICFRM 13, Nice, 10– 14 December 2007. [7] P. Aubert, F. Janin, E. Diegele, Y. Poitevin. Welding state of art for EUROFER 97 application to test blanket module for ITER reactor. In: 13th International Conference on Fusion Reactor Material, ICFRM 13, Nice, 10–14 December 2007. [8] E. Rigal, G. de Dinechin, G. Rampal, G. Laffont, J.-F. Salavy, Y. Poitevin, Fus. Eng. Des. 83 (2008) 1268–1272. [9] Ph. Aubert, F. Tavassoli, M. Rieth, E. Diegele, Y. Poitevin. Review of candidate welding processes for RAFM steels for test blanket modules for ITER and DEMO breeder blankets. In: 14th International Conference on Fusion Reactor Material, ICFRM 14, Sapporo, Japan, 6–11 September 2009. [10] Michael Rieth, Specific welds for test blanket modules, in: 13th International Conference on Fusion Reactor Material, ICFRM 13, Nice, 10–14 December 2007. [11] M. Rieth, J. Rey, J. Nucl. Mater. 386 (2009) 789–792. [12] M. Rieth, Fus. Eng. Des. 84 (2009) 1602. [13] L.P. Jones, P. Aubert, V. Avilov, W.F. Daenner, T. Jokinen, K.R. Nightingale, M. Wykes, Fus. Eng. Des. 69 (2003) 215–220. [14] F. Coste, Ph. Aubert, L. Jones, Nd: YAG laser welding of 60 mm thickness 316L parts using multiple passes, in: Proceedings of ICALEO’2001. Conference Jacksonville, USA. [15] Ph. Aubert, G. de Dinechin, F. Janin, Soudage hybride, pour des applications de soudage en chanfrein étroit, Matériaux 2006 congrès, Dijon (F), 14 November 2006. [16] F. Tavassoli, J. Nucl. Mater. 302 (2000) 73. [17] B. van der Schaaf, F. Tavassoli, C. Fazio, E. Rigal, E. Diegele, R. Lindau, Fus. Eng. Des. 69 (2003) 197–203. [18] Private Discussions with Pierre-Alexandre LEGAIT, Technical Assistance Engineer, Böhler Thyssen Welding. [19] A. Strang, Vodárek, Precipitation processes in martensitic 12 CrMoVNb steels during high temperature creep, in: A. Strang (Ed.), Micro Structural Development and Stability in High Chromium Ferritic Power Plant Steels, The Institute of Materials, 1997, pp. 31–51. [20] Ph. Aubert, Coating Qualification. TW2-TTBC003-D05 EFDA Task, 2007 Fusion Annual report.