- Email: [email protected]
Structural materials joints for ITER in-vessel components
Fusion Engineering and Design 39 – 40 (1998) 253 – 261
Structural materials joints for ITER in-vessel components G. Le Marois a,*, H. Burlet a, R. Solomon b, B. Marini c, J.M. Gentzbittel a, L. Briottet a a
CEA-Grenoble, DEM/SGM, 38054 Grenoble Cedex 9, France b SCM Metals Products, RTP, NC 277709 -2166, USA c CEA-Saclay, DECM/SRMA, Saclay, France
Abstract 316LN stainless steel (SS) and Glidcop AL-25 (DS-Cu) are primary candidates as structural materials for international thermonuclear experimental reactor (ITER) in-vessel components. Their joining is currently being studied at CEA/CEREM. This paper summarises recent progress on structural materials joining using a solid hot isostatic pressing (HIP) technique. The properties of the materials used and the effect of HIP cycles on the metallurgical quality of the SS are reported. A new specification for the DS-Cu is proposed. Different materials and surfaces preparation, the use of interlayers and different HIP conditions are presented and analysed in relation to the mechanical properties of the joints. Characterisation of the bi-metallic joints such as SS/DS-Cu is discussed and further developments with various mode loadings are briefly reported. Some preliminary modelling results regarding stress/strain distribution arising from manufacturing and thermal load conditions are presented. A visco-plastic material database is currently being developed for that purpose. © 1998 Published by Elsevier Science S.A. All rights reserved.
1. Introduction The international project ITER has to demonstrate the technical and scientific feasibility of controlled fusion on an industrial scale: a typical tokamak reactor is designed for that purpose. The internal components facing the plasma are modular made. The structural parts of these modules are made of 316LN stainless steel (SS) and a copper alloy heat sink (DS-Cu). Both are in* Corresponding author. E-mail: [email protected]
ternally cooled. The double curvature of the modules, double containment of the cooling and low leaks level requirement are the main design constraints. For their manufacture, solid hot isostatic pressing (HIP) has been proposed as it allows these drawbacks to be overcome. A review of the current research and development on SS/SS and Cu/ SS HIP joining performed at CEA is presented. A tentative specification of Cu is proposed. Some issues and improvements of the joints are discussed.
0920-3796/98/$19.00 © 1998 Published by Elsevier Science S.A. All rights reserved. PII S0920-3796(98)00108-2
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254
Table 1 Chemical composition of 316LN stainless steel billets Lot
Cr
Mo
Ni
C
Mn
Si
Cu
N2
S
P
PM130 T5091 IG Spec
17.45 17.5 17–18
2.5 2.4 2.3–2.7
12.23 12.16 12–12.5
0.024 0.02 0.015– 0.03
1.81 1.73 1.6 – 2
0.39 0.41 B0.5
0.28 0.23 B0.1
0.067 0.068 0.06 – 0.08
0.0011 0.001 0.005 – 0.01
0.022 0.025 B0.025
2. Materials
2.1. 316LN As-forged 316LN stainless steel (SS) billets have been provided by Tecphy. The first one (ref PM130) was produced under air, while the second one (ref T5091) was produced under vacuum. Except for Cu, their chemical composition (Table 1) are within ITER specifications. The mechanical properties of the two billets are slightly different at room temperature (RT). Both of them respect the RCC-MR recommendations (Table 2).
tect it against oxidation and to limit the formation of heterogeneities during cross-rolling. Cross-rolling reduction rates are improved to break particle alignment. As far as HIPing is concerned, the final annealing treatment is not specified, and outgassing at high temperature under vacuum is recommended prior to joining. Control: a test procedure is defined to control the size and distribution of the recrystallised grains and particles, in order to guarantee a good dispersion of the strengthened particles and to lower material and joints properties deviation.
2.2. Glidcop ® AL-25 3. SS/SS joining [2]
2.2.1. IG0 grade The IG0 grade, now considered as the reference grade for the DS-Cu alloy family, has been defined in collaboration with SCM Metals Products. The material provided by SCM is a Glidcop DS-Cu LOX AL-25, made using −80 mesh powder, extruded, declad, cross-rolled and stress relief annealed at approximately 1000°C. IG0 specification has been detailed [1]. It covers plates, 10–40 mm thick. IG0 plates 20 mm thick have been used in the present study. 2.2.2. IG1 grade Some bad metallurgical quality on declad material previous to the cross-roll operation was, however, observed, and CEA therefore proposed to SCM a new IG specification. The IG1 specification includes following improvements of the IG0 one: Manufacturing process: during the fabrication process, the material remains cladded to pro-
3.1. Manufacturing 3.1.1. Surface treatment The surfaces are finely machined (down to 1 mm) and degreased. A 304 stainless steel canister is welded around the parts to be bonded. 3.1.2. HIP conditions HIP parameters are as follows: temperature, 1090°C (PM130) and 1100°C (T5091); pressure, 120 MPa (PM130) and 100 MPa (T5091); and holding time, 2 h. 3.2. Metallographic examination of the joint (PM130 and T5091) Metallographic examinations of the joints (Fig. 1) show some recrystallised grains through the joint, indicating that diffusion has correctly occurred. Nevertheless, some precipitates identified
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Table 2 Mechanical properties of stainless steel billets at room temperature
RCC-MR PM130 T5091
YS (MPa)
UTS (MPa)
TE (%)
KCU (J cm−2)
\220 232–235 293
\525 549 – 550 551
\45 68.8 – 71.1 62
\120 266 – 270 272
as oxides can be observed along the joint (mainly for PM130).
3.3. Tensile, impact and fatigue tests (PM130) Testing specimens were cut out of the test blocks. The joint interface is located at the centre of the specimen, perpendicularly to the tensile axis. Tests have been performed at RT and 300°C, on joints and on base metal submitted to the same HIP treatment. Some specimens present a brittle fracture at the joint. This was observed at both temperatures. In that case, a high concentration of oxides is observed at the fracture interface. For all others specimens, the ultimate tensile stress of the bonded sample was nearly equal to that of the base ageing material. Nevertheless, it seems that the striction cannot occur on or close to the joint (Table 3). Impact test results show significant scatter, probably due to the presence of oxides at the interface (Table 3). At least, joint fatigue properties are within the scatter band of the base metal (Fig. 2).
4. DS-Cu/SS joining [3]
4.1. Manufacturing A low surface roughness (Ra 0.8 mm) machining is achieved. Surface preparations include chemical etching or chemical cleaning. In case of chemical cleaning, the DS-Cu parts were afterwards submitted to a heat treatment at 1000°C for 30 min, to reduce the copper oxides. Outgassings at various temperatures – times have been performed before sealing the can. HIP conditions used are summarised in Table 4.
4.1.1. Use of interlayers Various nickel-base interlayers have been tested. The objective is to limit the precipitation of Fe,Cr into the copper alloy, and to prevent the surface from further oxidation and/or contamination. Three different kinds of interlayers have been tested: ion plating coating, electrolytic coating and foils, with thickness in the range 5–100 mm. 4.2. Metallographic examination of the joints 4.2.1. Joint without interlayer Fig. 3 shows a typical microstructure of the joint after chemical etching. A diffusion layer parallel to the bond interface can be observed over a distance of about 40 mm. It is characterised by the presence of quasi-spherical particles with a diameter of about 1 mm. These particles are rich in Fe, Cr and Cu. Ni diffused in solid solution over a small distance. In SS, a slight depletion of chromium can be measured. Copper diffuses in SS in solid solution over a few micrometres. 4.2.2. Joint with an interlayer The use of an Ni interlayer limits the diffusion of Fe and Cr into DS-Cu. When the Ni thickness exceeds 10 mm, no precipitates are detected in the copper alloy. Ni diffuses in the DS-Cu over about 80 mm. 4.3. Mechanical T5091)
testing
(DS-Cu/PM130 and
4.3.1. Experimental procedure Tensile tests have been carried out on smooth axisymmetrical specimens of diameter 4 mm and gauge length 20 mm, at RT and at 300°C under
256
G. Le Marois et al. / Fusion Engineering and Design 39–40 (1998) 253–261
Fig. 1. Metallography of 316/316 joints.
vacuum. As the testing specimen’s axis is perpendicular to the interface, results have to be compared with those of DS-Cu in the short transverse direction. The deformation of the bi-material specimen being heterogeneous (DS-Cu undergoes higher deformation than 316LN), the ‘strain’ of the specimen has no physical meaning. In the following, it represents the ratio of the amount of total displacement over the initial gauge length. The strain rate was 10 − 4 s − 1.
4.3.2. Results and discussion All results obtained on joint without interlayers are listed in Table 5. IG* refers to a plate that was warm-rolled and heat-treated before being declad. The main results are the following: the maximum stress reached by the tensile specimen corresponds to the UTS of the DS-Cu itself. At RT, high ductilities obtained with the plate IG* specimen can be correlated to high necking located at about 4–5 mm. In this case, the deformation of the DS-Cu side is similar to that of the bulk DS-Cu. Numerical calculations simulating tensile tests on 316LN/DS-Cu specimens are in good agreement with these results. They predicted no stress concentration near the interface, due to similar mechanical behaviours of the two base materials. For IG0 specimens, most of the failures occurred within 1 mm of the interface. Necking was not observed in this case. The location of the
damage is probably related to the deep oxidation of the declad IG0 plate. At 300°C, although the UTS is reached in the DS-Cu within the joint, failures occur closer to the interface, and with a DS-Cu strain smaller than the total strain of a bulk DS-Cu. Numerical calculations show a high concentration of axial stress in the vicinity of the joint, on the edge of the DS-Cu part. This high stress could explain the abnormally rapid failure by a ‘delamination’ phenomenon of the DS-Cu plate. Tensile data on joints with Ni interlayers are summarised in Table 6. Obviously, ultimate tensile strengths and ductilities are worst in this case. The role of the nickel in the mechanical properties is not yet clearly understood. All the mechanical tests performed on smooth axisymmetrical specimens led to failure in the DS-Cu itself. To characterise the strength of the bonded interface itself, other specimens must be used. Axisymmetrical notched specimens, compact tension, disymmetric CT and specimens with internal flaws are planned in order to develop stress concentration along the interface and to generate various mode I/mode II ratio loadings.
4.4. Mock-up manufacturing and testing Mock-ups with different geometries and shapes (straight and bent), including for some armour material, have been manufactured, following a
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Table 3 Tensile and impact tests results on 316LN/316LN joints (PM130) Specimen
YS (MPa)
UTS (MPa)
TE (%)
UE (%)
KCU (J cm−2)
Joint, 20°C Base metal, 20°C (after HIP cycle) Joint, 300°C Base metal, 300°C (after HIP cycle)
246 – 255 260 162 – 161 160
563 – 566 580 425 – 441 446
45 – 58 59 31 – 44 42
44 – 51 50 31 – 40 35
62 – 122 274 – 287 47 – 314 254 – 264
relevant HIP manufacturing route. T8 (DS-Cu/SS with two SS cooling tubes, 1 mm thick) and T232 (W/DS-Cu/SS with three SS cooling tubes 0.5 mm thick) mock-ups have been tested under high heat flux (HHF) and thermal fatigue conditions in an electron gun facility.
4.4.1. HHF test A HHF test, up to 9.7 MW m − 2 absorbed flux, has been performed on the T8 mock-up. On the T232 mock-up, 250 cycles under 9 MW m − 2 have been performed. 4.4.2. Modelling and DT analysis In order to predict residual stresses distribution and permanent strain after the HHF test, an elasto-plastic calculation has been performed on the T8 mock-up and compared with experimental strain results. The main solicitations in a complex mode are shown to be at the SS tubes/DS-Cu joints: calculation gives 1.17 and 0.80% EPSE in DS-Cu and SS, respectively. Comparison with experimental measurement of the deformation indicates that improvement of the database is still required (Fig. 4). Further prediction improvements are still ongoing through development of the database, taking into account actual materials properties in an extended temperature range including visco-plastic data.
5. Effect of HIP cycles on SS properties For baffle fabrication, a manufacturing route has been proposed [4]. In order to provide full controllability of the joints, the component is
achieved in four HIP cycles: two cycles of 4/10/4 at 1095°C (4 h heating up, 10 h at 1095°C and 4 h cooling down) for the SS parts, followed by two cycles of 3/2/3 at 930°C for the heat sink, are performed. The effect of the four cycles on PM130 and T5091 properties has been analysed [5]. After one cycle, the grain size distribution appears quite heterogeneous. During the following cycles, grain growth enhances grain sizes scattering. The ASTM grain size number lies between 4 and 5 after one cycle and between 1 and 5 after four cycles. Moreover, some ferrite lines appeared in both materials, although in a lower proportion for T5091. A strong decrease of the hardness is observed after one cycle (200–140 HV40). The influence of the following cycles is less important ( 120 after four cycles). Tensile tests (PM130) show no influence of the cycles at room temperature. However, at 300°C a small decrease of the yield and ultimate strength is noticed. Finally, the effect of the HIP cycle on Charpy-U toughness (PM130) and fatigue resistance is not significant.
6. Observations and conclusions (i) The properties of the SS and DS-Cu joints are sensitive to the quality of the materials used. The presence of oxides and clusters at the interface weakens and embrittles the joints. (ii) Using SS and DS-Cu with improved quality, tensile and impact properties of the joints are similar to those obtained on the weaker material.
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Fig. 2. Fatigue resistance of SS HIP joint at 300°C (PM130). Comparison with 316LN SPH data.
The use of Ni interlayers does not improve the mechanical properties of the joints and is not recommended. (iii) For characterisation of bi-metallic joints such as DS-Cu/SS, it is necessary to generate various mode I/II loadings and to develop stress concentration along the interface. Testing of different specimens including compact tension, disymmetric CT are underway. (iv) SS and DS-Cu plate-to-plate and tube-toplate joints are able to sustain high strain/stress conditions during thermal fatigue testing. A material database is being developed in an extended
temperature range to predict stress/strain distribution arising from manufacturing and high heat load conditions. (v) When various HIP cycles are applied to simulate an ITER module relevant fabrication route, SS grain growth and grain size scattering are observed.
Table 4 HIP parameters for DS-Cu/SS joining Temperature (°C)
Time (h)
Pressure (MPa)
920 930 930 980
3/3/3 2/4/1 3/2/3 2/2/2
120 100 120 100
Fig. 3. Metallographic examination of a 316LN/DS-Cu joint (×1000). HIP parameters: 920°C, 3 h, 120 MPa.
CE CE CE CC
PM130 PM130 T5091 T5091
IGa IGa IGa IG0 IG0 IG0 No 1000 – 0.5
No No
Material treatm., T (°C) – t (h)
250 – 15
400 – 12 400 – 12
Outgassing of the can, T (°C) – t (h)
930/100/4 980/100/2 930/4 930/120/2 920/120/3 930/2
HIP cycle, T/P/t (°C/ MPa/h)
–389/18
265/386/16/C 255/391/14/C 340/393/17 268/38.3/6.5/C 280/377/7.6/C 256–337/320
20°C
159/190/1.6/CJ 155/195/1.7/CJ 191/205/10 166/192/1.5/C 176/202/1.7/C 179/194/6
300°C
Tensile results: YS/UTS/TE/rupturea (MPa/MPa/%)
C means rupture in the DS-Cu above 300 mm. CJ means rupture in the DS-Cu but close to the interface below 300 mm.
Surfaces prep.
316LN
DS-Cu
a
Joining parameters
Materials
Table 5 Typical tensile tests results on DS-Cu/316LN bonded specimens
G. Le Marois et al. / Fusion Engineering and Design 39–40 (1998) 253–261 259
Ni PVD-5 Ni PVD-10 Ni foil-30 FN42-100 Ni electro
IG0 IG0 IG0 IG0 IG0
CE CE CE CC CC
Surfaces prep.
Interlayr nature 316LN e (mm)
DS-Cu
T5091 T5091 T5091 T5091 T5091
Joining parameters
Materials
No No No 1000 – 0.5 1000 – 0.5
Material treatment, T (°C) –t (h)
Table 6 Tensile tests results on DS-Cu/SS joints using various interlayers
250 – 15 250 – 15
Outgassing of the can, T (°C) –t (h)
930/120/2 930/120/2 930/120/2 920/120/3 920/120/3
HIP cycle, T/P/t (°C/MPa/h)
265/293/8/C 273/359/4.1/CJ 264/361/4.7/CJ 283/385/9/C+J 281/374/7/C
20°C
161/175/0.6/CJ 167/178/0.5/CJ 159/174/1/CJ 173/192/1.8/C 176/191/0.9/C
300°C
Tensile results: YS/UTS/TE/rupturea (MPa/MPa/%)
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References
Fig. 4. Deformation (×20) of the T8 mock-up section after one cycle at 9.7 MW m − 2.
.
[1] G. Le Marois, J.M. Gentzbittel, J. Troxell, R. Solomon, Glidcop AL-25 specification for ITER first wall blanket heat exchanger material, Draft G, May, 1996. [2] B. Marini, ITER 214. Irradiation testing of stainless steel including weldments and rewelding of irradiated materials. Subtasks CEA1 and 2. Progress report, CEA report NT SRMA 97-2217, 1996. [3] H. Burlet, J.M. Gentzbittel, F. Bernier, P. Mourniac, C. Labonne, ITER DPI task 212, development and testing of Cu alloys/316LN SS joints by solid HIP, CEA report NT DEM 97/11. [4] G. Le Marois, P. Revirand, ITER task 232, baffle fabrication integrated concept, CEA report NT DEM 14/96. [5] L. Briottet, O. Bouaziz, H. Burlet, UT-M-A13, HIP and brazing technologies development, CEA report NT DEM 62/96.
Structural materials joints for ITER in-vessel components G. Le Marois a,*, H. Burlet a, R. Solomon b, B. Marini c, J.M. Gentzbittel a, L. Briottet a a
CEA-Grenoble, DEM/SGM, 38054 Grenoble Cedex 9, France b SCM Metals Products, RTP, NC 277709 -2166, USA c CEA-Saclay, DECM/SRMA, Saclay, France
Abstract 316LN stainless steel (SS) and Glidcop AL-25 (DS-Cu) are primary candidates as structural materials for international thermonuclear experimental reactor (ITER) in-vessel components. Their joining is currently being studied at CEA/CEREM. This paper summarises recent progress on structural materials joining using a solid hot isostatic pressing (HIP) technique. The properties of the materials used and the effect of HIP cycles on the metallurgical quality of the SS are reported. A new specification for the DS-Cu is proposed. Different materials and surfaces preparation, the use of interlayers and different HIP conditions are presented and analysed in relation to the mechanical properties of the joints. Characterisation of the bi-metallic joints such as SS/DS-Cu is discussed and further developments with various mode loadings are briefly reported. Some preliminary modelling results regarding stress/strain distribution arising from manufacturing and thermal load conditions are presented. A visco-plastic material database is currently being developed for that purpose. © 1998 Published by Elsevier Science S.A. All rights reserved.
1. Introduction The international project ITER has to demonstrate the technical and scientific feasibility of controlled fusion on an industrial scale: a typical tokamak reactor is designed for that purpose. The internal components facing the plasma are modular made. The structural parts of these modules are made of 316LN stainless steel (SS) and a copper alloy heat sink (DS-Cu). Both are in* Corresponding author. E-mail: [email protected]
ternally cooled. The double curvature of the modules, double containment of the cooling and low leaks level requirement are the main design constraints. For their manufacture, solid hot isostatic pressing (HIP) has been proposed as it allows these drawbacks to be overcome. A review of the current research and development on SS/SS and Cu/ SS HIP joining performed at CEA is presented. A tentative specification of Cu is proposed. Some issues and improvements of the joints are discussed.
0920-3796/98/$19.00 © 1998 Published by Elsevier Science S.A. All rights reserved. PII S0920-3796(98)00108-2
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Table 1 Chemical composition of 316LN stainless steel billets Lot
Cr
Mo
Ni
C
Mn
Si
Cu
N2
S
P
PM130 T5091 IG Spec
17.45 17.5 17–18
2.5 2.4 2.3–2.7
12.23 12.16 12–12.5
0.024 0.02 0.015– 0.03
1.81 1.73 1.6 – 2
0.39 0.41 B0.5
0.28 0.23 B0.1
0.067 0.068 0.06 – 0.08
0.0011 0.001 0.005 – 0.01
0.022 0.025 B0.025
2. Materials
2.1. 316LN As-forged 316LN stainless steel (SS) billets have been provided by Tecphy. The first one (ref PM130) was produced under air, while the second one (ref T5091) was produced under vacuum. Except for Cu, their chemical composition (Table 1) are within ITER specifications. The mechanical properties of the two billets are slightly different at room temperature (RT). Both of them respect the RCC-MR recommendations (Table 2).
tect it against oxidation and to limit the formation of heterogeneities during cross-rolling. Cross-rolling reduction rates are improved to break particle alignment. As far as HIPing is concerned, the final annealing treatment is not specified, and outgassing at high temperature under vacuum is recommended prior to joining. Control: a test procedure is defined to control the size and distribution of the recrystallised grains and particles, in order to guarantee a good dispersion of the strengthened particles and to lower material and joints properties deviation.
2.2. Glidcop ® AL-25 3. SS/SS joining [2]
2.2.1. IG0 grade The IG0 grade, now considered as the reference grade for the DS-Cu alloy family, has been defined in collaboration with SCM Metals Products. The material provided by SCM is a Glidcop DS-Cu LOX AL-25, made using −80 mesh powder, extruded, declad, cross-rolled and stress relief annealed at approximately 1000°C. IG0 specification has been detailed [1]. It covers plates, 10–40 mm thick. IG0 plates 20 mm thick have been used in the present study. 2.2.2. IG1 grade Some bad metallurgical quality on declad material previous to the cross-roll operation was, however, observed, and CEA therefore proposed to SCM a new IG specification. The IG1 specification includes following improvements of the IG0 one: Manufacturing process: during the fabrication process, the material remains cladded to pro-
3.1. Manufacturing 3.1.1. Surface treatment The surfaces are finely machined (down to 1 mm) and degreased. A 304 stainless steel canister is welded around the parts to be bonded. 3.1.2. HIP conditions HIP parameters are as follows: temperature, 1090°C (PM130) and 1100°C (T5091); pressure, 120 MPa (PM130) and 100 MPa (T5091); and holding time, 2 h. 3.2. Metallographic examination of the joint (PM130 and T5091) Metallographic examinations of the joints (Fig. 1) show some recrystallised grains through the joint, indicating that diffusion has correctly occurred. Nevertheless, some precipitates identified
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Table 2 Mechanical properties of stainless steel billets at room temperature
RCC-MR PM130 T5091
YS (MPa)
UTS (MPa)
TE (%)
KCU (J cm−2)
\220 232–235 293
\525 549 – 550 551
\45 68.8 – 71.1 62
\120 266 – 270 272
as oxides can be observed along the joint (mainly for PM130).
3.3. Tensile, impact and fatigue tests (PM130) Testing specimens were cut out of the test blocks. The joint interface is located at the centre of the specimen, perpendicularly to the tensile axis. Tests have been performed at RT and 300°C, on joints and on base metal submitted to the same HIP treatment. Some specimens present a brittle fracture at the joint. This was observed at both temperatures. In that case, a high concentration of oxides is observed at the fracture interface. For all others specimens, the ultimate tensile stress of the bonded sample was nearly equal to that of the base ageing material. Nevertheless, it seems that the striction cannot occur on or close to the joint (Table 3). Impact test results show significant scatter, probably due to the presence of oxides at the interface (Table 3). At least, joint fatigue properties are within the scatter band of the base metal (Fig. 2).
4. DS-Cu/SS joining [3]
4.1. Manufacturing A low surface roughness (Ra 0.8 mm) machining is achieved. Surface preparations include chemical etching or chemical cleaning. In case of chemical cleaning, the DS-Cu parts were afterwards submitted to a heat treatment at 1000°C for 30 min, to reduce the copper oxides. Outgassings at various temperatures – times have been performed before sealing the can. HIP conditions used are summarised in Table 4.
4.1.1. Use of interlayers Various nickel-base interlayers have been tested. The objective is to limit the precipitation of Fe,Cr into the copper alloy, and to prevent the surface from further oxidation and/or contamination. Three different kinds of interlayers have been tested: ion plating coating, electrolytic coating and foils, with thickness in the range 5–100 mm. 4.2. Metallographic examination of the joints 4.2.1. Joint without interlayer Fig. 3 shows a typical microstructure of the joint after chemical etching. A diffusion layer parallel to the bond interface can be observed over a distance of about 40 mm. It is characterised by the presence of quasi-spherical particles with a diameter of about 1 mm. These particles are rich in Fe, Cr and Cu. Ni diffused in solid solution over a small distance. In SS, a slight depletion of chromium can be measured. Copper diffuses in SS in solid solution over a few micrometres. 4.2.2. Joint with an interlayer The use of an Ni interlayer limits the diffusion of Fe and Cr into DS-Cu. When the Ni thickness exceeds 10 mm, no precipitates are detected in the copper alloy. Ni diffuses in the DS-Cu over about 80 mm. 4.3. Mechanical T5091)
testing
(DS-Cu/PM130 and
4.3.1. Experimental procedure Tensile tests have been carried out on smooth axisymmetrical specimens of diameter 4 mm and gauge length 20 mm, at RT and at 300°C under
256
G. Le Marois et al. / Fusion Engineering and Design 39–40 (1998) 253–261
Fig. 1. Metallography of 316/316 joints.
vacuum. As the testing specimen’s axis is perpendicular to the interface, results have to be compared with those of DS-Cu in the short transverse direction. The deformation of the bi-material specimen being heterogeneous (DS-Cu undergoes higher deformation than 316LN), the ‘strain’ of the specimen has no physical meaning. In the following, it represents the ratio of the amount of total displacement over the initial gauge length. The strain rate was 10 − 4 s − 1.
4.3.2. Results and discussion All results obtained on joint without interlayers are listed in Table 5. IG* refers to a plate that was warm-rolled and heat-treated before being declad. The main results are the following: the maximum stress reached by the tensile specimen corresponds to the UTS of the DS-Cu itself. At RT, high ductilities obtained with the plate IG* specimen can be correlated to high necking located at about 4–5 mm. In this case, the deformation of the DS-Cu side is similar to that of the bulk DS-Cu. Numerical calculations simulating tensile tests on 316LN/DS-Cu specimens are in good agreement with these results. They predicted no stress concentration near the interface, due to similar mechanical behaviours of the two base materials. For IG0 specimens, most of the failures occurred within 1 mm of the interface. Necking was not observed in this case. The location of the
damage is probably related to the deep oxidation of the declad IG0 plate. At 300°C, although the UTS is reached in the DS-Cu within the joint, failures occur closer to the interface, and with a DS-Cu strain smaller than the total strain of a bulk DS-Cu. Numerical calculations show a high concentration of axial stress in the vicinity of the joint, on the edge of the DS-Cu part. This high stress could explain the abnormally rapid failure by a ‘delamination’ phenomenon of the DS-Cu plate. Tensile data on joints with Ni interlayers are summarised in Table 6. Obviously, ultimate tensile strengths and ductilities are worst in this case. The role of the nickel in the mechanical properties is not yet clearly understood. All the mechanical tests performed on smooth axisymmetrical specimens led to failure in the DS-Cu itself. To characterise the strength of the bonded interface itself, other specimens must be used. Axisymmetrical notched specimens, compact tension, disymmetric CT and specimens with internal flaws are planned in order to develop stress concentration along the interface and to generate various mode I/mode II ratio loadings.
4.4. Mock-up manufacturing and testing Mock-ups with different geometries and shapes (straight and bent), including for some armour material, have been manufactured, following a
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Table 3 Tensile and impact tests results on 316LN/316LN joints (PM130) Specimen
YS (MPa)
UTS (MPa)
TE (%)
UE (%)
KCU (J cm−2)
Joint, 20°C Base metal, 20°C (after HIP cycle) Joint, 300°C Base metal, 300°C (after HIP cycle)
246 – 255 260 162 – 161 160
563 – 566 580 425 – 441 446
45 – 58 59 31 – 44 42
44 – 51 50 31 – 40 35
62 – 122 274 – 287 47 – 314 254 – 264
relevant HIP manufacturing route. T8 (DS-Cu/SS with two SS cooling tubes, 1 mm thick) and T232 (W/DS-Cu/SS with three SS cooling tubes 0.5 mm thick) mock-ups have been tested under high heat flux (HHF) and thermal fatigue conditions in an electron gun facility.
4.4.1. HHF test A HHF test, up to 9.7 MW m − 2 absorbed flux, has been performed on the T8 mock-up. On the T232 mock-up, 250 cycles under 9 MW m − 2 have been performed. 4.4.2. Modelling and DT analysis In order to predict residual stresses distribution and permanent strain after the HHF test, an elasto-plastic calculation has been performed on the T8 mock-up and compared with experimental strain results. The main solicitations in a complex mode are shown to be at the SS tubes/DS-Cu joints: calculation gives 1.17 and 0.80% EPSE in DS-Cu and SS, respectively. Comparison with experimental measurement of the deformation indicates that improvement of the database is still required (Fig. 4). Further prediction improvements are still ongoing through development of the database, taking into account actual materials properties in an extended temperature range including visco-plastic data.
5. Effect of HIP cycles on SS properties For baffle fabrication, a manufacturing route has been proposed [4]. In order to provide full controllability of the joints, the component is
achieved in four HIP cycles: two cycles of 4/10/4 at 1095°C (4 h heating up, 10 h at 1095°C and 4 h cooling down) for the SS parts, followed by two cycles of 3/2/3 at 930°C for the heat sink, are performed. The effect of the four cycles on PM130 and T5091 properties has been analysed [5]. After one cycle, the grain size distribution appears quite heterogeneous. During the following cycles, grain growth enhances grain sizes scattering. The ASTM grain size number lies between 4 and 5 after one cycle and between 1 and 5 after four cycles. Moreover, some ferrite lines appeared in both materials, although in a lower proportion for T5091. A strong decrease of the hardness is observed after one cycle (200–140 HV40). The influence of the following cycles is less important ( 120 after four cycles). Tensile tests (PM130) show no influence of the cycles at room temperature. However, at 300°C a small decrease of the yield and ultimate strength is noticed. Finally, the effect of the HIP cycle on Charpy-U toughness (PM130) and fatigue resistance is not significant.
6. Observations and conclusions (i) The properties of the SS and DS-Cu joints are sensitive to the quality of the materials used. The presence of oxides and clusters at the interface weakens and embrittles the joints. (ii) Using SS and DS-Cu with improved quality, tensile and impact properties of the joints are similar to those obtained on the weaker material.
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258
Fig. 2. Fatigue resistance of SS HIP joint at 300°C (PM130). Comparison with 316LN SPH data.
The use of Ni interlayers does not improve the mechanical properties of the joints and is not recommended. (iii) For characterisation of bi-metallic joints such as DS-Cu/SS, it is necessary to generate various mode I/II loadings and to develop stress concentration along the interface. Testing of different specimens including compact tension, disymmetric CT are underway. (iv) SS and DS-Cu plate-to-plate and tube-toplate joints are able to sustain high strain/stress conditions during thermal fatigue testing. A material database is being developed in an extended
temperature range to predict stress/strain distribution arising from manufacturing and high heat load conditions. (v) When various HIP cycles are applied to simulate an ITER module relevant fabrication route, SS grain growth and grain size scattering are observed.
Table 4 HIP parameters for DS-Cu/SS joining Temperature (°C)
Time (h)
Pressure (MPa)
920 930 930 980
3/3/3 2/4/1 3/2/3 2/2/2
120 100 120 100
Fig. 3. Metallographic examination of a 316LN/DS-Cu joint (×1000). HIP parameters: 920°C, 3 h, 120 MPa.
CE CE CE CC
PM130 PM130 T5091 T5091
IGa IGa IGa IG0 IG0 IG0 No 1000 – 0.5
No No
Material treatm., T (°C) – t (h)
250 – 15
400 – 12 400 – 12
Outgassing of the can, T (°C) – t (h)
930/100/4 980/100/2 930/4 930/120/2 920/120/3 930/2
HIP cycle, T/P/t (°C/ MPa/h)
–389/18
265/386/16/C 255/391/14/C 340/393/17 268/38.3/6.5/C 280/377/7.6/C 256–337/320
20°C
159/190/1.6/CJ 155/195/1.7/CJ 191/205/10 166/192/1.5/C 176/202/1.7/C 179/194/6
300°C
Tensile results: YS/UTS/TE/rupturea (MPa/MPa/%)
C means rupture in the DS-Cu above 300 mm. CJ means rupture in the DS-Cu but close to the interface below 300 mm.
Surfaces prep.
316LN
DS-Cu
a
Joining parameters
Materials
Table 5 Typical tensile tests results on DS-Cu/316LN bonded specimens
G. Le Marois et al. / Fusion Engineering and Design 39–40 (1998) 253–261 259
Ni PVD-5 Ni PVD-10 Ni foil-30 FN42-100 Ni electro
IG0 IG0 IG0 IG0 IG0
CE CE CE CC CC
Surfaces prep.
Interlayr nature 316LN e (mm)
DS-Cu
T5091 T5091 T5091 T5091 T5091
Joining parameters
Materials
No No No 1000 – 0.5 1000 – 0.5
Material treatment, T (°C) –t (h)
Table 6 Tensile tests results on DS-Cu/SS joints using various interlayers
250 – 15 250 – 15
Outgassing of the can, T (°C) –t (h)
930/120/2 930/120/2 930/120/2 920/120/3 920/120/3
HIP cycle, T/P/t (°C/MPa/h)
265/293/8/C 273/359/4.1/CJ 264/361/4.7/CJ 283/385/9/C+J 281/374/7/C
20°C
161/175/0.6/CJ 167/178/0.5/CJ 159/174/1/CJ 173/192/1.8/C 176/191/0.9/C
300°C
Tensile results: YS/UTS/TE/rupturea (MPa/MPa/%)
260 G. Le Marois et al. / Fusion Engineering and Design 39–40 (1998) 253–261
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261
References
Fig. 4. Deformation (×20) of the T8 mock-up section after one cycle at 9.7 MW m − 2.
.
[1] G. Le Marois, J.M. Gentzbittel, J. Troxell, R. Solomon, Glidcop AL-25 specification for ITER first wall blanket heat exchanger material, Draft G, May, 1996. [2] B. Marini, ITER 214. Irradiation testing of stainless steel including weldments and rewelding of irradiated materials. Subtasks CEA1 and 2. Progress report, CEA report NT SRMA 97-2217, 1996. [3] H. Burlet, J.M. Gentzbittel, F. Bernier, P. Mourniac, C. Labonne, ITER DPI task 212, development and testing of Cu alloys/316LN SS joints by solid HIP, CEA report NT DEM 97/11. [4] G. Le Marois, P. Revirand, ITER task 232, baffle fabrication integrated concept, CEA report NT DEM 14/96. [5] L. Briottet, O. Bouaziz, H. Burlet, UT-M-A13, HIP and brazing technologies development, CEA report NT DEM 62/96.