- Email: [email protected]
Advanced examination techniques applied to the qualification of critical welds for the ITER correction coils
Fusion Engineering and Design 98–99 (2015) 2072–2075
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Advanced examination techniques applied to the qualification of critical welds for the ITER correction coils Stefano Sgobba a,∗ , Stefanie Agnes Elisabeth Langeslag a , Paul Libeyre b , Dawid Jaroslaw Marcinek c , Aline Piguiet a , Alexandre Cécillon d a
CERN, CH-1211 Genève, Switzerland ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067 St. Paul Lez Durance Cedex, France c Tadeusz Ko´sciuszko Cracow University of Technology, 31-155 Kraków, Poland d RX Solutions, ZAC Altaïs, Chavanod, France b
h i g h l i g h t s • X-ray computed tomography (CT) was successfully applied to inspect the full weld volume of critical qualification welds for the ITER correction coils. • These welds featuring a complex geometry are virtually uninspectable with sufficient resolution by conventional Non-Destructive Examination (NDE) techniques.
• The applied advanced examinations allowed an exhaustive identification of weld imperfections and a thorough understanding of their nature, position and size.
• A substantial progress in weld quality could be achieved during the weld qualification phases.
a r t i c l e
i n f o
Article history: Received 18 September 2014 Accepted 6 May 2015 Available online 29 May 2015 Keywords: Non-destructive examinations Computed tomography ITER correction coils
a b s t r a c t The ITER correction coils (CCs) consist of three sets of six coils located in between the toroidal (TF) and poloidal field (PF) magnets. The CCs rely on a Cable-in-Conduit Conductor (CICC), whose supercritical cooling at 4.5 K is provided by helium inlets and outlets. The assembly of the nozzles to the stainless steel conductor conduit includes fillet welds requiring full penetration through the thickness of the nozzle. Static and cyclic stresses have to be sustained by the inlet welds during operation. The entire volume of helium inlet and outlet welds, that are submitted to the most stringent quality levels of imperfections according to standards in force, is virtually uninspectable with sufficient resolution by conventional or computed radiography or by Ultrasonic Testing. On the other hand, X-ray computed tomography (CT) was successfully applied to inspect the full weld volume of several dozens of helium inlet qualification samples. The extensive use of CT techniques allowed a significant progress in the weld quality of the CC inlets. CT is also a promising technique for inspection of qualification welds of helium inlets of the TF magnets, by far more complex to examine due to their larger dimensions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The ITER correction coils (CCs) are an integral part of the ITER magnet system. They are used to produce asymmetric fields aimed at correcting error fields due to imperfections in positioning and geometry of the Toroidal Field (TF) coils, the Central Solenoid (CS) and Poloidal Field (PF) coils [1,2]. Some of the CCs will be among the first components to be installed during assembly of the magnet
∗ Corresponding author. Tel.: +41 22 7679401. E-mail address: [email protected] (S. Sgobba). http://dx.doi.org/10.1016/j.fusengdes.2015.05.009 0920-3796/© 2015 Elsevier B.V. All rights reserved.
system, hence requiring early delivery to the ITER site. Extensive qualification of the manufacturing processes is in progress [3]. The CCs rely on a Cable-in-Conduit Conductor (CICC), whose supercritical cooling circuit at 4.5 K includes helium inlets and outlets. The assembly of the nozzles to the stainless steel conductor conduit involves manual fillet welds requiring full penetration through the thickness of the nozzle. Static and cyclic stresses have to be sustained by the inlet welds during operation. Severe constraints are imposed during welding, both in terms of position (welding on vertical conductor) and of maximum allowed temperature not to be exceeded on the superconducting (SC) strands during welding. Moreover, these welds operating at 4.5 K are submitted to stringent
S. Sgobba et al. / Fusion Engineering and Design 98–99 (2015) 2072–2075
quality levels of weld imperfections according to standards in force. The entire volume of helium inlet and outlet welds is virtually uninspectable with sufficient resolution by conventional Non-Destructive Examination (NDE) techniques, in particular by conventional or computed Radiographic Testing (RT) or by Ultrasonic Testing (UT). On the other hand, X-ray Computed Tomography (CT) was successfully applied to inspect the full weld volume of several dozens of helium inlet qualification samples. CT requires a relative movement of the X-ray source and the detector with respect to the inspected object. For CC inlet welds, the technique is applied in laminographic “shear” mode (X-ray laminography). Laminographic inspections allow exploring, layer after layer following image reconstruction, the entire volume of the welds. Weld imperfections, including the thinnest ones such as planar defects, can be easily and exhaustively imaged and their position can be precisely determined in the volume. Defects can finally be quantified and assessed according to most stringent levels of the weld standards in order to check compliance with ITER specifications. Laminographic inspections were already applied at CERN for in situ inspections of LHC interconnections by a portable CT device [4]. The tests performed on the CC inlets are on the other hand carried out with the help of a fixed microtomographic unit already successfully applied to inspect other CC components [5], allowing the weld imperfections in the volume to be detected with a very high resolution. 2. Experimental The examination concerns the manual TIG fillet weld of the CC helium outlet and inlet pipes (weld designated as B1 in Fig. 1a). The weld is required to ensure a leak-tight connection between the square-in-square CC conduit (in 316 L stainless steel) and the inlet nozzle (316 L), identified as no. 1 and no. 2 in the weld diagram (Fig. 1a), respectively. The dimensions of the workpiece are shown in Fig. 1b. X-ray laminography is performed on an EasyTom [6] automated X-ray machine, designed and built by RX Solutions for laboratory
Fig. 1. (a) Weld diagram. The filler material for the fillet weld is an AWS SFA5.9 ER316L. The CC conduit is a 19.2 mm square, 2.2 mm thick pipe with rounded corners. (b) Drawing of the CC helium inlet.
2073
Fig. 2. Setup for the laminographic scan.
and industrial application. The six motion axis, 14 bits and 16,000 grey levels CT unit allows full inspection of specimens up to 300 mm in diameter and 500 mm in length with a micro-focus sealed X-ray tube at a maximum tension of 130 kV and a detection rate of the sensor up to 60 images per second. CT is performed as planar tomography in shear mode (laminography), achieved by moving the X-ray source and the detector with respect to each other in order to access the entire weld volume (Fig. 2). The specimens are placed on a fixed stage, while the Xray source and the detector accomplish a relative motion in two directions, vertical and horizontal. Image detection is performed with use of a HR area image sensor with a resolution of 1920 pixels × 1536 pixels. The four sides of the inlet are identified by A, B, C and D, where A and B are the two long, and C and D the two short sides, respectively. The scan is performed from B to A (Fig. 3). A copper wire is attached to one of the outer corners for image reference purposes. Following reconstruction and data post-processing, dataacquisition results in several hundreds of separate radioscopies. Using the reconstruction software, radioscopies are combined to generate laminographic slices. The scans feature an anisotropic voxel size between 0.010 mm and 0.015 mm in x- and y-direction and between 0.015 mm and 0.020 mm in z-direction. The full volume, reconstructed in slices, is combined into a z-stack to compile a final 3D object reconstruction, which is collected in a video running through the entire z-stack. Weld examination is performed by an EN ISO 9712 RT level 2 CERN inspector by the observation of the reconstructed z-stack video, based on the individual laminographic slices, and the corresponding z-stack images on a 24 in. high-resolution (1920 pixels × 1080 pixels) computer screen. All observed imperfections are noted. Classification is subsequently performed following standard ISO 6520-1 and the imperfections are judged with respect to ISO 5817 – level B. Dimensions of imperfections in z-direction are estimated based on the voxel depth.
Fig. 3. Picture of the helium inlet weld and details of the progression of the CC helium inlet scanning.
2074
S. Sgobba et al. / Fusion Engineering and Design 98–99 (2015) 2072–2075
Laminographic results are checked against conventional or computed RT, carried out following ISO 17636-1 and -2, respectively and systematically performed for comparison purposes prior to laminography. RT has the advantage of being fully normalized and covered by standards in force such as EN 14784-2. Optimized testing parameters were set up based on extensive campaigns applied to several inlets. A classic test arrangement for double-wall penetration might detect certain lacks of fusion but is very poor in resolution and not suitable as an inspection method. The most suitable configuration is a single wall technique, by placing the film inside the nozzle and the X-ray source at an angle ˛ = 0◦ for views of the A and B side (with reference to Fig. 3) and ˛ = 3◦ for views C and D. Due to the small radius of the curved part of the nozzle, the film can only cover a fraction of the inner perimeter of the weld root, hence only 70% of the volume of the weld can be inspected by the single wall RT. Visual testing (VT) following EN ISO 17637 and EN 13018 is also applied in several cases as a complement of the above inspections. A Keyence VHX-1000 digital microscope with a maximum resolution of 54 MPixel, equipped with 0–50×, 20×–200× and 100×–1000× objectives, respectively, was used with the mere purpose of recording the relevant imperfections identified by direct VT. In specific cases direct observations by destructive microoptical investigations is carried out in fine.
3. Experimental results Fig. 4a shows a helium inlet (n◦ 5) of an early production that was subjected to computed RT and X-ray laminography. Only lack of penetration could be detected by RT on the side B of this inlet (defect n◦ 402 following ISO 6520-1, unacceptable for the stringent quality level B of ISO 5817), and a porosity (defect n◦ 2011) of an acceptable size. Although applicable, RT is limited in resolution for the detection of such imperfections and does not cover the entire weld volume. On the other hand, X-ray laminography allows imperfections to be accessed in the whole weld with higher resolution than through computed RT. Single laminar views featuring imperfections can be exported from the recorded tomographic movies. Fig. 4b shows an example of a single laminographic shot extracted from the tomographic inspection. Porosity (left) and crack (right) are identified. This laminar view corresponds to a longitudinal cut of the weld contained in the B wall with reference to Fig. 4c (the plane is visualized by a dashed row). The two imperfections are located in the same longitudinal plane. The observed porosity is of the type of the one identified by RT. On the other hand the crack, unacceptable by ISO 5817, was not identified by RT. Fig. 4d shows an example of a metallographic cut performed post mortem to confirm the detected defects in inlet n◦ 5, exactly in correspondence of the plane where X-ray laminography identified the defects shown in Fig. 4c. To the extent of the destructive tests performed, metallographic cuts systematically confirmed all the imperfections detected by X-ray laminography in the expected positions. An impressive one-to-one correspondence was demonstrated between the individual laminographic cuts, which allow the thinnest imperfections such as planar defects to be detected in the volume, and micro-optical observations. During their life, correction coils will be submitted to fatigue cycling [1,2]. Components such helium inlets shall be qualified by demonstrating that their fatigue life exceeds 534,000 cycles in the range of 5 K in liquid He, while keeping leak tight when tested with a sensitivity of 10−8 Pa m3 /s following the fatigue cycling. Full size samples have been manufactured by company A for qualification by strain driven fatigue testing between a strain of 4.5 × 10−4 and 9.0 × 10−4 . In order to monitor the possible presence of prior weld imperfections in the as welded inlet, six as-welded helium inlet witness samples were submitted to VT, computed RT and
Fig. 4. Inlet (a) submitted to non-destructive followed by destructive inspections. A single laminar view (b) featuring two imperfections in the same plane has been exported from the recorded tomographic inspection of the inlet. The view is relative to a laminographic “cut” of the weld (dashed arrow in c). Porosity and crack are identified. The metallographic section (d) coincides with the laminographic cut in (b). Porosity (e and f) and cracks (g and h) are confirmed exactly in the plane and the positions identified by laminography prior to cutting.
X-ray laminography. The two first techniques were also applied for contractual acceptance or rejection based on the highest requirement (level B) of ISO 5817. In this specific case, X-ray laminography was mainly aimed at identifying the full population of the weld imperfections in the six welds in order to monitor, by a second laminographic inspection following fatigue testing, their possible evolution under the applied cycling strain. Fig. 5 shows an example of two laminographic slices selected from the thousands produced during the 100% inspection of the six as-welded helium inlet witness samples, prior to fatigue testing. Different types of imperfections can be identified. Several of them, such as lack of fusion, incomplete penetration, weld cracks are susceptible to evolve following fatigue cycling. Although not requested by the standard in the severity class B, the size of these linear defects is systematically assessed in order to quantify through an
S. Sgobba et al. / Fusion Engineering and Design 98–99 (2015) 2072–2075
2075
Table 1 Compared detectability of imperfections by VT, RT and X-ray laminography (CT), respectively. Acceptance follows ISO 5817 class B. Unacceptable imperfections and reference to ISO 6520
VT
Shrinkage grooves Crack Surface porosity Root porosity Intermittent undercut Copper inclusions
x
5013 100 2017/2018 516 5012 304/3042
Acceptable imperfections and reference to ISO 6520 Wormhole Porosity (gas pores) Fig. 5. Laminographic views extracted from the examination of one of the inlets to be submitted to fatigue testing, prior to the test. All defects are identified. The dimensions of the defects susceptible to grow are measured. For linear defects their depth ı is also estimated, based on the number of laminographic slices affected by the single weld imperfection. d is the diameter, h the height of the imperfection, s is the thickness of the weld metal as defined by ISO 5817.
Fig. 6. Complementary VT observations of weld imperfections recorded by digital microscopy. Classification of imperfections follows ISO 6520-1.
additional laminographic inspection their possible growth following fatigue cycling. In addition to X-ray laminography and computed RT, CERN applied 100% VT of the weld crown and root of this set of welds. Since the root is also accessible, VT is performed as a direct inspection. Nonetheless, digital microscopy is applied to record the imperfections observed by VT. Examples of such imperfections are shown in Fig. 6. Imperfections can be sharply recorded on an extended depth thanks to the extended focus capability of digital microscopy. 4. Discussion and conclusions Table 1 shows as an example the list of defects identified in one of the helium-inlets inspected by VT, computed RT as well as Xray laminography. The latter is the most exhaustive technique to identify the weld imperfections, in particular the linear ones for which RT features limited resolutions. If RT is able in some cases to identify lack of fusion, cracks are in general not detected by RT, while X-ray laminography is able to detect cracks at least as narrow as 20 m and as short as 100 m. The exhaustive identification of weld imperfections enabled by the high lateral resolution, full
2016 2011
RT
x x x x x x
x
VT
CT
RT
CT
x
x x
volume examination provided by X-ray laminography has allowed a significant progress of the quality of the welds, thanks to the identification of the relevant imperfections and the adapted measures taken to reduce them. Nonetheless, the experience based on several dozens of tested inlets, produced by two different manufacturers, has proven that achieving the most stringent quality level B in this type of fully penetrated, manual welds carried out in a constraining position (welding on vertical conductor) is extremely difficult. It has to be taken into account that the maximum temperature allowed on the SC strands during welding is 250 ◦ C. This imposes several weld stops and starts, which are the source of most weld defects. On this basis, taking also into account the outcome of mechanical calculations and pending results of fatigue tests, a request to degrade the quality level of the weld to ISO 5817 class C (intermediate) is being examined. For defects that would still not fulfil class C a non-conformity would be opened: acceptance “as is” or weld repairs might be imposed by ITER Organization (IO). Defects such as root surface cracks, copper depositions or inclusions might also be tolerated subject to non-conformity. In conclusion, advanced examination techniques such as VT associated to digital microscopy, computed RT and X-ray laminography and their combination have not only allowed an exhaustive identification of weld imperfections of these critical welds, but also a thorough understanding of their nature, position and size and a substantial progress in weld quality that could be achieved during the qualification phases. Acknowledgments Thanks are owed to the partner companies and institutes involved by IO and the ITER Chinese Domestic Agency (CNDA) for supplying the weld samples and for extensive discussions throughout this campaign, and to A. Porret for many of the results of computed RT. “The views and opinions expressed herein do not necessarily reflect those of the ITER Organization”. References [1] [2] [3] [4]
A. Foussat, et al., IEEE Trans. Appl. Supercond. 20 (2010) 402. A. Foussat, et al., IEEE Trans. Appl. Supercond. 22 (2012) 4201205. P. Libeyre, et al., Fusion Eng. Des. 88 (2013) 1478–1481. C. Sauerwein, et al., Proceedings of the 1st International Particle Accelerator Conference (IPAC ’10), 23–28 May 2010, Kyoto, Japan, also CERN/ATS document 2010-143, 2010. [5] S. Sgobba, et al., IEEE Trans. Appl. Supercond. 24 (2014) 4201904. [6] RX Solutions, Technical description of the EasyTom device, http://media.wix. com/ugd/56ca9d f8d6be41febedff088f8153d2040ff3e.pdf
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Advanced examination techniques applied to the qualification of critical welds for the ITER correction coils Stefano Sgobba a,∗ , Stefanie Agnes Elisabeth Langeslag a , Paul Libeyre b , Dawid Jaroslaw Marcinek c , Aline Piguiet a , Alexandre Cécillon d a
CERN, CH-1211 Genève, Switzerland ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067 St. Paul Lez Durance Cedex, France c Tadeusz Ko´sciuszko Cracow University of Technology, 31-155 Kraków, Poland d RX Solutions, ZAC Altaïs, Chavanod, France b
h i g h l i g h t s • X-ray computed tomography (CT) was successfully applied to inspect the full weld volume of critical qualification welds for the ITER correction coils. • These welds featuring a complex geometry are virtually uninspectable with sufficient resolution by conventional Non-Destructive Examination (NDE) techniques.
• The applied advanced examinations allowed an exhaustive identification of weld imperfections and a thorough understanding of their nature, position and size.
• A substantial progress in weld quality could be achieved during the weld qualification phases.
a r t i c l e
i n f o
Article history: Received 18 September 2014 Accepted 6 May 2015 Available online 29 May 2015 Keywords: Non-destructive examinations Computed tomography ITER correction coils
a b s t r a c t The ITER correction coils (CCs) consist of three sets of six coils located in between the toroidal (TF) and poloidal field (PF) magnets. The CCs rely on a Cable-in-Conduit Conductor (CICC), whose supercritical cooling at 4.5 K is provided by helium inlets and outlets. The assembly of the nozzles to the stainless steel conductor conduit includes fillet welds requiring full penetration through the thickness of the nozzle. Static and cyclic stresses have to be sustained by the inlet welds during operation. The entire volume of helium inlet and outlet welds, that are submitted to the most stringent quality levels of imperfections according to standards in force, is virtually uninspectable with sufficient resolution by conventional or computed radiography or by Ultrasonic Testing. On the other hand, X-ray computed tomography (CT) was successfully applied to inspect the full weld volume of several dozens of helium inlet qualification samples. The extensive use of CT techniques allowed a significant progress in the weld quality of the CC inlets. CT is also a promising technique for inspection of qualification welds of helium inlets of the TF magnets, by far more complex to examine due to their larger dimensions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The ITER correction coils (CCs) are an integral part of the ITER magnet system. They are used to produce asymmetric fields aimed at correcting error fields due to imperfections in positioning and geometry of the Toroidal Field (TF) coils, the Central Solenoid (CS) and Poloidal Field (PF) coils [1,2]. Some of the CCs will be among the first components to be installed during assembly of the magnet
∗ Corresponding author. Tel.: +41 22 7679401. E-mail address: [email protected] (S. Sgobba). http://dx.doi.org/10.1016/j.fusengdes.2015.05.009 0920-3796/© 2015 Elsevier B.V. All rights reserved.
system, hence requiring early delivery to the ITER site. Extensive qualification of the manufacturing processes is in progress [3]. The CCs rely on a Cable-in-Conduit Conductor (CICC), whose supercritical cooling circuit at 4.5 K includes helium inlets and outlets. The assembly of the nozzles to the stainless steel conductor conduit involves manual fillet welds requiring full penetration through the thickness of the nozzle. Static and cyclic stresses have to be sustained by the inlet welds during operation. Severe constraints are imposed during welding, both in terms of position (welding on vertical conductor) and of maximum allowed temperature not to be exceeded on the superconducting (SC) strands during welding. Moreover, these welds operating at 4.5 K are submitted to stringent
S. Sgobba et al. / Fusion Engineering and Design 98–99 (2015) 2072–2075
quality levels of weld imperfections according to standards in force. The entire volume of helium inlet and outlet welds is virtually uninspectable with sufficient resolution by conventional Non-Destructive Examination (NDE) techniques, in particular by conventional or computed Radiographic Testing (RT) or by Ultrasonic Testing (UT). On the other hand, X-ray Computed Tomography (CT) was successfully applied to inspect the full weld volume of several dozens of helium inlet qualification samples. CT requires a relative movement of the X-ray source and the detector with respect to the inspected object. For CC inlet welds, the technique is applied in laminographic “shear” mode (X-ray laminography). Laminographic inspections allow exploring, layer after layer following image reconstruction, the entire volume of the welds. Weld imperfections, including the thinnest ones such as planar defects, can be easily and exhaustively imaged and their position can be precisely determined in the volume. Defects can finally be quantified and assessed according to most stringent levels of the weld standards in order to check compliance with ITER specifications. Laminographic inspections were already applied at CERN for in situ inspections of LHC interconnections by a portable CT device [4]. The tests performed on the CC inlets are on the other hand carried out with the help of a fixed microtomographic unit already successfully applied to inspect other CC components [5], allowing the weld imperfections in the volume to be detected with a very high resolution. 2. Experimental The examination concerns the manual TIG fillet weld of the CC helium outlet and inlet pipes (weld designated as B1 in Fig. 1a). The weld is required to ensure a leak-tight connection between the square-in-square CC conduit (in 316 L stainless steel) and the inlet nozzle (316 L), identified as no. 1 and no. 2 in the weld diagram (Fig. 1a), respectively. The dimensions of the workpiece are shown in Fig. 1b. X-ray laminography is performed on an EasyTom [6] automated X-ray machine, designed and built by RX Solutions for laboratory
Fig. 1. (a) Weld diagram. The filler material for the fillet weld is an AWS SFA5.9 ER316L. The CC conduit is a 19.2 mm square, 2.2 mm thick pipe with rounded corners. (b) Drawing of the CC helium inlet.
2073
Fig. 2. Setup for the laminographic scan.
and industrial application. The six motion axis, 14 bits and 16,000 grey levels CT unit allows full inspection of specimens up to 300 mm in diameter and 500 mm in length with a micro-focus sealed X-ray tube at a maximum tension of 130 kV and a detection rate of the sensor up to 60 images per second. CT is performed as planar tomography in shear mode (laminography), achieved by moving the X-ray source and the detector with respect to each other in order to access the entire weld volume (Fig. 2). The specimens are placed on a fixed stage, while the Xray source and the detector accomplish a relative motion in two directions, vertical and horizontal. Image detection is performed with use of a HR area image sensor with a resolution of 1920 pixels × 1536 pixels. The four sides of the inlet are identified by A, B, C and D, where A and B are the two long, and C and D the two short sides, respectively. The scan is performed from B to A (Fig. 3). A copper wire is attached to one of the outer corners for image reference purposes. Following reconstruction and data post-processing, dataacquisition results in several hundreds of separate radioscopies. Using the reconstruction software, radioscopies are combined to generate laminographic slices. The scans feature an anisotropic voxel size between 0.010 mm and 0.015 mm in x- and y-direction and between 0.015 mm and 0.020 mm in z-direction. The full volume, reconstructed in slices, is combined into a z-stack to compile a final 3D object reconstruction, which is collected in a video running through the entire z-stack. Weld examination is performed by an EN ISO 9712 RT level 2 CERN inspector by the observation of the reconstructed z-stack video, based on the individual laminographic slices, and the corresponding z-stack images on a 24 in. high-resolution (1920 pixels × 1080 pixels) computer screen. All observed imperfections are noted. Classification is subsequently performed following standard ISO 6520-1 and the imperfections are judged with respect to ISO 5817 – level B. Dimensions of imperfections in z-direction are estimated based on the voxel depth.
Fig. 3. Picture of the helium inlet weld and details of the progression of the CC helium inlet scanning.
2074
S. Sgobba et al. / Fusion Engineering and Design 98–99 (2015) 2072–2075
Laminographic results are checked against conventional or computed RT, carried out following ISO 17636-1 and -2, respectively and systematically performed for comparison purposes prior to laminography. RT has the advantage of being fully normalized and covered by standards in force such as EN 14784-2. Optimized testing parameters were set up based on extensive campaigns applied to several inlets. A classic test arrangement for double-wall penetration might detect certain lacks of fusion but is very poor in resolution and not suitable as an inspection method. The most suitable configuration is a single wall technique, by placing the film inside the nozzle and the X-ray source at an angle ˛ = 0◦ for views of the A and B side (with reference to Fig. 3) and ˛ = 3◦ for views C and D. Due to the small radius of the curved part of the nozzle, the film can only cover a fraction of the inner perimeter of the weld root, hence only 70% of the volume of the weld can be inspected by the single wall RT. Visual testing (VT) following EN ISO 17637 and EN 13018 is also applied in several cases as a complement of the above inspections. A Keyence VHX-1000 digital microscope with a maximum resolution of 54 MPixel, equipped with 0–50×, 20×–200× and 100×–1000× objectives, respectively, was used with the mere purpose of recording the relevant imperfections identified by direct VT. In specific cases direct observations by destructive microoptical investigations is carried out in fine.
3. Experimental results Fig. 4a shows a helium inlet (n◦ 5) of an early production that was subjected to computed RT and X-ray laminography. Only lack of penetration could be detected by RT on the side B of this inlet (defect n◦ 402 following ISO 6520-1, unacceptable for the stringent quality level B of ISO 5817), and a porosity (defect n◦ 2011) of an acceptable size. Although applicable, RT is limited in resolution for the detection of such imperfections and does not cover the entire weld volume. On the other hand, X-ray laminography allows imperfections to be accessed in the whole weld with higher resolution than through computed RT. Single laminar views featuring imperfections can be exported from the recorded tomographic movies. Fig. 4b shows an example of a single laminographic shot extracted from the tomographic inspection. Porosity (left) and crack (right) are identified. This laminar view corresponds to a longitudinal cut of the weld contained in the B wall with reference to Fig. 4c (the plane is visualized by a dashed row). The two imperfections are located in the same longitudinal plane. The observed porosity is of the type of the one identified by RT. On the other hand the crack, unacceptable by ISO 5817, was not identified by RT. Fig. 4d shows an example of a metallographic cut performed post mortem to confirm the detected defects in inlet n◦ 5, exactly in correspondence of the plane where X-ray laminography identified the defects shown in Fig. 4c. To the extent of the destructive tests performed, metallographic cuts systematically confirmed all the imperfections detected by X-ray laminography in the expected positions. An impressive one-to-one correspondence was demonstrated between the individual laminographic cuts, which allow the thinnest imperfections such as planar defects to be detected in the volume, and micro-optical observations. During their life, correction coils will be submitted to fatigue cycling [1,2]. Components such helium inlets shall be qualified by demonstrating that their fatigue life exceeds 534,000 cycles in the range of 5 K in liquid He, while keeping leak tight when tested with a sensitivity of 10−8 Pa m3 /s following the fatigue cycling. Full size samples have been manufactured by company A for qualification by strain driven fatigue testing between a strain of 4.5 × 10−4 and 9.0 × 10−4 . In order to monitor the possible presence of prior weld imperfections in the as welded inlet, six as-welded helium inlet witness samples were submitted to VT, computed RT and
Fig. 4. Inlet (a) submitted to non-destructive followed by destructive inspections. A single laminar view (b) featuring two imperfections in the same plane has been exported from the recorded tomographic inspection of the inlet. The view is relative to a laminographic “cut” of the weld (dashed arrow in c). Porosity and crack are identified. The metallographic section (d) coincides with the laminographic cut in (b). Porosity (e and f) and cracks (g and h) are confirmed exactly in the plane and the positions identified by laminography prior to cutting.
X-ray laminography. The two first techniques were also applied for contractual acceptance or rejection based on the highest requirement (level B) of ISO 5817. In this specific case, X-ray laminography was mainly aimed at identifying the full population of the weld imperfections in the six welds in order to monitor, by a second laminographic inspection following fatigue testing, their possible evolution under the applied cycling strain. Fig. 5 shows an example of two laminographic slices selected from the thousands produced during the 100% inspection of the six as-welded helium inlet witness samples, prior to fatigue testing. Different types of imperfections can be identified. Several of them, such as lack of fusion, incomplete penetration, weld cracks are susceptible to evolve following fatigue cycling. Although not requested by the standard in the severity class B, the size of these linear defects is systematically assessed in order to quantify through an
S. Sgobba et al. / Fusion Engineering and Design 98–99 (2015) 2072–2075
2075
Table 1 Compared detectability of imperfections by VT, RT and X-ray laminography (CT), respectively. Acceptance follows ISO 5817 class B. Unacceptable imperfections and reference to ISO 6520
VT
Shrinkage grooves Crack Surface porosity Root porosity Intermittent undercut Copper inclusions
x
5013 100 2017/2018 516 5012 304/3042
Acceptable imperfections and reference to ISO 6520 Wormhole Porosity (gas pores) Fig. 5. Laminographic views extracted from the examination of one of the inlets to be submitted to fatigue testing, prior to the test. All defects are identified. The dimensions of the defects susceptible to grow are measured. For linear defects their depth ı is also estimated, based on the number of laminographic slices affected by the single weld imperfection. d is the diameter, h the height of the imperfection, s is the thickness of the weld metal as defined by ISO 5817.
Fig. 6. Complementary VT observations of weld imperfections recorded by digital microscopy. Classification of imperfections follows ISO 6520-1.
additional laminographic inspection their possible growth following fatigue cycling. In addition to X-ray laminography and computed RT, CERN applied 100% VT of the weld crown and root of this set of welds. Since the root is also accessible, VT is performed as a direct inspection. Nonetheless, digital microscopy is applied to record the imperfections observed by VT. Examples of such imperfections are shown in Fig. 6. Imperfections can be sharply recorded on an extended depth thanks to the extended focus capability of digital microscopy. 4. Discussion and conclusions Table 1 shows as an example the list of defects identified in one of the helium-inlets inspected by VT, computed RT as well as Xray laminography. The latter is the most exhaustive technique to identify the weld imperfections, in particular the linear ones for which RT features limited resolutions. If RT is able in some cases to identify lack of fusion, cracks are in general not detected by RT, while X-ray laminography is able to detect cracks at least as narrow as 20 m and as short as 100 m. The exhaustive identification of weld imperfections enabled by the high lateral resolution, full
2016 2011
RT
x x x x x x
x
VT
CT
RT
CT
x
x x
volume examination provided by X-ray laminography has allowed a significant progress of the quality of the welds, thanks to the identification of the relevant imperfections and the adapted measures taken to reduce them. Nonetheless, the experience based on several dozens of tested inlets, produced by two different manufacturers, has proven that achieving the most stringent quality level B in this type of fully penetrated, manual welds carried out in a constraining position (welding on vertical conductor) is extremely difficult. It has to be taken into account that the maximum temperature allowed on the SC strands during welding is 250 ◦ C. This imposes several weld stops and starts, which are the source of most weld defects. On this basis, taking also into account the outcome of mechanical calculations and pending results of fatigue tests, a request to degrade the quality level of the weld to ISO 5817 class C (intermediate) is being examined. For defects that would still not fulfil class C a non-conformity would be opened: acceptance “as is” or weld repairs might be imposed by ITER Organization (IO). Defects such as root surface cracks, copper depositions or inclusions might also be tolerated subject to non-conformity. In conclusion, advanced examination techniques such as VT associated to digital microscopy, computed RT and X-ray laminography and their combination have not only allowed an exhaustive identification of weld imperfections of these critical welds, but also a thorough understanding of their nature, position and size and a substantial progress in weld quality that could be achieved during the qualification phases. Acknowledgments Thanks are owed to the partner companies and institutes involved by IO and the ITER Chinese Domestic Agency (CNDA) for supplying the weld samples and for extensive discussions throughout this campaign, and to A. Porret for many of the results of computed RT. “The views and opinions expressed herein do not necessarily reflect those of the ITER Organization”. References [1] [2] [3] [4]
A. Foussat, et al., IEEE Trans. Appl. Supercond. 20 (2010) 402. A. Foussat, et al., IEEE Trans. Appl. Supercond. 22 (2012) 4201205. P. Libeyre, et al., Fusion Eng. Des. 88 (2013) 1478–1481. C. Sauerwein, et al., Proceedings of the 1st International Particle Accelerator Conference (IPAC ’10), 23–28 May 2010, Kyoto, Japan, also CERN/ATS document 2010-143, 2010. [5] S. Sgobba, et al., IEEE Trans. Appl. Supercond. 24 (2014) 4201904. [6] RX Solutions, Technical description of the EasyTom device, http://media.wix. com/ugd/56ca9d f8d6be41febedff088f8153d2040ff3e.pdf