Plasma and Fusion Research: Regular Articles

Volume 14, 3405062 (2019)

Study on Butt Weld Joint of Thick Plate Superconducting Coil
Structure to Reduce Welding Residual Deformation∗)
Arata NISHIMURA
National Institute for Fusion Science, Toki 509-5292, Japan(Received 27 December 2018 / Accepted 23 February 2019)In a large fusion experimental device, the magnetic field in the coil is rather high to create higher magnetic
field at the plasma center, and the coil current is very large resulting in generation of huge electromagnetic force.
To sustain such huge electromagnetic force, thick section components are required for the coil structure of whichmaterial is generally an austenitic stainless steel to avoid magnetization. On the process of the welding design,
the welding residual deformation must be taken into account to keep the position of the current center of the coiland to avoid the degradation of the superconducting conductor by the excessive welding residual strain. In thispaper, a partial welding which was used for the LHD construction will be focused and the image of the conceptual
welding design of ITER TF coil butt joints will be discussed for an example of the fusion application.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: partial welding, welding residual stress, welding residual deformation, LHD, ITER, TF coil
DOI: 10.1585/pfr.14.3405062

1. Introduction
When a large-scale plasma device or a fusion DEMO
is designed conceptually, it is very important to consider
the assembly process including welding procedures at thesame time. As the size scale of the plasma device or the
DEMO becomes large, the electromagnetic force which is
generated by the coil current and the magnetic field becomes large and the stiﬀer support structure is requested tomaintain the magnetic surface of the plasma. This meansthat the welding of the thicker components will be necessary, and the non-destructive inspection must be considered to ensure the soundness of the weld joint. In addition, the welding deformation must be taken into accountwhen the tolerance of the manufacturing and assemblingprocesses are determined. Therefore, the welding design
to reduce the welding residual deformation must be investigated during the design activities.
When a full penetration welding is performed on a
thick section weld joint with a normal welding groove suchas J-shape groove and a general multi-pass welding process, the shrinkage of the weld joint in the vertical direction
to the welding line will become the same level as one-halfof the plate thickness. Also, the angular distortion will occur at the weld joint, which will cause an out-of-plane deformation and produce a large mismatch at the end of the
product. Generally, it is diﬃcult to correct the angular distortion after the welding. Therefore, consideration of thewelding process during design activity is very important.
When the Large Helical Device (LHD) was designed,
author’s e-mail: nishimura.arata@toki-fs.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

the assembly procedure was investigated and considered asa part of design activity. The partial weld joint with deep
narrow grooves was proposed to reduce the shrinkage, andthe symmetric welding process with small welding heat input was adopted to prevent the angular distortion.
In this paper, the countermeasures taken in the
LHD design and construction to reduce the welding
shrinkage and the residual deformation are summarized
succinctly [1]. The conceptual design on the butt weld
joint of ITER TF coil cases will also be discussed andthe weld joint image will be presented based on the recent
ITER NEWSLINE [2].

2. Outline of LHD Support Structure
The LHD design activity started in 1991 and the firstplasma was produced in March 1998 [3]. The LHD project
was lasted 8 years, and the practical design activity andmany research and development eﬀorts were carried out inthe early stage of the project. The plasma experiments arecontinuing, and D-D experiments started in March 2017and interesting results have been obtained [4, 5].
The LHD has one pair of helical coils and three pairsof poloidal field coils. All these coils are superconducting
and have been operated for almost 20 years, since 1998.
All coils are supported by a cryogenic support structure
called a support shell. A lower shell and an upper shellwere manufactured separately. Figure 1 shows the trialassembled lower shell in the LHD Experimental Hall. The
support shell consists of ten sectors, and the shell wasformed by butt-joining of these sectors. The outer diameter was about 11 m. After setting the pair of helical coilsin the lower shell, the upper shell was put on the lower

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c 2019 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 14, 3405062 (2019)

Fig. 3 Sketches of weld groove of each butt joint.
Fig. 1 Field assembly of lower support shell of LHD.

Fig. 4 Configuration of thick plate butt weld joint and residual
stress distribution.

Fig. 2 Cross section of LHD support structure and coils.

shell and the equator line was welded. The cross section of
the LHD is shown in Fig. 2. The left side is a device centerand the right side is out-board side of the LHD. There arepairs of Inner Vertical (IV) coils, Inner Shaping (IS) coils,
and Outer Vertical (OV) coils outside of the support shell,
and the helical coils are located inside the shell.
As shown in Fig. 1, the inside of the support shell wasmachined out in the factory after being trial-assembled bytack welding. Then, the shell was disassembled and transferred to the LHD experimental building. Since the innersurfaces of all sectors were machined out with a severe tolerance, the welding residual deformation must be reduced
to as small as possible at the construction site. Therefore,
a special welding groove was investigated and designed,
as shown in Fig. 3 (a). The plate thickness was divided
into three parts. The middle one-third of the plate was ametal-touched zone with narrow grooves, and the remaining cross-sections were fully welded. The expected residual stress distribution and the role of the narrow grooves
will be discussed below.
After setting up the lower support shell and the uppersupport shell, the equator line was welded from outside
because the helical coils were already installed inside thesupport shell. The welding groove at the equator is shownin Fig. 3 (b). The upper side had triangular groove and thelower side was flat, which made the horizontal welding
easier. The inner side (left side of the drawing) was cut
out in a semicircle to make welding ripples when the firstweld pass was performed and to avoid the concentration of
the welding residual stress.

3. Design of Partial Welding Joint
When the butt joint is welded by arc welding, the joint
will shrink due to the melting filler metal and high temperature beyond the mechanical melting point. In addition,
the residual stress will remain after cooldown, for the multipass welding will cause the temperature diﬀerence which
may generate angular distortion, i.e., out-of-plane deformation. Therefore, the butt joint weld groove was designed
taking account of the practical welding process.
The images of the butt joint configuration and theresidual stress distribution are shown in Fig. 4. In the midsection of the joint there is a metal-touched area and both
surface sides are weld metals. The narrow oval grooves
are located at both ends of the metal-touched area. Regarding the welding process, it was assumed that the same
weld passes on both grooves would be welded at the sametime, though the welding heat input would be diﬀerent depending on the welding position, for example, flat positionor overhead position. Under these conditions, the residual
stress distribution was supposed, as shown in Fig. 4. The
distribution will be symmetric about the plate center andthe tensile stress remains near both surfaces and the com-

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Fig. 5 Weld grooves of butt joint of support structure on equator(a), and butt joint on horizontal port (b).

pressive stress exists in the midsection of the plate.
When the first pass was welded on both grooves, theweld ripples were formed on the back surface of the weld
pass in the narrow oval grooves, and these grooves would
reduce the stress concentration and prevent creation of thecrack-like defect at the weld root. If the first passes on bothgrooves would be welded at the same time, both weld metals would tend to shrink. However, the metal-touched area
would deform only elastically because of low temperaturerise, and that prevents the shrinkage of the weld metal. As
the results, the weld metal could not shrink enough, and it
would be pulled. On the other hand, the metal-touched area
would be pushed. Therefore, the tensile and compressive
residual stress would be generated in the weld metal andthe metal-touched area, respectively. Since the total tensilestress must be equal to the total compressive stress, it is expected that the stress around the narrow groove would becompressive. If the residual stress distribution should not
be symmetric about the plate center, this means that there
is a bending stress component and the plate will be bent by
the residual stress. This is the so-called angular distortion
which is out-of-plane deformation and known as one typeof the welding residual deformation.
The welding grooves of the butt joints are shown in
Fig. 5. Figure 5 (a) shows the welding groove of the uppersupport shell which has the triangular-shape groove on theequator. Figure 5 (b) shows the butt joint on the horizontal
port after temporary assembly. The top surface is flat, andthe narrow oval grooves and J-shape grooves are very clear.
To achieve the symmetric residual stress distribution,
the symmetric welding process was adopted in the practical manufacturing. The welding heat input was reduced,
and the small bead was formed little by little. Finally, theupper and the lower support shells were constructed withan excellent manufacturing tolerance.

Fig. 6 First assembling of in-board and out-board coil cases of
TF coil (Republic of Korea, 2017).

Fig. 7 Integration of in-board and out-board coil cases andwinding pack. (Italy, 2018).

4. Butt Joint of ITER TF Coil Case
Generally, a large-scale superconducting magnet isdesigned with a very tight tolerance to fit the principal devices. ITER TF coil case was also designed with the severe
manufacturing tolerance of +/− 1 mm order, although themagnet height is approximately 14 m, and the width is approximately 9 m. Therefore, the ITER TF coil case will betaken here as an example of a thick section welded structure.
The first assembly of the in-board coil case and theout-board coil case was carried out in the Republic of Korea at the end of 2017. The assembly was succeeded, and it
was announced that both components could be adjusted toless than 1 mm. The success photo is shown in Fig. 6 [2].
In autumn 2018, the coil cases were integrated with thewinding pack of the superconductor as shown in Fig. 7 [6].
The winding pack set at the center and the in-board andthe out-board coil cases were placed separately. The inboard and out-board coil cases were shifted slowly to thewinding pack, and the winding pack was inserted in both

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coil cases. At the bottom of the winding pack (right side in
Fig. 7), there is a large rectangular space, and all conductor
joints and the cooling pipes were placed in this space. The
butt joints at the top and the bottom of the coil cases will
be welded, and the cover plates of the winding pack will
be attached by welding.
The butt joint at the top of the coil case is shown in
Fig. 8. The configuration of the joint is U-shape and narrow grooves for coil case cooling pipes can be seen. The
bottom view of the coil case is shown in Fig. 9. The rectangular space is for the conductor joints and the coolingpipes of the winding pack, and the butt joints are located at
the upper and the lower plates of the space.
As discussed above, there are two types of the welding
residual deformation. One is the shrinkage in-plane and theother is the angular distortion of out-of-plane. The residual
deformation would cause the following matters.
(1) The shrinkage in-plane will deform the winding packand degrade the critical current of the coil when the winding pack is supported by the coil case with some GFRP
spacers.
(2) The angular distortion will generate a large mismatchand make it diﬃcult to connect with the neighboring coilcases with keys and bolts. Additional field work to matchthe joints may become necessary.
(3) Both deformations will shift the coil current center andproduce unexpected magnetic fields. If the winding pack is
supported by only the resin which will be filled in the gap
between the winding pack and the coil case, the coil current

center must be evaluated after the integration work of thewinding pack and the coil cases, and the results must be
used for the adjustment of the divertor set-up. The positionof the coil current center would be diﬀerent in each TF coil.

5. Conceptual Design of Butt Joint
The butt joints of the TF coil case were investigatedand designed conceptually. Mainly, it was taken accountto avoid and reduce the welding residual deformation. The
proposed weld joint shapes are shown in Fig. 10 for the top
joint and Fig. 11 for the bottom joint.
In both figures, the dashed part shows a metal-touchedarea to reduce the shrinkage in-plane around the joint, andthe J-grooved area will be welded. The welding can bedone only from the outside, because the winding pack isalready installed inside the coil case. Therefore, a large
groove was designed for one side welding. Since the total weld metal will become a large amount, the welding
must be carried out slowly keeping the interpass temperature lower, and the welding must be performed symmetrically about the coil case center. The order of the welding
will be important. The end part of the side plate will be

Fig. 10 Proposal of weld groove for the top joint of TF coil case.

Fig. 8 Butt joint of the top of the TF coil case.

Fig. 9 Bottom view of TF coil case.

Fig. 11 Proposal of weld groove for the bottom joint of TF coilcase.

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Volume 14, 3405062 (2019)welded first following the corner part welding in the caseof the top joint (Fig. 10). All first pass must be formed,
and then the color check will be carried out to check thecracking on the bead. After that, the second pass will bewelded. The generation of the symmetric residual stress
distribution about the joint center is one of the key issues
to reduce the angular distortion. The welding heat input
also must be controlled to be lower for all welding positions.
Since the metal-touched area does not support theshear stress, and the area is supposed to be about one-thirdof the total cross-section of the joint, the splice plate will
be required to maintain enough cross-section against thetorsion. Therefore, two splice plates were attached outside
the side plates of the coil case in these figures.
As the welding of the coils case can be done only fromoutside and the space for welding machine and for nondestructive inspection (NDI) is needed, the weld joints will
be welded first and then the splice plates will be welded.
After the NDI is performed and the soundness of the weld
joints of the coil case is confirmed, the reinforcement of
the weld joints will be removed, and the splice plate will
be welded. To reduce the total welding time, a large plate
will be attached as a splice plate and the both ends will
be fillet welded. As the large torsion stress will act on thefillet weld joints, the throat thickness of the fillet joint musthave enough margin.
After welding of the coil case joints and the fillet weld
joints, the NDI of the ultrasonic test (UT) and the radiographic test (RT) is required to certify the soundness of
the weld joint. Generally, the attenuation of the ultrasonicwave is remarkable in the thick section component of theaustenitic stainless steel. Therefore, the phased array UTprocedure will be applicable to achieve high resolution.
Also, the RT is useful as a volumetric test. The identification minimum dimension must be clarified when the RT
is carried out.

6. Summary
The large-scale superconducting magnet generates
huge electromagnetic force and the large support structurewith thicker components are required. Generally, the thicksection welding causes the large shrinkage and severe angular distortion, and the welding residual deformation cannot be corrected because the winding pack is already installed inside the coil case. Therefore, the welding groove
design and the welding procedure are very important foravoiding and reducing the residual deformation. In thispaper, the experience learned in the LHD design and construction was introduced and the basic design concept of
the thick section welding was summarized. Then, the conceptual design on the weld joints of the ITER TF coil casewas carried out based on the partial welding and the symmetric welding procedure.
On the design of the LHD, the special welding groove

was designed based on the consideration of the symmetric residual stress distribution about the plate center. There
was a metal-touched zone at the midsection of the plate.
The size is about one third of the plate thickness. And
the narrow oval grooves were machined at both ends of themetal-touched zone to produce the welding ripples to avoid
the crack-like defect at the weld root and to release thestress concentration. Both surface sides of about one-thirdof the thickness were welded symmetrically to generate thesymmetric residual stress distribution. The metal-touchedarea would reduce the shrinkage of the joint, and the symmetric residual stress distribution would prevent the bending of the joint so called “angular distortion.” Regarding
the welding of the joint, the order of welding, the welding
heat input, and the weld bead length were controlled carefully, and both grooves were welded symmetrically. The
welding of the support shell of the LHD was successful
and achieved an excellent tolerance.
The weld joints of the ITER TF coil case were investigated and the images of the joint were presented. The
joint of the top of the coil was U-shaped, and three partsof the metal-touched zone were designed. The length ofeach zone would be about one-third of the plate width. The
metal-touched area would resist the shrinkage of the weld
metals and reduce the total shrinkage of the joint. Also, thewelding order was considered in that the edge part of theside plate will be welded first and the corner of the joint
will be welded second. In the case of the bottom joint, twoplates are welded at the same time to keep the symmetricresidual stress distribution.
In both cases, the cross section must be consideredagainst the torsion. Since the metal-touched area does notsupport the share stress, the splice plates will be requested
to attach the side plates.
The non-destructive inspection on the weld joints isimportant to certify the soundness of the weldment. The
tests will be rather complicated because of the thick section
joints, but they must be carried out.

Acknowledgement
The author would like to express his thanks to the
LHD project which has been constructed under the leadership of Professor Atsuo Iiyoshi, the first Director General of NIFS. Also, he thanks to ITER project which is undergoing at Cadarache in France as an international fusion
program.
[1] O. Motojima et al., Fusion Eng. Des. 20, 3 (1993).
[2] ITER home page; https://www.iter.org/newsline/-/2891[3] A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999).
[4] Y. Takeiri, J. Plasma Fusion Res. 94, 18 (2018).
[5] H. Takahashi, Nucl. Fusion 58, 106028 (2018).
[6] Fusion for Energy home page; http://fusionforenergy.
europa.eu/mediacorner/newsview.aspx?content=1296

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