the investigation on welding processes for sus316ln tubes used in superconducting magnetic system of...

The Investigation on Welding Processes for SUS316LN Tubes Used in Superconducting

Magnetic System of EAST

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2005 Plasma Sci. Technol. 7 2932

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Plasma Science & Technology, vo1.7, yo.4, Aug. 2005

The Investigation on Welding Processes for SUS316LN Tubes Used in Superconducting Magnetic System of EAST*

Wu Jiefeng(%*@), Chen Siyue(R,#,@), Weng Peide($M@), Gao Daming(B a) Institute of Plasma Physics, Chinese Academy of Sciences. Hefei 230031, China

Abstract The force flow cooled superconducting cable-in-conduit conductor (CICC) is used in both of EAST toroidal field (TF) and poloidal field (PF) coils. The conductor consists of multi-stage NbTi superconducting cable and 1.5 mm thick square jacket. The cable is pulled through in a thin wall circular jacket and then compacted to square cross-section conductor. The jacket material is SGS316LN austenitic stainless steel seamless tubes (about 10 m each), which is assembled by butt-welding up to 600 m. The results of the welding procedure investigation and quality assurance procedures carrying out are described in this paper.

Keywords: cable-in-conduit conductor. SUS316LN austenitic stainless steel, automatic welding and quality assurance

PACS: 52.55.Fa, 85.25.Am

1 Introduction

Cable-in-conduit conductor has been investigated and widely used in large-scale magnets, especially in the TF and PF magnets of superconducting Tokamaks. It is desirable that winding coils have long enough con- ductors to reduce the number of joints and get a good superconducting property. There are two ways to fabri- cate such conductors: one is a cable overlapped by steel or other alloys tape, which is welded into a jacket by a longitudinal welding; the other is a cable inserting into the conduit with a full length jacket formed by using seamless tube and butt-weld technology. In both these technologies, the assembled conduit and cable will be compacted to the required shape and dimensions.

EAST is a full superconducting Tokamak to be constructed in Institute of Plasma Physics, Chinese Academy of Sciences. The TF and PF magnets of EAST using CICC and the conductors have all been produced by the Hefei 600 m CICC jacketing line within two years. The total length of CICCs is about 35 km.

The cross-section of conductor is shown in Fig. 1. The original size of conduit is 26 mmx 1.5 mm and 8 m 12 m long, the material is 316Ln' stainless steel. Every piece of conductor is less than 600 m [l].

The welding procedure and quality assurance are the two key issues of CICC fabrication and must corre- spond to ASME pressure vessel code for class lcom- ponents and GB3233 standard. The welding quality must be verified by using X-ray, ultrasonic and pen- etrant inspection as well as proof-test of helium leak- age for each joint. The leak rate should be less than 1 x 10-1°Pa.m3/sec and the internal helium pressure should be less than 3 MPa.

2 Choice of the method and main technological parameters of welded joints

Non-rotating joints welding technology of automatic gas tungsten arc welding (GTAW) for thin-wall tubes is adopted. In the complete set of equipment for GTAW, the following items are included 1'1:

A tube-to-tube automatic TIG welding machine (Tubemaster model 511) with the control equipment. It has the following functions:

a.

(1) IBM compatible Microprocessor (2) 100 welding programs on-board memory (3) Program transfer on non-volatile data card (4) Independently settable override limits for each

( 5 ) Whole circle possible to be divided into 6 seg-

(6) Capability of welding 12.7" N 63.5 mm OD b.

welding parameter

ments with different parameters

The welding head C-25 (see Fig. 2) for welding of non-rotating joints

c . The system for alignment and fixture of tube on staple. d. The system for arrangement of gas lines,

manometers, pressure controller, gas cylinders and de- vice for internal and external gas-flow of a weld.

e. The cut fitting for cutting the ends of tubes. There are nearly 3300 joints in all the EAST conduc-

tors, and each joint should be qualified. Moreover, in order to let the 600 m cable pull through the conduit, the maximum welding reinforcement jut on the inside surface has to be less than 0.1 mm. So to get a good butt joint, a great deal of experiments is to be done.

* The project supported by the National Meg-Science Engineering Project of the Chinese Government

Fig.1 The CICC sections

Wu Jiefeng et al. : Welding Processes for SUS316LN Tubes Used in Superconducting Magnetic System

Weld

Argon_

Fig.3 The inside argon flow kit

3.1 X-ray and ultrasonic inspections

During the welding of the conduit, X-ray and ultra- sonic (USC) non-destructive inspections are very im- portant in finding the inner defects. X-ray and USC inspections can detect defects such as crack, gas pores, incomplete penetration and lack of fusion. Weldability of full austenitic stainless steel is very good. As the crystalline grains of welding seams are very large, it is very difficult for USC inspection to detect the inner defects of welding seams[3]. For the USC amplitude of vibration from the defect and the weld microstruc- ture is nearly the same, a lot of experiments and many standard specimens must be done. By comparing the demonstrated figures on the ultrasonic screen reflected from the defect or crystal boundary, defects can be iden- tified from the noisy signals. X-ray inspection is a direct observable non-destructive investigation. Its films can also directly show the defect. So it is a better inspec- tion way, especially to the full austenite stainless steel welding seam. The disadvantage of X-ray inspection is its insensitive to the defects which are perpendicular to the X-ray. But it is just the advantage of UsC inspec- tion. The combination of both the X-ray and the USC is a good solution for the welding inspection.

Fig.2 C-25 welding head

At the first stage of research, the top quality weld- ing of short samples is conducted by adjusting a series of welding parameters, such as the welding current of pulse, the pulse length, the current of base, the welding speed, the argon flow both outside and inside the tube samples, the gap between the ends of the electrode and the welded tube. Through these experiments, a set of optimized welding parameters can be obtained. The 3.2 Pressure control and vacuum welding is carried out without filler in both the con- tinuous mode and pulsing mode. The whole welding procedure is divided into two steps. The first step uses pulsing mode, and the second takes continuous one. The distinctive features of this equipment are: real- ization of the welding method in automatic mode, cor- rection of the position of tungsten electric pole across the joint with stationary part and availability of the fixture of alignment of the tubes. The specific welding parameters are given in Table 1. Besides using high purity argon, a special adaptation kit inside the tube has been designed and manufactured for protection of the weld on both the external and the internal surfaces (see Fig. 3).

tightness tests

The pressure control and the vacuum tightness tests are divided into two steps: one being the inspection to the girth welds on line, and the other is the check to the vacuum leak rate of a whole CICC. The deter- mination of the vacuum tightness of the welds is con- ducted by one of the kinds of mass-spectrometric meth- ods, a helium flow method. Its makeup is a device, subject to the control, with a high-sensitivity leak de- tector l x 10-10Pa,m3/sec) 230D. When the chamber outside the conduit or CICC is evacuated with a turbo- molecule pump up to lo-' Pa, the helium of 0.3MPa N 3.0 MPa is filled inside the conduit or CICC. If there is any leakage of more than 1 x 10-l' Pa.m3/sec in the weld, the helium gets into the leak detector cavity and

3 Method for quality assurance is caught by it.

of welded joints 3.3 Endoscopy, dye penetrant inspec-

tions and welding protrusion test Any defect at the joints might be fatal to the EAST magnet, and thus each joint ought to be strictly in- spected. The following nondestructive inspections have been performed.

The most probable defects of the girth welds are the protrusions on the internal surface, displacement of

a. X-ray and Ultrasonic inspections; the welded edges, incomplete penetration of the welded b. Pressure and Vacuum tightness tests; joints and superficial tiny crack. Endoscopy and dye c . penetrant inspections can respectively find the inside

or the outside superficial defects, and the welding pro-

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Endoscopy, Dye penetrant inspections and weld- ing protrusion test

Plasma Science & Technology, vo1.7, Ko.4, Aug. 2005

Table 1. The welding parameters of butt joint of tube Pulse welding Base welding Welding velocity Time

Pass1 Pass2 Pass1 Pass2 Pass1 Pass2 Pass1 Pass2 Weld current (A) current (A) (r/min) ( s )

Start 30 Level 1 59 40 20.7 1.56 6.4 Level 2 59 39 20 1.56 6.4 Level 3 58 38 19.7 1.56 6.4 Level 4 58 36 19.7 1.56 6.4 Level 5 56 36 19 1.56 6.4 Level 6 55 37 18.7 1.55 6.3 Final 15

Table 2. Chemical composition of SUS316LN Tube Element content (wt%) sample Fe Cr Xi C S P Mn Si MO iX a26 x 1.5 base 17.54 13.24 0.019 0.002 0.012 1.43 0.27 2.46 0.13

Table 3. Mechanical properties of tube samples State Base metal Weld sample

(Auto TIG) T (k) 293 4.2 293 4.2 u0.2 (MPa) 343.6 1241 339.1 1145 ut, (MPa) 780.4 1474 778.7 1316 6 (%o) 45 42 38 29 Place of Break Break fracture over weld over weld

trusions or displacement can be measured with a series of standard gauges.

4 Mechanical properties and mi- crostructures of welding joint and base metal

Taking the CICCs undergoing a large electromag- netic force into account. the mechanical properties of conduit and the welded joints are very important. The chemical composition of the tube is shown in Table 2.

Table 3 presents the results of the mechanical testing of short specimens at room and liquid helium temper- ature.

Fig. 4 and Fig. 5 show the tesion curve of the base metal and the weld samples a t 4.2 K.

Fig. 6, Fig. 7 and Fig. 8 respectively demonstrate the microstructures of the base metal, the weld junction and welding seam. In Fig. 8, it can be seen that the structure of the base metal is full austenite. The mi- crostructure consists of small grains of equal axis's.

But the microstructures of the weld junction (Fig. 7) and the welding seam (Fig. 8) are characterized by large columnar grain. The direction of columnar grain is parrelled to cooling temperature gradient. To full austenitic stainless steel, according to related theo- ries L41 that the drive force and the speed of grain growth are in inverse relation with the grain size, and the smaller the grain size is, the faster the HAZ (heat af- fected zone) grain growth speed will be. Plenty previous welding experiments also testify it. Meanwhile, the size

35t

3 o i 7 I

-200 0 200 400 600 800 1000 1200 1400 1600 Time / S

Fig.4 2.24 mm. width- 10.00 mm

Tension curve of base mate1 at 4.2 K, thickness-

J J I

25 30 t

-100 0 100 200 300 400 500 600 700 Time / S

Fig.<' Tension curve of weld at 4.2 K, thickness- 2.20 mm, width- 10.00 mm

Fig.6 Base metal (1OOx)

of the grain is in direct proportion to the welding heat input for full austenitic stainless steel. So during weld- ing the conduit of CICC, the welding parameters of

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Wu Jiefeng et al. : Welding Processes for SUS316LN Tubes Used in Superconducting Magnetic System

of TF coils and 24 CICCs of PF coils have been man- ufactured. Moreover, 11 TF coils and one CS (central solenoid ) coil have been tested successfully. All these tests have been demonstrated that the welding joints of conduits are reliable.

6 Conclusions a. The SUS316LN austenitic stainless steel can be

successfully welded by the GTAW technology with ap- propriate welding parameters.

To the welding of non-rotating joints of thin- walled tubes, automatic gas tungsten arc welding is a better method, for it can obtain satisfactory welding joints and surface appearance, especially for it can con- trol the maximum welding reinforcement jut inside the tube.

c . In the jacketing line of CICC, the non-destructive inspections can effectively assure the welding qualities.

b.

References 1 Chen Siyue, Wu Jiefeng, Chen Xingqian, et al. Plasma

Science & Technology, 2002, 4(1): 1163 N 1170 2 Edited by John Emmerson. Operating and Mainte-

nance Instruction Manual For Tubemaster Welding System Controller Model 511 with Tubermaster C-25 Weld Head Model 811A. Magnatech Limitied Partner- ship press in 1998, East Granby, Connecticut U.S.A. Rychagov A V, Filatov 0 G, Egorov S A. Fusion En- gineering and Design, 1999, 45: 209 N 216 Zhang Fuju, Xu Weigang, Wang Yutao, et al. China Welding, 2003, 12(2): 122 N 127 (in Chinese)

3

4

(Manuscript received 7 July 2004) E-mail address of Wu jiefeng: jfw@ipp.ac.cn

Fig.8 Welding seam ( 1 0 0 ~ )

smaller heat input are selected. In Fig. 7 and Fig. 8, there is a little delta ferrite diffuse at the boundaries of ausentite grains. Although the delta ferrite in- creases magnetic conductivity, it prevents the welding hot cracks from occurring.

5 Experimental results Up to now, the optimized welding parameters

through a series of welding experiments have been taken to weld about 3300 butt welding joints, and each joint has passed through the non-destructive inspec- tions above mentioned. At the same time, 34 CICCs

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