Physics Procedia 67 ( 2015 ) 1022 – 1027

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1875-3892 © 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of ICEC 25-ICMC 2014doi: 10.1016/j.phpro.2015.06.195

ScienceDirect

25th International Cryogenic Engineering Conference and the International Cryogenic Materials

Conference in 2014, ICEC 25–ICMC 2014

Analysis on full-size ITER TF jacked tubes after tensile test at 300, 77 and 4.2 K

Huihui Yanga, Hao Zhanga, Xiangdong Xua, Qiang Liua,b, Laifeng Lia* aState Key Laboratory of Technologies in Space Cryogenic Propellants, Technical Institute of Physics and Chemistry, Chinese Academy of

Sciences, Beijing, 100190, P.R. China bUniversity of Chinese Academy of Sciences, Beijing 10080, P.R. China

Abstract

The ITER TF jacket tube is made of a modified 316LN stainless steel and we demonstrate a detailed analysis after tensile tests at 300, 77 and 4.2 K for the full size tubes. The tensile test results at 4.2 K satisfy the ITER IO requirements. In contrast to fractured tubes after 300 and 77 K tests, the 4.2 K tested tubes show obvious structural deformation. Two different kinds of fractured tubes for 4.2 K tests are comparatively discussed regarding VSM, XRD and TEM data. The maximum phase deformation occurs at fracture and a gradual transformation of into ' state occurs parallel the longitudinal direction.

© 2014 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the organizing committee of ICEC 25-ICMC 2014.

Keywords: ITER; TF jacket tube; 316LN stainless steel; tensile test; cryogenics

1. Introduction

The International Thermonuclear Experimental Reactor (ITER) project is a worldwide research experiment that aims to explore nuclear fusion as a viable source of energy for the ages to come, see reports from Brumfiel [1] and Vandenplas [2]. The ITER magnetic system is used to confine the plasma, which includes 18 Toroidal Field (TF) Coils, 6 Poloidal Field (PF) coils, 6 Central Solenoid (CS) modules and 18 Correction Coils, Federici [3] and Perkins [4]. All coils are designed based on using Cable-in-Conduit Conductors (CICC). The TF coils are

* Corresponding author. Tel.: +86-10-82543698; fax: +86-10-82543700. E-mail address: laifengli@mail.ipc.ac.cn

© 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of ICEC 25-ICMC 2014

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constructed using Nb3Sn CICCs due to the requirement of high magnetic field. The 316LN stainless steel material has been chosen as the TF jacket, due to its good mechanical properties at elevated temperatures, its excellent corrosion resistance and fabric-ability. Since the conductor needs special qualifications because it has to go through several process steps to form the final superconducting coil, as compaction, stretching and long-term reaction heat treatment (RHT) after inserting the super conducting cable into the jacket.

Recent results from Yang [5-6] and Huang [7] indicate that, during the superconducting magnet construction program of ITER, various cryogenic components need to be tested verifying their design. Furthermore, in order to form the Nb3Sn superconductor within CICCs, conductors have to be reaction heat treated at a temperature up to 650 for over 200 h. On the other hand, the TF conductor jacket need to sustain high electromagnetic forces during operation and the TF coils are practically non-repairable. Furthermore, the CICCs are operated at 4.2 K and it is therefore necessary to evaluate the mechanical properties at low temperature and it is important to assure their reliability, following by Lo [8], Weiss [9] and Lee [10].

In order to reflect the true mechanical performance of the jacket material at 4.2 K and to evaluate its suitability as ITER TF jacket, tensile tests on the full-size TF conductor jacket tubes were carried out at 4.2, 77 and 300 K. The 4.2 K test results show 28.8% for EL, 1100 MPa for 0.2% YS and 1490 MPa for the ultimate tensile strength, which are over the ITER requirements. Indeed, the full-size tubes failed not only along the slip band but also across the slip band.

In this paper, we present thorough tests of the full size tube after tensile test, and provide a possible mechanism underlying the stress-strain and free energy linking to structure transformation and reveal a profound connection between the grain morphology changes and phase deformation.

2. Experimental procedure

The tensile tests were carried out on a testing machine (MTS-SANS CMT5000), which was conducted in displacement control with a strain rate of 5×10-4 1/s in accordance with ASTM E1450. The cryogenic environment was obtained through immersing the whole full-size tube into liquid helium or nitrogen.

Specimens in the form of discs with 6 mm diameter were cut from the tensile fractured tube parallel the longitudinal directions by wire-electrode cutting, named as site 1 to 5 (in Fig. 1.), then were examined by using in-situ temperature dependent X-ray diffractometer (XRD). Specimens (1.5 × 4 mm2) for magnetic measurements were also taken from the tensile fractured tubular TF conductor jacket, with the same position as sites 1 to 5. Selected specimens were also detected by optical microscopy and transmission electron microscopy (TEM) analysis.

Fig. 1.Optical images of the fractured full size tubes at 4.2, 77 and 300 K.

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3. Results and discussions

The length of the full-size tube for tensile test is 260 mm and the gauge length is 100 mm. Usually, it is not easy to do the tensile test for the hollow structure as the ends of the tube will be crushed due to the heavy strain during the tensile test. Here, we developed a special design. First, the ends of the tube were welded to another ring, which guaranteed the tightly hold. Second, we put an additional stuff inside the ends to guarantee the successful tensile test, as shown in Fig. 1.

After tensile test at 300, 77 and 4.2 K, the optical images of the fractured full-size tubes are given in Fig. 1. Note that, some ripples are visible in the structure of the fractured tube after the 4.2 K test; whereas, the ripples are not

Table 1. The chemical composition of the TF conductor jacket material after tensile test at 4.2 K.

Specimen Ms

(Am2/kg)Mr

(Am2/kg)Hc

(Oe)V '

(%)S1 1 40.2 3.0 64 26S1 2 28.5 2.8 77 18S1 3 14.5 2.1 113 9S1 4 5.2 1.0 145 3S1 5 0.4 0.2 333 0.3

S2 (1) 10.5 1.74 123 6.8S2 1 14.2 2.13 111 9.2S2 2 11.1 1.82 123 7.2S2 3 7.4 1.36 140 4.8S2 4 0.9 0.13 242 0.6S2 5 0.3 0 0.2

Fig. 2. Magnetization loops for S1 (a) and S2 (b), the magnetic field was increased from 80000 Oe up to +80000 Oe and againback to 80000 Oe.

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present on the other two tubes. At the 4.2 K test, the tube failed not only along the slip band but also across the slip band, and the appearance of macroscopic slip bands going with the occurrence of load jumps is clearly observed. Further, for all 4.2 K tests, the fractured TF tubes show apparent magnetic property, which is of fundamental and practical interest.

We have chosen two different kinds of fractured tubes from the 4.2 K test to do more detailed analysis. For the first one (S1), the tensile breaking site was almost in the middle place of the tube, whose test result was more reliable without doubt. In contrast, for the second one (S2), the breaking site was slightly off-center due to the uneven strain between the inside stuff and the ends of the tube. Their test results are used for comparison. Actually, for the tensile test at 4.2 K, the elongation is 28.8% for S1 and 22% for S2, meaning that the S2 tube undergoes a much heavier deformation.

Both S1 and S2 showed magnetic property. To explore possible structural changes along the tube, we tested specimens from several sites. The saturation magnetization Ms, remanent magnetization Mr, and coercive force Hc are listed in Table 1, and the loops are shown in Fig. 2. While analyzing the Ms, we exclude the paramagnetic interference. As expected for S1, the maximum Ms value (ca. 40.2 Am2/kg) is for S1-1, then it decreases gradually. Ms is 5.2 Am2/kg for S1-4, meaning only a small amount of ferromagnetic phase exists in the specimen. S1-5 gives almost sole paramagnetic behavior. As for S2, the Ms for the tensile breaking position at S2-(1) is about 10.5 Am2/kg. The maximum Ms in the S2 sample is 14.2 for S2-1, and then decreases from S2-2 to S2-5. Magnetic properties are highly sensitive to the structure of materials, which provides information with regard to the whole sample including both surface and interior of the bulk. As known, the magnetic property has no relation with the -austenite structure of 316LN SS, whereas it is due to the force induced '-martensite structure, according to Tavares [11] and Das [12]. Taken together, we think after tensile test at 4.2 K only that the structure of the tube has changed. Irrespective of the breaking position, the maximum deformation is thought to be at around the middle site of the tube, then it decreases along the axis direction from the fracture to the ends.

The intrinsic value of Ms for the 100% '-martensite phase is 154 Am2/kg, from Mumtaz [13]. Thus it is possible to calculate the content of '-martensite (V ') in the specimen, which is listed in Table 1. The V ' of S1-1 is 26%, meaning a significant of structure deformation occurred under tensile test at 4.2 K for the modified 316 LN SS.

In order to identify the deformation in the structure, XRD experiments were performed, which is demonstrated in Fig. 3. Distinctly, the original specimen and specimens from fractured tubes after tensile test at 300 and 77 give typical of a face centered cubic lattice (fcc) structure of -austenite peaks. However, the XRD patterns of specimens

Fig. 3. XRD patterns of specimens from site 1 5, S1 (a), S2 (b).

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Fig. 4. TEM images of the specimen at site 1, of S1 (a) and (b), of S2 (c) and (d).

S1 and S2 exhibit not only -austenite peaks but also tensile stress induced '-martensite peaks. Peaks at 2 =44.56° and 82.72° of the body centered cubic (bcc) structure correspond to the plane spacing of the ferrite (d=0.2032 and 0.1171nm). Its peak intensity is strong, which demonstrates a certain amount of induced '-martensite. In contrast to S1, peaks of '-martensite in S2 is clearly weaker, meaning that less force induced '-martensite deformation in S2. The peaks of the remaining -austenite decrease slightly, whereas do not shift. Interestingly, the intensity of the ' peak (mainly in (110) and (211)) has a tendency to decrease gradually.

The bright-field images of specimens S1 and S2 were investigated by TEM and results are shown in Fig. 4. Pronounced deformations are identified in both images, except that the deformed bands are much more obvious for S1. Additionally, the deformed twins, martensite microstructure and intersection between martnesite and deformation twins are visible as a typical phenomenon taking place on intersecting {111} planes. The bands may be produced by slip on closely spaced slip planes.

4. Conclusion

Full-size tubes of ITER TF jacket have been analyzed after tensile test at 300, 77 and 4.2 K. In the optical images, some ripples have been observed in the structure of the fractured tube after 4.2 K test; whereas, the ripples cannot be seen on the other two tubes. Detailed analysis shows that the results of the fractured tube at 4.2 K, whose tensile breaking site is in the exact middle place of tube, are more reliable. Consequently, the maximum structure

Huihui Yang et al. / Physics Procedia 67 ( 2015 ) 1022 – 1027 1027

deformation is always found in the exact middle position of the tube, the same is true for the XRD results and TEM image, and then decrease gradually along the axis direction from the fracture to the ends.

Acknowledgements

This work was supported by the National Magnetic Confinement Fusion Science Program of China (2011GB112004), and National Natural Sciences Foundation of China (grant No.51377156).

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