Jacket profile

The CHSN01 material needs to be produced using medium-frequency induction furnace steelmaking, followed by a suitable refining process aimed at minimizing the presence of impurity gases, such as hydrogen and oxygen. To achieve the desired properties, appropriate refining techniques such as vacuum oxygen decarburization and ESR must be employed.

CHSN01 pipes manufactured by Jiuli Company in China undergo a hot extrusion process followed by multiple drawing steps. Subsequently, an intermediate annealing treatment is required before subjecting them to a final solution annealing (SA) at temperatures ranging from 1120 °C to 1200 °C. The outer and inner hole diameters of a circle-in-square pipe are 37.7 mm and 29.5 m, respectively. A 30 t CHSN01 jacket with a total length of approximately 5000 m has been produced. Mass production of the CHSN01 jacket is shown in Fig. 2. Each batch in the solution furnace is considered as a separate entity, with a total of 20 batches.

Fig. 2Full-screen View

(Color online) Mass production of the CHSN01 jacket

The surface roughness of the jackets, as measured by R_a, is below 1.6 μm, whereas the equivalent permeability is below 1.01. In addition, each individual jacket measures at least 9 m in length. The jacket grain size number, as per ASTM standards, exceeds four, and intergranular corrosion is not observed. Furthermore, micrographs at a magnification of 500x reveal the absence of discernible ferrite traces.

During CICC coil manufacturing, the jacket must be compressed (the inside of the jacket is fitted to a cable), bent + straightened + wound (conducive to transportation), and treated to Nb₃Sn superconductor reaction aging [22, 23]. The compaction process uses a cold-rolling machine to reduce the outer dimensions from 37.7 mm × 37.7 mm to 35.1 mm × 35.1 mm, thereby ensuring precise dimensional control. Subsequently, a series of cold working (CW) operations are performed on the jackets, which result in an overall deformation of approximately 4%. Following this cold-working stage, the Nb₃Sn conductor jackets are annealed in a vacuum furnace. A heat-treatment solution specifically designed for ITER Nb₃Sn is used to age the jackets [24, 25]. The duration of the Nb₃Sn superconductor reaction aging treatment cycle is as follows: 210 °C for 50 h + 340 °C for 25 h + 450 °C for 25 h + 575 °C for 100 h + 650 °C for 100 h. During the heat treatment process, vacuum is always maintained at approximately 5×10^-3 Pa.

Mechanical test samples and procedures

Plate-like tensile samples were obtained by cutting the corner and face of a jacket within the space of limited thickness. The total length of the flaky tensile sample was 67 mm, length of the parallel section was 26 mm, thickness was 3 mm, and original gauge length was 20 mm. Tensile test specimens were fabricated according to the ASTM E1450 guidelines [26]. Compact tension (CT) specimens for fracture toughness testing were prepared with a width (W) of 25 mm and thickness of 3 mm, without any side grooves. These CT specimens adhered to the JIS Z 2284 standard for fracture toughness [27]. FCGR CT specimens with a width of 30.6 mm and thickness of 3 mm were prepared. The dimensions of the FCGR specimens were in accordance with the specifications in ASTM E647 [28]. The direction of crack processing of the fracture toughness and FCGR samples was parallel to the length direction of the jacket. The locations of the test samples are shown in Fig. 3.

Fig. 3Full-screen View

(Color online) Positions of the test samples for each batch of jacket

The total number of batches was 20, and each batch contained approximately 27 jackets. One jacket was required per batch for sampling purposes, and each batch was tested for two tensile strengths (corner and face), one fracture toughness, and one FCGR strength. The test samples were consistently enclosed within a thermally insulating sleeve and continuously immersed in liquid helium throughout the experiment [29, 30]. A thermometer was placed above a sample to ensure that the sample was adequately cooled, and the temperature was guaranteed to be less than 7 K during the test. The tensile tests were conducted under displacement control at a strain rate below 5×10^-4 s^-1. During the tests, the strain was measured using a clip-on extensometer (Epsilon model 3542-025M-50-LT) in the range of 10%. The mechanical properties of the samples, including the YS, EL, and ultimate tensile strength (UTS), were determined by testing. Each fracture toughness specimen was precracked at room temperature at a frequency of 3 Hz.

Weld proof samples

In the production process of CICCs, butt welding of the jackets is necessary to ensure a sufficient conductor length. Subsequently, a proof sample (one per machine) is welded using the same machine used for conductor production to assess the welding quality. Tungsten inert gas (TIG) welding technology is used for joint welding, and the shielding gas is a mixture of 70% argon and 30% helium to obtain a fine grain size. The circular area around the inner hole of a jacket is welded using an automatic track-welding machine, except for the first line, which is welded by self-fusion. The corners are manually TIG-welded using a 1.6 mm diameter electrode. Refer to the previous development of jacket-welding techniques and welding processes [31].

The quality of the welded joints must be ensured by nondestructive testing, which includes conducting penetration tests, examining X-rays, and performing helium leak tests. Subsequently, the conductor is subjected to necking and extrusion after being pulled through the conduit during the manufacturing of the superconducting cables. Therefore, during the conductor preparation process, subjecting both the jacket and its welded joints to CW and aging procedures is imperative. In this study, five groups of welded joints were prepared, and ten tensile samples of the welded joints were obtained (two tensile samples for each group). The size and test procedure for the tensile samples were the same as those described in Sect. 2.2. According to the requirements of the ITER, the gauge length of the tensile specimen of a welded joint was the sum of twice the thickness of the sample and the width of the weld. The difference was that the original gauge length of the welded joint tensile sample was 10 mm. The YS of the welded joint exceeded 1400 MPa and the elongation exceeded 20% at 4.2 K.

Physical property test

To obtain the basic physical properties of the material, we used a physical property measurement system (Quantum Design Inc., dynacool) to measure the resistivity, specific heat capacity, and thermal conductivity of the CHN01 jacket [32]. A jacket was operated under a strong magnetic field at 4.2 K. The properties of the material were studied at low temperatures and under magnetic fields. Resistivity and thermal expansion test samples with volumes of 8 mm × 2 mm × 2 mm were prepared via wire cutting from the jacket after CW and heat treatment with aging (HT). The resistivity test required ohmic contact between the sample and electrode. The sample size for testing the specific heat capacity was 2 mm × 1 mm × 1 mm, and the mass was 30.4 mg. The sample was affixed to a mounting platform using a thin layer of Apiezon N grease to precisely measure the specific heat capacity. To ensure accurate results, the heat capacities of the sample platform and grease were also determined [33, 34].

Mechanical test results

The results, including the YS, EL, and K_IC, are shown in Fig. 4. The CHSN01 samples (after undergoing CW + HT) exhibit a YS exceeding 1500 MPa and an elongation greater than 25% under 4.2 K test conditions. Forty tensile samples were tested in twenty batches of jackets. Figure 4a shows that the average YS of the CHSN01 jacket is 1560 MPa and the standard deviation is 42.2. The average UTS of the CHSN01 jacket is 1828 MPa. The average EL of all the CHSN01 jacket samples is 32.7% and the standard deviation is 2.4. The strength at the corner position is slightly higher than that at the face position, owing to the uneven force of the jackets during CW. The test results exhibit excellent repeatability and consistency.

Fig. 4Full-screen View

(Color online) Jacket mechanics test results at 4.2 K. (a) The yield strength and elongation of the samples. (b) The yield strength and K_IC of the samples. (c) Test curve of the FCGR. All the curves are for CHSN01 except that of the center solenoid model coil (CSMC), and the serial numbers represent different furnace batches in the legend

The YS shown in Fig. 4b is the average YS of the corner and face positions of each batch of jackets. Twenty fracture toughness samples were tested in twenty batches of jackets. J_IC was acquired and transformed into K_IC(J) according to the guidelines in JIS Z 2284. The relationship between the YS and fracture toughness K_IC(J) of the CHSN01 jacket is shown in Fig. 4b. The average K_IC(J) of all CHSN01 jacket samples is 220 MPa·m^1/2 and the standard deviation is 24.9. The fracture toughness tends to decrease with increasing strength. Noticeably, the fracture toughness of the jackets satisfies the requirement of exceeding 130 MPa·m^1/2 and has a certain safety margin.

The relationship between the rate of fatigue crack propagation da/dN and range of stress intensity factor ΔK in the Paris regime is shown in Fig. 4c for a CHSN01 jacket operating at 4.2 K. The FCGR of the CHSN01 jacket is lower than that of the CFEDR center solenoid model coil (CSMC) jacket (modified 316LN material). When ΔK is less than 35 MPa·m^1/2, the ability of the CHSN01 jacket to prevent crack propagation is very strong, i.e., the threshold value of the crack growth of CHSN01 is high, and the 316LN jacket begins to expand.

The fracture morphology of a jacket tensile specimen was characterized using scanning electron microscopy, and the fracture mode was assessed. As depicted in Fig. 5, micrographs of the fracture surface exhibit a characteristic ductile fracture pattern distinguished by numerous dimples. During low-temperature tensile deformation, deformation twins are produced, which increase the work-hardening rate, delay the occurrence of necking, and increase the strength and elongation of CHSN01.

Fig. 5Full-screen View

(Color online) Fracture morphology of a jacket tensile specimen

The mechanical properties of the mass produced CHSN01 jacket exhibit good stability. The exceptional strength of the jacket is attributed to work hardening during cold deformation in conjunction with meticulous material design. The mechanical performance test results of all the samples meet the technical requirements of the CFEDR. The CHSN01 jacket has a 40% higher YS, comparable plasticity and toughness, and better fatigue resistance compared with a 316LN jacket. The CHSN01 jacket for a CICC meets the requirements of future fusion reactor applications and has great potential. The jacket has high strength, toughness, and plasticity, which are the basis for the safe operation of future devices.

Welding test result

The welding test results, including the YS and EL, are shown in Fig. 6. The welded joint samples of a CHSN01 jacket (after undergoing CW + HT) exhibit a YS exceeding 1400 MPa and an EL exceeding 20% under 4.2 K test conditions. Ten tensile samples were tested. Figure 6 shows that the average YS of the CHSN01 jacket welded joints is 1445 MPa and the standard deviation is 45. The average EL of all the CHSN01 jacket welded joint samples is 22.3% and the standard deviation is 3.1. The strength of a welded joint is 93% of that of the base material. Currently, the preparation of Nb₃Sn CICC conductors is predominantly simulated, and the stability of the welding joint performance has been observed. The consistency of the welded joint performance is an important index to prove the welding quality and evaluate the application of the jacket mass.

Fig. 6Full-screen View

(Color online) Mechanics test results at 4.2 K of the welded joints of a jacket

Physical property test results

Figure 7 shows the thermal expansion (ΔL/L) of CHSN01 steel from 4 K to 300 K. Thermal contraction primarily occurs within the temperature range of 300 K– 100 K, exhibiting minimal thermal shrinkage below 50 K owing to the absence of lattice vibrations. The thermal shrinkage behaviors of CHSN01 and 316LN become nonlinear at low temperatures and tend toward linearity as the temperature is elevated to ambient conditions. The thermal contraction of the 316LN jacket between 300 K and 4 K is approximately 0.3117%, whereas that of the CHSN01 jacket is approximately 0.2726%, indicating a smaller magnitude than that of the former [35]. The thermal contraction of the Nb₃Sn wire between 300 K and 4 K is approximately 0.21% [36]. The shrinkage coefficient of the material is mainly related to its Ni–Cr equivalent ratio. The Ni_eq/Cr_eq ratio of CHSN01 is higher than that of 316LN. In general, the larger the Ni–Cr equivalent ratio, the smaller the thermal shrinkage of the material. For the CICC conductor structure, the thermal shrinkage coefficients of the jacket and superconducting materials are approximately the same, which prevent the influence of thermal stress on the superconducting material. A low coefficient of thermal expansion is more suitable for Nb₃Sn CICC jackets, which can effectively reduce the influence of the thermal shrinkage stress on the performance of superconducting cables.

Fig. 7Full-screen View

(Color online) The thermal expansion of the CHN01 jacket from 4 K to 300 K

Figure 8 shows the resistivity of CHSN01 steel from 4 K to 300 K. Figure 8b shows the relationship between the resistivity and magnetic field of CHSN01 steel at 4.2 K. Two magnetic fields parallel and perpendicular to the sample were tested.

Fig. 8Full-screen View

(Color online) Resistivity test curve of the CHSN01 jacket. (a) The relationship between resistivity and temperature. (b) The relationship between resistivity and magnetic field

The CHSN01 jacket resistivity is 6.7×10^-7 Ω·m at 4.2 K. The resistivity of the CHSN01 jacket decreases gradually with decreasing temperature, and the trend is similar to that of the 316LN jacket. Both CHSN01 and 316LN are austenitic stainless steels and exhibit negligible differences in resistivity.

The background magnetic field strength increases from 0 T to 12 T, and the resistivity of the material does not change significantly. The resistivity of the CHSN01 jacket does not change significantly under a strong magnetic field. No new substances are produced in the steel under extremely low temperatures and strong magnetic fields. The resistivity of the material is not affected by the strength of the magnetic field; therefore, it is suitable for applications in high-magnetic-field devices.

The specific heat capacity of the CHSN01 jacket without a magnetic field and with a backfield strength of 10 T was tested; the test results are shown in Fig. 9. A strong correlation exists between the specific heat capacity and temperature. Similar to most metallic materials, the specific heat capacity decreases with decreasing temperature. In the temperature range from 50 K to 4 K, the slope of the curve gradually decreases, thereby indicating that the rate of change of specific heat capacity with temperature decreases. The test curves with and without background magnetic field coincide. The specific heat capacity of the CHSN01 material is not affected by the magnetic field. At 4 K, the CHSN01 material specific heat capacity is 2.0 J/(kg·K).

Fig. 9Full-screen View

(Color online) The relation of the specific heat capacity with temperature

Figure 10 shows the relationship between the thermal conductivity and temperature of the CHSN01 material. In the temperature range of 350 K–100 K, the thermal conductivity decreases with decreasing temperature, but the decrease is not large. Thermal conductivity tests were conducted at the Institute of Solid-State Physics, Chinese Academy of Sciences. The obtained results for the thermal conductivity without a background magnetic field are in substantial agreement with those previously reported by the Technical Institute of Physics and Chemistry [33]. The thermal conductivity of CHSN01 is not affected by the magnetic field.

Fig. 10Full-screen View

(Color online) The relation of thermal conductivity with temperature

For the design of CICC superconducting magnets, the resistivity, specific heat capacity, and thermal conductivity of the jacket are the key input parameters. Comprehensive testing of the physical properties of a CHSN01 jacket, including resistivity, thermal shrinkage coefficient, specific heat capacity, and thermal conductivity, was performed over a temperature range from 4 K to 300 K. Remarkably, the magnetic field had no discernible impact on these physical properties, thereby rendering the material suitable for deployment in environments characterized by low temperatures and strong magnetic fields. Fundamental data pertaining to the physical properties of CHSN01 can serve as valuable reference information for its application in low-temperature research.