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Review

Development of Cryogenic Structural Steels for Magnetic Confinement Fusion

by
Jingjing Dai
1,2 and
Chuanjun Huang
2,*
1
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
*
Author to whom correspondence should be addressed.
Cryo 2025, 1(4), 13; https://doi.org/10.3390/cryo1040013
Submission received: 9 July 2025 / Revised: 18 September 2025 / Accepted: 11 October 2025 / Published: 30 October 2025
(This article belongs to the Special Issue Low Temperature Physics, Pioneer of Chinese Low Temperature Physics and Cryogenics—In Memory of Prof. Dr. Chaosheng Hong (C.S. Hung))
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Abstract

With the growth in global energy demand and increasing concern over the environmental issues associated with fossil fuels, magnetic confinement fusion (MCF) has gained widespread attention as a clean and sustainable energy solution. The superconducting magnet systems in MCF devices operate under liquid helium temperature of 4.2 K and strong magnetic fields, requiring structural materials to possess exceptional high strength, high toughness, and non-magnetic properties. This paper reviews recent research advances in cryogenic high-strength and high-toughness austenitic stainless steels (ASSs) for MCF devices, focusing on modified grades like 316LN and JK2LB used in the International Thermonuclear Experimental Reactor (ITER) project, as well as China’s CHN01 steel developed for the China Fusion Engineering Test Reactor (CFETR) project. The mechanical properties at 4.2 K (including yield strength (Rp0.2), fracture toughness (K(J)Ic), and elongation at fracture (e)), microstructural evolutions, weldability, and manufacturing challenges of these materials are systematically analyzed. Finally, the different technical approaches and achievements in material development among Japan, the United States, and China are compared, the current limitations of these materials in terms of weld integrity and manufacturability are discussed, and future research directions are outlined.

1. Introduction

The global energy crisis driven by population growth and industrialization has exposed the limitations of fossil fuels, namely, volatile supply chains and catastrophic greenhouse gas emissions [1]. Fusion energy emerges as an alternative solution by fusing deuterium (D) and tritium (T) to helium (He) and a neutron at extremely high temperatures, offering a clean and virtually waste-free energy source [2].
Primary terrestrial approaches for controlled fusion are inertial confinement (laser-driven pellet compression) and magnetic confinement (using magnetic fields). Among these, magnetic confinement, particularly the tokamak design, represents the most mature and dominant pathway towards practical fusion power. Invented in the 1950s at the Kurchatov Institute, the tokamak is a toroidal device whose key enabling systems for plasma confinement include a vacuum chamber, a magnet system, and the structural supports that must withstand immense electromagnetic forces [3]. Tokamak devices create the conditions for fusion reactions by controlling high-temperature plasma through a powerful magnetic confinement system. The International Thermonuclear Experimental Reactor (ITER), as a collaborative effort among the Europe, the United States, Russia, Japan, China, South Korea, and India, aims to prove the feasibility of fusion for large-scale energy production. Parallel breakthroughs include the Toroidal Fusion Test Reactor (TFTR) in the US, which pioneered plasma confinement techniques [4]. Japan demonstration (JA-DEMO) in Japan and K-DEMO in South Korea target commercial reactor designs [5,6]. The Experimental Advanced Superconducting Tokamak (EAST) in China achieved a world-record high confinement plasma operation with a 403-s steady state in 2023 [7,8]. Notably, the China Fusion Engineering Test Reactor (CFETR) bridges experimental goals of the ITER with the commercialization roadmap of DEMO.
The ITER magnet system integrates four magnet systems, comprising Central Solenoid (CS) coils, which generate inductive current for plasma initiation, Toroidal Field (TF) Coils which confine plasma through a helical magnetic field, and Polar Field (PF) and Correction (CC) Coils, which finely tune plasma positioning and stability. These coils demand extreme currents up to 95.6 kA for TF coils and 60 kA for CS coils and magnetic fields as high as 17 T and 14.5 T in advanced devices like the CFETR to achieve fusion-grade plasma parameters [9,10]. Such conditions necessitate superconducting materials (Nb3Sn and NbTi) to eliminate resistive losses and enable ultra-high current densities. The chosen conductor architecture is the Cable-In-Conduit Conductor (CICC), wherein the superconducting cables are housed within a structural conduit and cooled by supercritical helium to maintain cryogenic stability. As illustrated in Figure 1, the CICCs used in different coils exhibited significant variations in terms of dimensions, cross-sectional configurations, and the layout of superconducting cable bundles. These structural variations directly influence the electromagnetic forces imposed on the ASS jackets, the manufacturing complexities, and the mechanical performance requirements. Consequently, a range of high-performance steels has been developed to meet these specific demands.
Austenitic stainless steels (ASSs), renowned for their excellent properties including high strength, high toughness, and non-magnetic at cryogenic temperatures, have become crucial structural materials for jackets of the CICCs and cases of coils in many large-scale and high-field tokamaks. However, traditional alloys like Fe-Mn-Cr and Fe-Ni-Cr steels faced a severe challenge posed by the requirement of withstanding huge electromagnetic forces while retaining sufficient ductility at 4.2 K [11]. Japan pioneered Fe-Mn-Cr alloys by optimizing Cr and Ni content (such as 25Cr-13Ni-0.4N and 27Cr-18Ni-0.35N), successfully enhancing nitrogen solubility for solid-solution strengthening while suppressing embrittlement [12]. The ITER has established the baseline for cryogenic structural steels; however, next-generation MCF devices demand even higher strength-toughness combinations and enhanced resistance to cyclic loading. Building on advancements that have already been made, China has initiated a program since 2015 to develop next-generation ASS for next-generation MCF projects, and a landmark achievement was the CHN01 steel, engineered by the Technical Institute of Physics and Chemistry (TIPC, CAS) in collaboration with the Hefei Institute of Physical Science (HFIPS, CAS). Unique composition of the CHN01 steel combining refined Cr/Ni ratios and nitrogen microalloying enabled outstanding cryogenic performance with Rp0.2 exceeding 1500 MPa and K(J)Ic more than 200 MPa·m1/2 at 4.2 K. The CHN01 steel was officially selected for the superconducting magnet system of the Burning Experimental Superconducting Tokamak (BEST) in 2022.
This paper comprehensively reviews recent advancements in cryogenic high-strength and high-toughness ASSs, which are critical for MCF devices. We highlight the important role of ASSs in superconducting magnets for the ITER, the CFETR, and the JA-DEMO, including the modified 316LN, JK2-Low C-B-added (JK2LB), JJ1, and the CHN01. Key discussions include mechanical performance under extreme conditions, phase stability, and fabrication challenges, alongside comparative analyses of international R&D efforts in Japan, the United States, and China. This paper underscores the significance of advanced structural ASSs in enabling sustainable fusion energy, and provides a reference for future research to bridge gaps between laboratory-scale achievements and industrial- scale for future commercial fusion reactor demands.

2. Austenitic Stainless Steels for the ITER Superconducting Magnets

2.1. Jackets of the CICCs of Coils

The ITER magnets consist of 18 TF coils, 6 individually powered CS coils, 6 PF coils, and 18 CC coils [13]. The distribution of manufacturing responsibilities for jackets of CICC of coils in the ITER coil is detailed in Table 1. China was responsible for 7.51% of jackets of the TF coils, 67% of jackets of the PF coils (PF 2/3/4/5), and 100% of jackets of the CC coils. The ITER project has developed a variety of cryogenic structural steels, among which the modified 316LN, JK2LB, modified 316L, and 316L steel were selected as the jackets of the CICCs of the TF coils, the CS coils, the PF coils, and the CC coils, respectively. Their chemical compositions and mechanical properties are displayed in Table 2 and Table 3. It should be noted that while the official ITER specification sets a maximum cobalt content limit of 0.1 wt.% [14], the actual levels in produced steels are typically maintained within a much stricter range of 0.01 to 0.06 wt.% [15]. A comparative analysis of the mechanical properties in Table 3 reveals that all ITER-grade steels meet or significantly exceed the project’s specifications at 4.2 K. For instance, the JK2LB (CS coils) achieves an exceptional KIc of 209 MPa·m1/2, far above the requirement of 130 MPa·m1/2. Notably, the newly developed CHN01 steel targeting the CFETR, demonstrates superior performance with a yield strength of 1600 MPa and K(J)Ic of 265 MPa·m1/2, setting a new benchmark for next-generation reactors.
The modified 316LN austenitic stainless steel has been adopted for the jackets of CICCs of TF coils due to its exceptional mechanical performance at cryogenic temperatures and non-magnetic properties [16]. Extensive studies [17,18,19,20,21] on sub-sized and full-scale samples confirm that its mechanical performance at 4.2 K including a Rp0.2 exceeding 950 MPa, e greater than 20%, and KIc over 150 MPa·m1/2, meets ITER’s specifications even after compaction and aging treatments. The high nitrogen and low carbon content in modified 316LN significantly enhanced Rp0.2 and Rm for metastable ASS without significant impact on the total e at cryogenic temperatures [22]. In general, increased cold working elevated the Rp0.2 at 4.2 K of both base material and weldments but reduced e [23,24,25]. Conversely, those subjected to cold working and aging treatment exhibited an improvement in e from 37% to 41% and from 23% to 38%, respectively [26,27]. Short-duration Nb3Sn reaction heat treatments (625 °C for ≤100 h) showed no detrimental effect on the cryogenic mechanical properties of modified 316LN. In contrast, prolonged aging treatments (625 °C for 696 h) have been found to be able to further improve strength [18,20,28]. A significant challenge arose from the ultrathin thickness of jackets (~2 mm), which prevented standardized KIc and fatigue crack growth rate (FCGR) tests. Researchers replaced KIc with a ductility-based criterion (e more than 20% below 7 K) and evaluated grain boundary sensitization through fracture surface analysis [29]. While the ductility-based criterion provided a practical workaround for quality control, it remains an indirect and potentially less conservative measure of fracture resistance compared to direct KIc measurement. The critical gap between standardized evaluations and real-world constraints of thin-walled jackets could be solved by more sophisticated and non-destructive evaluation techniques (such as Acoustic Emission [30]). These practical challenges have intensified research into the microstructural origins of fracture in modified 316LN. However, clear contradictions existed in the literature: some studies using EBSD and TEM confirmed the presence of strain-induced γ → α′ martensitic transformation at cryogenic temperatures and high deformation levels [19,21,31,32], while others, employing similar techniques, reported no martensite formation and attributed deformation structures solely to dense dislocation networks and stacking faults [17,33,34,35,36,37]. Notably, magnetic data have suggested that the amount of α′-martensite decreased in a linearly proportional manner from the fracture origin to the root on the parallel section [25,38,39]. This obvious discrepancy cannot be dismissed as mere observational error and suggests that subtle differences in material processing, thermomechanical history, or local compositional gradients may critically influence the deformation mechanism of modified 316LN. Resolving this inconsistency is essential for a predictive understanding of fracture behavior and requires systematic reinvestigation under carefully controlled and documented conditions.
Table 1. Production proportions of each country for coils in the ITER (Data from [40]).
Table 1. Production proportions of each country for coils in the ITER (Data from [40]).
TFCSPF1&6PF2–5CC
Assignment
proportion
(%)		CN7.5 100100
EU20.2 39.8
JA25.0 60.2
KO20.2
RF19.3
US7.8100
Table 2. Chemical compositions (wt%) and size of jackets of the CICCs of the ITER (Data from [14,15]).
Table 2. Chemical compositions (wt%) and size of jackets of the CICCs of the ITER (Data from [14,15]).
TFCSPF1&6PF2–5CC
Material typesModified 316LNJK2LBModified 316LAISI 316L
Chemical
compositions
(wt%)		C<0.015<0.02≤0.03≤0.03
Si<0.75<0.28≤0.75≤0.75
Mn<2.020.50–22.50≤2.00≤2.00
P<0.04<0.008≤0.03≤0.045
S<0.03<0.008≤0.01≤0.030
Cr16.0–18.012.00–14.0016.00–18.5016.0–18.0
Ni11.0–14.08.0–10.011.0–14.010.0–14.0
Mo2.0–3.00.5–1.52.00–2.502.00–3.00
N0.14–0.180.09–0.15 0.1–0.16
B 0.001–0.004
Co<0.1<0.1≤0.1
Coil shapesCircleCircle in squareCircle in squareSquare
Coil sizes
(mm)		Outer diameter43.749.053.851.919.2
Inner diameter39.732.637.735.314.8
Table 3. Mechanical properties of jackets of CICCs of the coils in the ITER and CFETR at 4.2 K (Data from [15,41,42]) specification (real value).
Table 3. Mechanical properties of jackets of CICCs of the coils in the ITER and CFETR at 4.2 K (Data from [15,41,42]) specification (real value).
Modified 316LNJK2LBModified 316L316LCHN01
Yield strength (Rp0.2) (MPa)4.2 K>950 (1165)>850 (1076)>700 (797)>700 (813)>1500 (1600)
300 K(530)>700 (800)>173 (494)>173 (512)547
Tensile strength
(Rm) (MPa)		4.2 K(1537)>1150 (1450)>1000 (1525)>1000 (1582)>1800 (1930)
300 K(746)>1000 (1550)>483 (626)>483 (615)>480 (776)
Young’s modulus
(E) (GPa)		4.2 K(202)(201)>205 (209)(202)(216)
300 K(195)>180 (192)>190 (193)>190 (194)(184)
Elongation
(e) (%)		4.2 K>20 (34)>25 (43)>40 (45)>30 (39)>25 (34)
300 K(38)>30 (44)>35 (51)>35 (45)>35 (50)
Fracture toughness (K(J)Ic) (MPa·m1/2)4.2 K>150 (300)>130 (209)>150 (278) >130 (265)
Pre-test processingCompaction 3.5%; Tensile 2.5%; 650 °C (200 h)Compaction 3.5%; Tensile 2.5%;
650 °C (200 h)		Compaction 5%; Bending (radius 2 m); StraighteningCompaction 3.5%;
Tension 2.5%		Compaction 5%; Bending (radius 2 m); Straightening; 665 °C (50 h)
The CS coils critical for plasma current induction and shaping in tokamaks demand jackets that can balance cryogenic strength, fatigue resistance, and dimensional stability under extreme cyclic loading. In response to these stringent demands, the Japan Atomic Energy Agency (JAEA) and Kobe Steel developed JK2LB steel, a low-carbon, low-boron, nitrogen-strengthened ASS (0.03C-22Mn-13Cr-9Ni-1Mo-0.24N-B), as a superior alternative to conventional 316LN steel. Its reduced thermal shrinkage (0.21% vs. 0.29% for 316LN) provides critical thermal compatibility by minimizing compressive stresses on the brittle Nb3Sn strands during cooldown and operation [43,44,45,46]. The mechanical performance of JK2LB at 4.2 K is profoundly influenced by its thermal-mechanical processing history. While cold working prior to heat treatment could enhance subsequent precipitation hardening (18.5% cold working increasing Rp0.2 from 1007 MPa to 1442 MPa but reducing KIc from 144.1 MPa·m1/2 to 52.3 MPa·m1/2 [47,48]), specific optimized heat treatments (570 °C for 250 h and 650 °C for 100 h) have been developed to maintain high ductility (e > 30%) without decreasing strength [49]. This tailored approach was underpinned by microstructural optimizations, including reduced carbon (<0.03%) and controlled nitrogen (~0.2%) content, which suppressed detrimental chromium carbide (Cr23C6) precipitation and mitigated aging-induced embrittlement [50,51]. As a result, even after compaction and aging treatments, JK2LB maintains a Rp0.2 exceeding 850 MPa and KIc above 130 MPa·m1/2 [47,49,50,51,52,53,54,55]. Moreover, JK2LB ensured functional reliability through its antiferromagnetic behavior below 240 K, which guaranteed compatibility with high superconducting fields. JAEA implemented a rigorous non-destructive examination (NDE) regime for jackets of CS coils to ensure the integrity of the manufactured jackets against even minuscule defects that could initiate fatigue cracks under 60,000 operational cycles [56]. NDE system employs two complementary techniques: Eddy Current Testing (ECT) for detecting surface flaws on both the inner and outer jacket surfaces, and Phased Array Ultrasonic Testing (PAUT) for identifying embedded volumetric defects. The inspection criteria are based on a linear elastic fracture mechanics (LEFM) analysis, setting the maximum allowable defect size at 2 mm2 for surface flaws and between 5 and 7 mm2 for internal defects, depending on their location. NDE has been successfully validated on over a thousand jacket sections, demonstrating the capability to reliably detect critical flaws and ensuring the functional reliability of every production unit [57]. Together, these excellent thermal and mechanical properties establish JK2LB as a comprehensively engineered ASS capable of meeting the integrated demands of CS coil operation.
The PF magnet system consists of six poloidal coils from PF1 to PF6, which are connected to the outside of the TF coil box by flexible plates or sliding supports to stabilize the positional equilibrium of the plasma current and the vertical stability of the plasma [58]. For the critical PF2 to PF5 coils, a modified 316L steel was selected for the CICC jackets due to its balanced mechanical properties. To accurately simulate in-service conditions, the modified 316L steel underwent rigorous processing, including compaction, bending, and straightening, prior to comprehensive mechanical evaluation. Performance assessments across a range of temperatures (4.2 K, 77 K, and 300 K) confirmed that the modified 316L steel met ITER’s stringent requirements for tensile strength, impact toughness, and fatigue resistance [59,60,61]. Detailed investigation revealed that a 5.58% compaction treatment induced a strain-driven γ→α’ martensitic transformation, which enhanced Rp0.2 by 10% and improved impact toughness at 4.2 K. However, the strengthening mechanism came at the expense of ductility, with e reduced by 4% due to concurrent dislocation slipping and stacking fault formation [60,61]. Both the PF and CS coil jackets employed a combined strategy of Phased Array Ultrasonic Testing (PAUT) and Eddy Current Testing (ECT) for NDE. However, a specialized NDE procedure has been developed for the modified 316L jackets of the PF coils, accounting for their complex circular-in-square cross-section. The PAUT method utilized four different probes and performed 11 separate scans over each jacket surface to reliably identify internal volumetric defects, while multi-frequency ECT was specifically employed to detect transverse inner surface flaws with depths of up to 2–3 mm. The NDE for modified 316L jackets has achieved a high acceptance rate of 96.3%, providing essential quality assurance for conductor assembly. Note that the allowable defect sizes are larger (10 mm2 for surface defects and 20 mm2 for embedded defects at the corners) compared to those specified for the JK2LB jackets [62]. The NDE methodology can be extended and refined for next-generation MCF devices (CFETR), whose TF coils will face even greater electromagnetic loads and require higher-performance materials (CHN01 steel). The experience gained in inspecting complex geometries and qualifying stringent acceptance criteria is directly applicable to developing the inspection protocols for CFETR’s larger and more robust conduits, ensuring their reliability under more extreme operational conditions. Notably, the magnetic permeability, a critical parameter for non-magnetic fusion components, remained uncharacterized in these studies. Given that the formation of ferromagnetic α’-martensite could compromise the material’s non-magnetic performance, future research must comprehensively evaluate its magnetic characteristics to ensure full compliance with fusion application standards.
The CC coils comprise six Top Correction Coils (TCCs), six Bottom Correction Coils (BCCs), and six Side Correction Coils (SCCs) to compensate for field errors arising from manufacturing deviations, assembly tolerances, and winding distortions [63]. To ensure mechanical reliability under cyclic electromagnetic loads in this critical system, 316L steel was selected as the jacket material for the Cable-In-Conduit Conductors (CICCs) owing to its outstanding cryogenic performance, which includes a Rp0.2 exceeding 700 MPa and e greater than 35% at 4.2 K [64,65]. High strength coupled with exceptional ductility provides a critical foundation for resisting fatigue cracking in high-field environments. The high strength enables the material to withstand the significant cyclic stresses inherent to coil operation, while the superior ductility allows it to absorb deformation energy and mitigate crack initiation and propagation. However, while these tensile properties are strong indicators of fatigue resistance, the direct measurement of fatigue life under the relevant stress ranges (300–600 MPa) remains an essential objective for future validation to fully confirm the 316L’s performance for long-term operation.

2.2. Cases of the TF and CC Coils

For the cases of TF and CC coils of the ITER, the production tasks have been entirely undertaken by Japan and China, respectively. JJI and 316LN steel have been developed and utilized as materials for the coil cases, whose chemical compositions and mechanical properties are presented in Table 4 and Table 5. The compositional difference between JJ1 (TF coils) and 316LN (CC coils) was revealed in Table 4, which was referred to as high-Mn, high-Mo and high-N grade (9–11% Mn, 4–6% Mo, 0.21–0.27% N) were designed for JJ1.
The cases of the TF coils, designed to mechanically protect TF superconducting windings [66], endure severe in-plane static and out-of-plane cyclic stresses, particularly at the inward bending regions during Tokamak operation [67]. Material selection for this critical component prioritizes high strength, fatigue resistance, and weldability at 4.2 K, with Quantum Science and Technology (QST) recommending JJ1 steel for high-stress zones and graded 316LN variants including 316LNH (C + N ≥ 0.18%), 316LNM (C + N ≥ 0.13%), and 316LNL (C + N ≥ 0.08%) for moderate to low stress areas [68]. <JSME Codes for Fusion Facilities> was established for JJ1 and 316LN variants to reduce the high cost of cryogenic testing, mainly containing prediction curves for Rp0.2 and Rm at 4 K based on the (C + N) content and mechanical properties at 300 K [69,70]. Over 2400 materials were tested and registered, validating the approach despite an inherent scatter of ±10% in predictions. A detailed assessment of the reasons for the scattered values and how the values can be improved is a subject for a future study. However, the mechanical performance of its weld was constrained by geometric size sensitivity (miniaturized specimens showed crooked crack propagation) and welding-induced embrittlement. Narrow-gap tungsten inert gas welding (NG-TIGW) with a specially matched filler metal (Table 4) has been employed to enhance joint uniformity and strength by reducing inclusions [71]. Further studies showed that EBW was applicable but limited to 40 mm thickness, with vertical upward EB welding being the preferred method [72,73]. Nevertheless, aging treatments were found to promote detrimental carbide (M23C6) and nitride (Cr2N) precipitation, underscoring the need for careful thermal process control to avoid intergranular embrittlement [74,75]. In contrast, the 316LN variants demonstrated superior ductility (e >50%) critical for components under cyclic electromagnetic loads. And their welded joints exhibited sufficient safety margins under cyclic loading, as reflected in S-N curve performance [76,77]. This presents a fundamental design choice: JJ1 offers superior base metal toughness but requires meticulous control over welding and heat treatment, while 316LN variants provide more predictable weldability and reliability.
Table 4. Production countries and chemical compositions of TF and CC cases in the ITER (wt%) (Data from [69,75,76,78]).
Table 4. Production countries and chemical compositions of TF and CC cases in the ITER (wt%) (Data from [69,75,76,78]).
CASE of TF CASE of CC
Assignment proportions (%)CN 100%
JA100%
Material typesJJ1Filler material316LNH316LNM316LNL316LNFiller material
Chemical compositionsC<0.030.016<0.03<0.03<0.03<0.030.018
Si<0.750.52<0.75<0.75<0.75<1.00.46
Mn9.0–11.010.17<2.0<2.0<2.0<2.07.01
P<0.0350.004<0.03<0.03<0.03<0.040.017
S<0.0150.004<0.02<0.02<0.02<0.030.003
Cr11.0–13.012.2716.0–18.516.0–18.516.0–18.516.0–18.020.59
Ni11.0–13.014.1610.0–14.010.0–14.010.0–14.010.0–14.015.37
Mo4.0–6.04.972.0–3.02.0–3.02.0–3.02.00–3.002.83
N0.21–0.270.1620.17–0.220.12–0.170.10–0.12<0.180.156
Table 5. Mechanical properties of cases of TF and CC cases in the ITER (Data from [68,76,79,80]) specification (real value).
Table 5. Mechanical properties of cases of TF and CC cases in the ITER (Data from [68,76,79,80]) specification (real value).
JJ1316LNH316LNM316LNL316LN
Yield strength (Rp0.2) (MPa)4.2 K>1000 (1126)>900 (1093)>700 (868)>500 (713)>700 (1080)
300 K(330)(355)(289)(263)>250 (318)
Tensile strength
(Rm) (MPa)		4.2 K(1319)(1444)(1348)(1313)>1000 (1430)
300 K(665)(666)(590)(557)>480 (642)
Elongation
(e) (%)		4.2 K(42) >35
300 K(65) >40
Fracture toughness (K(J)Ic) (MPa·m1/2)4.2 K>200 (366) >200>180 (213)
The main characteristics of cases of the CC coils are compact cross-section (~240 mm × 147 mm), large overall dimensions (~7 m), and complex geometry (60° bend angle with an 8 m radius), which are used to strengthen the windings and thus to reduce the stress and deformation induced by electromagnetic loads [79,81]. The cases of the coils are constructed from 20-mm-thick 316LN ASS, chosen for its exceptional cryogenic strength and ductility [80]. The 316LN ASS must comply with rigorous cryogenic standards, including the Rp0.2 at 4.2 K, which should be more than 700 MPa, the Rm should be more than 1350 MPa, the e should be more than 40%, the KIc should be more than 180MPa·m1/2, and the fatigue strength under 3000 cycles must be more than 500 MPa. Additionally, the welds of cases should satisfy that the Rp0.2 at 300 K should be more than 250 MPa, the Rm should be more than 480 MPa, and the KIc should be more than 180 MPa·m1/2 [82,83]. Due to the strict manufacturing tolerances, tungsten inert gas (TIG) welding was employed for sub-assembly, while laser beam welding (LBW) was used for the final enclosure welding. This combination ensures full penetration and minimizes defects [84]. Due to the strict manufacturing tolerances, tungsten inert gas (TIG) welding was employed for sub-assembly, while laser beam welding (LBW) was used for the final enclosure welding. This combination ensures full penetration and minimizes defects [67]. The Rm of TIG and LBW weldments reached 1555 MPa and 1522 MPa at 4.2 K, respectively, representing 99.7% and 97.6% of the base material’s strength. Both joints exhibit excellent ductility, with e exceeding 35%, and high K(J)Ic values of 210 and 241 MPa·m1/2 [78,85,86]. The assembly process involves fabricating two L-shaped sections for the Side Correction Coil (SCC) and a U-shaped base with a planar cover for the Bottom/Top Correction Coils (B/TCC) [87]. The geometric complexity necessitates precise alignment to avoid residual stress concentrations.
The outstanding mechanical properties of these structure ASSs for fusion are intrinsically governed by their microstructural evolution under 4.2 K. The deformation mechanisms of face-centered cubic (FCC) metals are strongly influenced by their stacking fault energy (SFE), which depends strongly on deformation temperature and chemical compositions. As the temperature decreases from 300 K to cryogenic temperatures, the SFE is significantly reduced to activate nano-sized twinning and strain-induced martensitic transformation. Mechanical twinning tends to dominate within an intermediate SFE range (20 < SFE < 45 mJ·m−2). When SFE falls below 20 mJ·m−2, martensite formation (γ→ε and γ→α′) becomes the preferred deformation mechanism.; when SFE exceeds 45 mJ·m−2, plastic deformation is controlled predominantly by dislocation glide [88,89]. For metastable austenitic alloys (modified 316L and 316LN), the strain-induced transformation of γ-austenite to α′-martensite provides a critical strengthening mechanism, though often causing an increase in magnetic permeability. In advanced nitrogen-alloyed steels (JK2LB and CHN01), high nitrogen content significantly increases phase stability by elevating SFE thereby preventing martensitic transformation. Dislocation and twinning formation are promoted ultimately to an exceptional combination of high strength and fracture toughness at cryogenic temperatures. Furthermore, the formation of nano-twins and phase boundaries strengthens the material by impeding dislocation motion, resulting in high strain hardening rates (SHR) and simultaneously enhances both strength and ductility, a phenomenon known as the ‘‘dynamic Hall-Petch’’ effect [90]. These microstructural adaptations are essential for maintaining performance under the extreme operational conditions encountered in fusion magnet systems.

3. Historical Development of Cryogenic Structural Steels in Japan and the United States

3.1. Japan

In the 1980s, the Japan Atomic Energy Research Institute (JAERI) spearheaded the development of cryogenic structural steels for high-field superconducting magnets for the fusion devices. Domestic steelmakers (Nippon and Kobe Steel) achieved an unprecedented yield strength–fracture toughness balance (Rp0.2 ≥ 1200 MPa, K(J)Ic ≥ 200 MPa·m1/2), known as the “JAERI-BOX” criterion, as shown in Figure 2 [91,92], which surpassed the limits of conventional AISI 300 series austenitic stainless steels. Through rigorous screening of over 80 candidate materials, including Ferritic Steels (FSs), High-Mn Austenitic Steels (HMASs), and others, as shown in Table 6, the Japanese Cryogenic Steels (JCS) series emerged [93], mainly high-Mn and nitrogen-strengthened CSUS-JN1/JK2 (22~24% Cr, 13~15% Ni, 0.2~0.3% N), optimized for higher Rp0.2 and Rm via solid-solution hardening, and low-Cr/Ni CSUS-JJ1 (12Cr-12Ni-5Mo-10Mn-0.2N), which excels in weldability and phase stability. Their chemical compositions are shown in Table 7. The JK2LB and JJ1 steels were ultimately selected for jackets of CICCs of the CS coils and cases of the TF coils in the ITER project due to their balanced cryogenic performance. Japan established a joint special design team in 2015 to enhance the coordination of JA-DEMO related research and development.

3.2. The United States of America

The Massachusetts Institute of Technology (MIT) and International Nickel Corporation (INC) jointly developed an age-hardened nickel-based alloy, INCOLOY 908 [94], whose chemical compositions and mechanical properties of the INCOLOY 908 could be found in Table 8 and Table 9. INCOLOY 908 showed excellent mechanical properties at 4.2 K: Rp0.2 increased to 1200 MPa and KIc increased to 200 MPa·m1/2; however, FCGR was faster than that of modified 316LN steel [95]. The thermal shrinkage of the INCOLOY 908 alloy from 923 K down to 4.2 K is 1.1% (close to 0.7% for Nb3Sn), which allows for higher pre-strain in Nb3Sn strands; however, the INCOLOY 908 alloy carries the risk of stress-accelerated grain boundary oxidation (SAGBO) during heat treatment, and ultimately the ITER project abandoned the application of the INCOLOY 908 in the jacket of CICCs of the CS coils. The problem of SAGBO was avoided by vacuum heat treatment when applied to the jacket of CICCs of the CS coils in the KSTAR [96,97].

4. Recent Advances in Austenitic Stainless Steels for Next-Generation MCF Devices

4.1. Japan

Japan proposed a conceptual design for a fusion demonstration reactor (JA-DEMO) in 2014, aiming to achieve stable power generation (fusion power (Pfus ~ 1.5 GW) and validate commercial viability by the 2030s [99]. Transitioning to the engineering design phase around 2025, JA DEMO sets significantly higher technical benchmarks compared to the ITER, including larger aperture and enhanced magnetic field of the TF coils, as shown in Table 10. These demands place higher requirements on the cryogenic mechanical properties of structural steels in the JA-DEMO (Rp0.2 more than 1600 MPa and K(J)Ic more than 120 MPa·m1/2 at 4.2 K) [100]. Japan is advancing next-generation nitrogen-strengthened austenitic steels, focusing on improving cryogenic strength.
Two advanced high-strength steels for JA-DEMO have been developed in Japan, i.e., an XM-19-derived Nb-containing material and a JN1-derived Nb-free material (chemical composition in Table 7) [101]. The ITER restricted Nb content to less than 0.05% due to irradiation embrittlement risks while Nb in XM-19 derivatives enhanced Rp0.2 through grain refinement. Fracture toughness evaluations revealed that the target of the JA DEMO aligns with a trend parallel to the reference line of the NIST, intersecting JN1’s data points at Rp0.2 more than 1200 MPa and KIc more than 200 MPa·m1/2 [101,102,103]. The chemical composition of the JN1 steel could be found in Table 6. Specimen No.1-3 can satisfy the performance target of the JA DEMO but exhibits unstable fracture risks as cracks terminate near specimen edges with minimal ligament retention. Consequently, the JA DEMO excludes Nb-containing steels for jackets of CICCs of TF coils, prioritizing Nb-free materials like JN1 derivatives to achieve high toughness (K(J)Ic > 120 MPa·m1/2) [104]. Japan’s prioritization of Nb-free steels for JA-DEMO, despite the attractive strength offered by Nb-containing grades, highlights a pivotal principle in fusion material development: prominent toughness is often more critical than marginal gains in strength. Any failure in a fusion magnet, often initiated from a microstructurally unstable region, can have catastrophic consequences, making reliability the paramount design criterion.

4.2. China

To bridge the gap between the ITER and the commercial fusion power plant, China plans to launch the CFETR project, aiming to establish the scientific and technological foundation for a fusion power plant prototype [1]. The primary objectives of CFETR include: (1) achieving a fusion power output of 50–200 MW; (2) sustaining effective full-power operation for 30–50% of the annual duty cycle; and (3) realizing tritium self-sufficiency through advanced neutron multiplication technologies in the breeding blanket [105,106]. To surpass the operational stability and power output of the ITER, the CFETR requires stronger magnetic fields (exceeding 13 T of the ITER) to confine high-energy plasmas, which imposes extreme mechanical demands on cryogenic structural steels. Specifically, the superconducting coils generate colossal cyclic electromagnetic forces, necessitating steels with Rp0.2 > 1350 MPa, Rm > 1650 MPa, e > 20% and K(J)Ic > 130 MPa·m1/2 at 4.2 K [107]. The CHN01 (modified N50), a next-generation ultra-cryogenic austenitic stainless steel optimized for high strength and phase stability, has been developed [95]. This material exemplifies China’s strategy to advance fusion engineering through tailored alloy design and rigorous performance validation.
Nitrogen-strengthened austenitic stainless steels were originally developed by Armco Steel at the beginning of the 21st century, and one of the most widely used nitro alloys is N50 (Nitronic 50, XM19, UNS S20910, 1.3964), which is a non-magnetic, corrosion-resistant, high-strength, and weldable (to a certain degree) nitrogen-strengthened austenitic stainless steel [108,109]. The N50 steel has been used in the form of large forgings of approximately 16 m in length for key connecting plate components of the ITER CS system [110], as shown in Figure 3. Figure 3 clearly demonstrates the full spectrum application potential of N50 and CHN01 steel, ranging from large-scale forgings (~16 m, Figure 3a) to medium-thickness coil boxes (~4.7 m, Figure 3b), and finally to ultra-thin jackets (~3 mm, Figure 3c), highlighting its versatility as the main structural material for the next-generation MCF devices. Initially, the N50 steel was used as a structural material without aging treatment and was strengthened with high carbon (0.06% max) and nitrogen (0.2–0.4%) contents. As a result, several problems arose during production including forging cracking during hot rolling and forging, carbide precipitation, and poor toughness.
The CHN01 steel was then developed domestically by completely removing δ-ferrite and strictly controlling the carbon (max. 0.01%) and oxygen (max. 20 ppm) content [85,86]. Moreover, the content of nickel has been increased compared to that of the N50. Chemical compositions of the CHN01 and the N50 are shown in Table 11. These modifications made the CHN01 significantly different from the N50, characterized by excellent mechanical properties with the Rp0.2 of the CHN01 jacket after cold working and aging being about 1600 MPa, the e was more than 29%, and the K(J)Ic was more than 200 MPa·m1/2 [42]. The CHN01 steel has been selected as a potential candidate for jackets of the CICCs of the TF and CS coils in the CFETR [95]. Further refinements, B microalloying (10 ~ 40 ppm) and δ-ferrite removal encouraged K(J)Ic by refining grain boundaries [111,112,113], while thermal deformation studies (950~1250 °C) revealed a transition from continuous to discontinuous DRX, guiding optimized hot-working processes [114]. The compatibility with ReBCO and Nb3Sn conductors of the CHN01 jacket has been validated for the TF and CS coils in the CFETR [95,115].
However, the performance of CHN01 welded joints remained a critical challenge for the application of Nb3Sn conductors despite the outstanding properties of the BM. The performance requirements for CHN01 welded joints in the CFETR are specified as follows: (1) a Rp0.2 > 1500 MPa and an e > 20%; (2) a K(J)Ic > 150 MPa·m1/2 at 4.2 K. The Institute of Plasma Physics (ASIPP) employed tungsten inert gas welding (TIGW) to join CHN01 BM using CHN01J filler material [42]. Subsequent cryogenic tensile and fracture tests revealed that the CHN01 welded joints after Nb3Sn ageing treatment exhibited degradation of Rp0.2, e, and K(J)Ic compared with that of CHN01 BM, as shown in Table 12, which could be attributed to the microstructural heterogeneity induced by the welding thermal cycle, (weld zone (WZ), heat affected zone (HAZ), and base metal (BM)). Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) results indicated microstructural coarsening and the formation of numerous large-sized precipitates in the WZ of CHN01 welded joints, which deteriorated its ductility, and thus failed to meet the requirements for jackets of Nb3Sn conductors. Severe degradation of mechanical properties reveals a critical weakness for CHN01 and similar high-performance alloys: the impressive properties of the BM are not transferable to welded structures, and the effective strengthening mechanisms that make the BM exceptional also make it inherently susceptible to welding embrittlement. Therefore, the future of the CHN01 application hinges not on further enhancing BM properties, but almost exclusively on solving the weldability challenge through innovative filler materials, advanced welding processes, and novel post-weld treatments. The application of CHN01 steel extends beyond thin jacket profiles (3–5 mm) to thick structural components like TF coil cases (up to 300 mm), as illustrated in Figure 3. While progress has been made in qualifying the base material for jackets, the properties of heavy-section forgings remain unsatisfactory, and their weldability is even less understood.
To elucidate the failure mechanisms of welded joints, the digital image correlation (DIC) technique was employed, which is an optical, non-contact deformation testing method by analyzing the grayscale patterns of speckle images on the surface of a specimen during the stretching process [117]. Combined with metallographic analysis, DIC mapping of CHN01 welded joints at 6 K localized the cryogenic strain concentration and confirmed that degradation of toughness and plasticity was predominantly confined to the local strain concentration phenomenon in the WZ, which is consistent with previous findings [42,116]. Furthermore, the fracture behavior of the CHN01 base material at 77 K has been systematically investigated using an integrated approach combining mechanical testing and acoustic emission (AE) analysis [118]. AE technique is a highly sensitive, non-destructive method for the in-situ monitoring of damage evolution in metallic materials [119]. By capturing transient elastic waves generated during crack initiation and propagation, the AE technique provides real-time, multidimensional insights into damage evolution, including localization and identification of damage types [120]. Given that the AE technique has proven effective in characterizing the base material, its application to CHN01 welded joints represents a promising direction, which could monitor the fracture process in the WZ and provide complementary acoustic data to the full-field strain information obtained by the DIC technique, thereby offering a more comprehensive understanding of the failure mechanisms.

5. Comparative Analysis and Future Perspectives

The fracture toughness versus yield strength properties at 4.2 K for structural steels developed by the United States, Japan, and China are compared as summarized in Figure 4. The data reveal a general trend where later-generation materials (indicated by the dotted arrows) achieve a more desirable combination of simultaneously higher strength and greater toughness. JAERI established an early lead with the JJ1 steels in the 1990s, setting a high benchmark for excellent Rp0.2-KIc combination (1126 MPa and 366 MPa·m1/2), which provided a foundation for their contribution to ITER (JK2LB steel for CS coils) and promoted their current development for JA-DEMO (JN1 steel targeting a higher Rp0.2 of 1288 MPa). Adopting a more pragmatic approach, the United States firstly explored advanced alloys like INCOLOY 908, which was abandoned owing to its complexity and susceptibility to SAGBO. And the U.S. converged on the well-understood modified 316LN for the jackets of CICCs of TF coils in the ITER, reflecting in its prioritizing manufacturability and reliability over ultimate performance. China’s path exemplified an accelerated development. Starting with developing the ITER-grade 316L, China rapidly progressed to indigenously develop CHN01. The CHN01 steel outstands at the forefront of cryogenic mechanical performance by achieving a remarkable Rp0.2 as 1600 MPa and K(J)Ic as 265 MPa·m1/2, making it a leading candidate for the CFETR.
Table 13 underscores an inverse correlation between strength–toughness and manufacturability-weldability. JK2LB of Japan, modified 316LN of the U.S., and modified 316L of China offer relatively excellent weldability and manufacturability, though at different performance levels. Their costs are tied to mature industrial processes. JJ1 of Japan and CHN01 of China, respectively, represent the extreme of cryogenic strength and cryogenic fracture toughness, which could be clearly seen in Figure 4. However, JJ1 steel suffers from severe welding embrittlement requiring special techniques like NG-TIGW while CHN01 steel exhibits significant degradation of strength and toughness in the weld region. The manufacturing of the CHN01 is also inherently difficult, requiring extreme purity control (C and O) and precise thermo-mechanical processing (vacuum oxygen de-carbonized and electro-slag remelting), which drastically increases cost and challenges reproducibility for large-scale components. Promotions in cryogenic mechanical performance are often achieved at the expense of weldability and manufacturability, which subsequently raise cost and complicate production. Steel production should be standardized to develop large-scale manufacturing routes and achieve commercialization for huge components in next-generation MCF devices. Implementing comprehensive qualification programs is as important as strictly testing full-scale prototypes under simulated service conditions, including fatigue and electromagnetic loads, to ensure economic viability for fusion energy.
The research focus for Japan’s JN1 and China’s CHN01 austenitic stainless steels differs significantly: JN1 emphasizes strengthening base materials, while CHN01 prioritizes anti-embrittlement of welded joints. For JN1 steel, dynamic recrystallization (DRX) techniques could be a promising approach to refine grain structure and reset dislocation densities for simultaneous strength-toughness improvement [121], along with hybrid alloy designs (B/Mo/Nb) that leverage synergistic microalloying effects to overcome conventional trade-offs [113,122,123]; For CHN01 steel, the development of novel filler materials is required. Anti-aging welding materials should be developed by augmenting the concentration of micro-alloy elements that impede weld tissue coarsening, refining the weld grain size, reducing C content in ingots, and enhancing molten steel cleanliness. Furthermore, residual stresses introduced during manufacturing processes like forging, welding, and cold working can be exacerbated at cryogenic temperatures, potentially accelerating fatigue crack initiation and compromising the structural integrity [124,125]. Post-weld heat treatments and advanced welding techniques are essential to mitigate these stresses. Cryogenic treatment (CT), a supplementary process to conventional heat treatment, has been demonstrated to improve mechanical properties [126] and dimensional stability [127] of ASSs. The process typically involves cooling specimens below −100 °C at a controlled rate, holding for a specified duration, and then reheating. During CT, martensitic transformation can be induced resulting in microstructural refinement, enhanced mechanical properties, and relief of residual stresses in weldments [128,129,130,131]. Although current research on cryogenic treatment of CHN01 steel is limited, applying cryogenic treatment to CHN01 welded joints offers a viable pathway to reduce detrimental residual stresses and enhance cryogenic performance via microstructural refinement.

6. Conclusions

Cryogenic high-strength and high-toughness ASSs serve as the critical structural material for superconducting magnet systems in MCF devices. The modified 316LN, JK2LB, and JJ1 steels currently employed in the ITER project have demonstrated reliability under extreme conditions. However, next-generation MCF devices like JA-DEMO and CFETR demand higher mechanical properties, requiring Rp0.2 exceeding 1500 MPa and K(J)Ic greater than 120 MPa·m1/2. Japan focuses on enhancing the fundamental strength of JN1 steel, while China concentrates on resolving performance degradation for CHN01 welded joints which exhibited outstanding Rp0.2 and K(J)Ic for base material. Future research must prioritize developing novel welding materials by microalloying element addition and improving controlled dynamic recrystallization, along with applying deep cryogenic treatment to enhance the cryogenic performance of WZ in CHN01 welded joints. International collaboration and standardized production will accelerate the transition of these advanced materials from laboratory to engineering applications, ultimately advancing the commercialization of fusion energy.

Author Contributions

Investigation, J.D.; writing—original draft preparation, J.D.; writing—review and editing, C.H.; supervision, C.H. All authors have read and agreed to the published version of the manuscript.

Funding

The development of CHN01 steel has been funded by the National Key R&D Program of China (Grant No.: 2017YFE0301400), and the key Program of the Chinese Academy of Sciences (Grant No.: ZDRW-CN-2021-2-4).

Data Availability Statement

No data was used for the research described in the article.

Conflicts of Interest

The authors declare no conflicts of interest.

Nomenclature

eElongation at Fracture
K(J)IcFracture Toughness
RmUltimate Tensile Strength
Rp0.2Yield Strength
AEAcoustic Emission
ASSAustenitic Stainless Steel
BESTBurning Experimental Superconducting Tokamak
BMBase Metal
CCCorrection Coil
CFETRChina Fusion Engineering Test Reactor
CICCCable-In-Conduit Conductor
CSCentral Solenoid
DRXDynamic Recrystallization
DICDigital Image Correlation
EASTExperimental Advanced Superconducting Tokamak
EBSDElectron Backscatter Diffraction
ECTEddy Current Testing
EDSEnergy Dispersive Spectroscopy
ENEAEuropean Nuclear Energy Agency
FCCFace-Centered Cubic
FCGRFatigue Crack Growth Rate
FSFerritic Steel
HAZHeat Affected Zone
HFMLHigh Field Magnet Laboratory
HMASHigh-Mn Austenitic Steel
HMASSHigh-Mn Austenitic Stainless Steel
HTSHigh Temperature Superconductivity
INCInternational Nickel Corporation
ITERInternational Thermonuclear Experimental Reactor
JA-DEMOJapan Demonstration
JAEAJapan Atomic Energy Agency
JAERIJapan Atomic Energy Research Institute
JCSJapanese Cryogenic Steel
JK2LBJK2-Low C-B-added
JT-60SAJapan Torus-60-Super Advanced
LBWLaser Beam Welding
LEFMLinear Elastic Fracture Mechanics
MITMassachusetts Institute of Technology
NAFASSYNational Facility for Superconducting Systems
NDENon-Destructive Examination
NG-MIGWNarrow-Gap Metal Inert Gas Weld
PAUTPhased Array Ultrasonic Testing
PFPolar Field
QSTQuantum Science and Technology
SAGBOStress Accelerated Grain Boundary Oxidation
SEMScanning Electron Microscope
SFEStacking Fault Energy
SHRStrain Hardening Rate
TEMTransmission Electron Microscope
TIGWTungsten Inert Gas Weld
TFToroidal Field
TFTRToroidal Fusion Test Reactor
VEBWVacuum Electron Beam Weld
WZWeld Zone

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Cryo 01 00013 g001
Figure 1. Cross-section of several conductors designed in the ITER: (a) PF of the ITER; (b) TF of the ITER; (c) CC of the ITER; (d) CICC for High-Temperature Superconducting (HTS) by ENEA (Note: this conductor is shown for comparative purposes as an example of next-generation HTS technology).
Figure 1. Cross-section of several conductors designed in the ITER: (a) PF of the ITER; (b) TF of the ITER; (c) CC of the ITER; (d) CICC for High-Temperature Superconducting (HTS) by ENEA (Note: this conductor is shown for comparative purposes as an example of next-generation HTS technology).
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Figure 2. Relationship between fracture toughness and yield strength of JCS at 4.2 K.
Figure 2. Relationship between fracture toughness and yield strength of JCS at 4.2 K.
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Figure 3. Schematic diagram of the application of large forgings: (a) N50, as a ~16 m long forging for key connecting plates in the ITER CS system; (b) CHN01, as a ~4.72 m long forging for casing components of the CFETR TF system; (c) CHN01, as a ~3 mm thin workpiece for jacket components of the CFETR TF system (Figure 3c has been enlarged ten times compared to its actual size to make it easier to view).
Figure 3. Schematic diagram of the application of large forgings: (a) N50, as a ~16 m long forging for key connecting plates in the ITER CS system; (b) CHN01, as a ~4.72 m long forging for casing components of the CFETR TF system; (c) CHN01, as a ~3 mm thin workpiece for jacket components of the CFETR TF system (Figure 3c has been enlarged ten times compared to its actual size to make it easier to view).
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Figure 4. Relationship between fracture toughness and yield strength at 4.2 K of structural steels developed by the United States, Japan and China. (Note: direction of the dotted arrows represents the progression of development time).
Figure 4. Relationship between fracture toughness and yield strength at 4.2 K of structural steels developed by the United States, Japan and China. (Note: direction of the dotted arrows represents the progression of development time).
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Table 6. Development process of new cryogenic structural steels in JA and the USA.
Table 6. Development process of new cryogenic structural steels in JA and the USA.
StepsExperimentsRequirementsNumber of Test Steels at 4.2 KTime
ASSHMASSHMSSFSTotal
1TensileRp0.2 ≥ 1200 MPa and
CVN ≥ 100 J		3140144891982
2Charpy Impact3135123811984
Next candidate materials1012 221986
3Fracture toughnessKIc ≥ 200 MPa·m1/21012 221988
4Fatigue properties~316LN 32 51990
ITER candidate materials11 2
Table 7. Chemical compositions of the JCS.
Table 7. Chemical compositions of the JCS.
JCSCSiMnPSNiCrMoNCuV
CSUS-JN10.0260.994.20.0260.00214.7424.2 0.340.700.30
CSUS-JKA10.0230.420.490.0060.00114.025.00.680.268
CSUS-JN20.0500.3422.40.0100.0023.2213.40.700.24
CSUS-JK20.050.3621.790.0130.0054.9412.82 0.212
CSUS-JJ10.0460.449.740.0200.00211.9212.214.890.203
Table 8. Chemical compositions of the INCOLOY 908 (wt%) (Data from [98]).
Table 8. Chemical compositions of the INCOLOY 908 (wt%) (Data from [98]).
FeNiCrNbTiAlNMnCCo
40.7493.982.921.740.930.0020.0410.01<0.1
Table 9. Mechanical properties of the INCOLOY 908 for CICC tests (Data from [98]).
Table 9. Mechanical properties of the INCOLOY 908 for CICC tests (Data from [98]).
Yield Strength
(Rp0.2) (MPa)		Tensile Strength
(Rm) (MPa)		Young’s Modulus
(E) (GPa)		Elongation at Fracture (e) (%)Fracture Toughness (KIc) (MPa·m1/2)
4.2 K1216169417722196
300 K1110130217122196
Table 10. Comparison of the ITER and JA-DEMO.
Table 10. Comparison of the ITER and JA-DEMO.
ITERJA-DEMO
SC StrandNb3SnNb3Sn
Number of TFC1816
Bmax11.8 T13.9 T
Conductor current68 kA83 kA
Number of turns per TFC134192
Design stress667 MPa800 MPa
Total magnetic energy41 GJ153 GJ
Width/Height of TFC8/12.6 m12/19 m
Table 11. Chemical compositions of the N50 and CHN01 steel (Data from [116]).
Table 11. Chemical compositions of the N50 and CHN01 steel (Data from [116]).
CNCrNiMnMoNbVSi
N50 (XM-19)0.0580.3023.2313.54.812.080.220.250.67
CHN010.0080.3122.314.65.182.10.090.190.30
Table 12. Comparison between mechanical properties of the CHN01 base material and welded joint for jacket application (Data from [42]).
Table 12. Comparison between mechanical properties of the CHN01 base material and welded joint for jacket application (Data from [42]).
CHN01 Base MaterialCHN01 Welded Joint
300 K77 K4.2 K300 K77 K4.2 K
Yield strength
(Rp0.2) (MPa)		546.51193.51600.0514.51103.01335.5
Tensile strength
(Rm) (MPa)		776.01550.01930.0699.51339.01622.5
Young’s modulus
(E) (GPa)		184.0196.5215.5218.5185.0219.5
Elongation
(e) (%)		49.238.533.829.014.519.1
Fracture toughness (K(J)Ic) (MPa·m1/2) 264.5 209.0
Table 13. A comparison of the development of structural steels for coils in Japan, the United States, and China. (Note: the number of stars and squares from the largest to the smallest represents the corresponding performance from excellent to poor).
Table 13. A comparison of the development of structural steels for coils in Japan, the United States, and China. (Note: the number of stars and squares from the largest to the smallest represents the corresponding performance from excellent to poor).
JapanUnited StatesChina
MaterialJK2LBJJ1JN1Modified 316LNINCOLOY 908Modified 316LCHN01
ApplicationITER CSITER TFJA-DEMO (Design)ITER TFCICC TestITER PFCFETR
(Design)
Development time2000s1990s2010s1990s1990s2010s2018
Fracture toughness (MPa·m1/2)209366210300196278265
Yield Strength
(MPa)		107611261288116512167971550
Manufacturability◊◊◊◊◊◊◊◊◊◊◊◊◊◊◊◊◊◊
WeldabilityCryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001Cryo 01 00013 i001
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Dai, J.; Huang, C. Development of Cryogenic Structural Steels for Magnetic Confinement Fusion. Cryo 2025, 1, 13. https://doi.org/10.3390/cryo1040013

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Dai J, Huang C. Development of Cryogenic Structural Steels for Magnetic Confinement Fusion. Cryo. 2025; 1(4):13. https://doi.org/10.3390/cryo1040013

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Dai, Jingjing, and Chuanjun Huang. 2025. "Development of Cryogenic Structural Steels for Magnetic Confinement Fusion" Cryo 1, no. 4: 13. https://doi.org/10.3390/cryo1040013

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Dai, J., & Huang, C. (2025). Development of Cryogenic Structural Steels for Magnetic Confinement Fusion. Cryo, 1(4), 13. https://doi.org/10.3390/cryo1040013

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