Low temperature physical properties of a high Mn austenitic steel JK2LB

High manganese austenitic stainless steel JK2LB is developed by the Japan Atomic Energy Agency for applications as a conduit material for superconducting cable-in-conduit conductors for the magnets of international thermonuclear experimental reactor (ITER). The low temperature physical property data of this material are very important to ITER magnet design. Therefore in this paper, our measurements of the physical properties including room temperature Young’s modulus and thermal expansion, magnetization, thermal conductivity, specific heat and resistivity at temperatures from room temperature down to 2 K are reported. We found that JK2LB is antiferromagnetic at low temperatures with a Néel temperature of 240 K. This is consistent with a prediction based on the chemical composition of the austenite stainless steel. The antiferromagnetic phase transition is also evident in the resistivity vs. T curve. Nevertheless, no anomalies are observable in its specific heat and thermal conductivity from 2 K to 300 K. The thermal expansion of this steel between 10 K and 300 K is about 0.22%. Its Young’s modulus, specific heat and thermal conductivity are comparable to that of 316LN stainless steel.     

Using the HELIOS facility for assessment of bundle-jacket thermal coupling in a CICC

In a Cable In Conduit Conductor (CICC) cooled by forced circulation of supercritical helium, the heat exchange in the bundle region can play a significant role for conductor safe operation, while remaining a quite uncertain parameter. Heat exchange between bundle and jacket depends on the relative contributions of convective heat transfer due to the helium flow inside the bundle and of thermal resistance due to the wrappings between the cable and the conduit.In order to qualify this thermal coupling at realistic operating conditions, a dedicated experiment on a 1.2 m sample of ITER Toroidal Field (TF) dummy conductor was designed and performed in the HELIOS test facility at CEA Grenoble. Several methods were envisaged, and the choice was made to assess bundle-jacket heat transfer coefficient by measuring the temperature of a solid copper cylinder inserted over the conductor jacket and submitted to heat deposition on its outer surface.The mock-up was manufactured and tested in spring 2015. Bundle-jacket heat transfer coefficient was found in the range 300–500 W m⁻² K⁻¹. Results analysis suggests that the order of magnitude of convective heat transfer coefficient inside bundle is closer to Colburn–Reynolds analogy than to Dittus–Boelter correlation, and that bundle-jacket thermal coupling is mainly limited by thermal resistance due to wrappings. A model based on an equivalent layer of stagnant helium between wraps and jacket was proposed and showed a good consistency with the experiment, with relevant values for the helium layer thickness.     

Design,development and fabrication of indigenous 30 kA NbTi CICC for fusion relevant superconducting magnet

The fusion relevant superconducting magnet is under development in India using a cable-in-conduit-conductor (CICC) with operating current of 30 kA at 5.5 T and 4.5 K. The 30 kA NbTi based CICC is designed on the basis of desired critical design parameters as well as mechanical fabrication considerations. The 30 kA CICC has been designed having square cross-section (30 mm × 30 mm) consisting NbTi as superconducting cable, SS316LN as jacket material and SS304 foil as wrapping around the cabled strands. The design configuration of 30 kA NbTi CICC has been discussed in this paper. The NbTi base high current carrying strands have been fabricated indigenously using direct extrusion and cold drawing process. The 100 m long NbTi–Cu strands twisting, insertion of cabled strands into a circular conduit has been developed with pull through technology. The welding process qualification and effects of cold work on jacket material at room temperature have been elaborated in this paper. The manufacturing parameters and quality procedures for development of CICC have been successfully established and demonstrated with fabrication of 100 m NbTi based CICC without any technical difficulties.     

Fabrication of ZK60 magnesium alloy thin sheets with improved ductility by cold rolling and annealing treatment

Thin ZK60 magnesium alloy sheets with ultrafine-grain structure were successfully fabricated by continuous cold rolling with proper intermediate annealing treatments at 503–523 K for 30 min. Meanwhile, microstructure uniformity and planar texture anisotropy were strikingly improved by rolling deformation and static recrystallization, resulting in continuous improvement in the strength anisotropy. Excellent ductility more than 30% in fracture elongation was achieved after further annealing treatment at a lower temperature of 473 K. This was primarily attributed to the significant weakening of the {0 0 0 2} pole intensity and grain refinement during the process. It is shown that mechanical properties of the final sheets could be closely controlled by the present process.     

Long term elongation of Kevlar-49 single fiber at low temperature

We have measured the rate of elongation of a loaded Kevlar-49 fiber as a function of time at 4.2 K. The result puts a worst case upper limit of 0.028% in the elongation rate ΔL/L for a 0.5 mm diameter fiber kept under a constant tension of 2.7 kg for 8 months. A value that is probably closer to reality is actually 0.004%. This result proves that Kevlar-49 can be safely used in cryogenic applications in which high mechanical stability under stress is required.     

Status of Nb3Sn strand development in Korea

Although there were many research activities for the development of superconducting Nb₃Sn strands, the major one started under KSTAR (Korea Superconducting Tokamak Advanced Research) project in 1996. After the success of a large scale production test of Nb₃Sn strand using the internal tin route, a new mass production facility is under operation since 2004.KAT (Kiswire Advanced Technology Ltd.), an affiliate of Kiswire Ltd., manufactured various types of Nb₃Sn strands using the internal tin process optimized for fusion magnets. For the Nb₃Sn strand of the KSTAR PF coil, each module has ～190 niobium filaments and 19 modules are restacked for the strand production. For the ITER TF strand, there are two types of basic design. One of them has 37and 19 modules with 169–219 niobium filaments in each module. The other has 19 modules with 164–190 niobium filaments in each module. Both the designs satisfy the requirements for ITER TF strand with enough margins. The characterization of the strands is performed by hysteresis loss measurement, RRR (Residual Resistivity Ratio), n-value, and critical current density measurement vs. temperature, magnetic field, and strain. The critical current density of the strands reached around 1100 A/mm² at 12 T and 4.2 K. A well defined quality assurance program helped to produce a high quality strand with a piece length of more than 15 km. KAT has been provided Nb₃Sn strand for KSTAR PF Coil and ready to produce the Nb₃Sn strand for ITER TF coil.In this paper, the design concept, the fabrication procedure and the result of the strand performance test are discussed.     

Mechanical tests on the ITER PF 316L jacket after compaction

This paper focuses on mechanical tests on the ITER PF jacket 316L stainless steel material. As requirement of ITER, the conductor will be compacted after cable insertion. Young’s modulus, yield strength (0.2% offset), ultimate tensile strength and elongation at failure shall be reported. For researching an effect of compaction on mechanical properties, the three jacket sections which were cut from the same jacket were compacted at 0%, 4.77% and 5.58% in cross section. The mechanical properties of materials were measured at 4.2 K, 77 K and 300 K. The effects of compaction on materials properties were discussed. TEM images showed that there were stress-induced martensite phase after compaction. It is concluded that the results are accordant with requirement of ITER.     

The effect of rolling direction on the creep process of Al–Cu bimetallic sheet

The study of the mechanical properties of aluminium–copper (Al–Cu) metal layered composite, formed by joining aluminium and copper sheets in the process of rolling have been presented in this paper. The influence of the rolling direction on the basic strength parameters and rheological properties of the composite was analysed. All tests were carried out on flat specimens cut from a sheet in the direction compatible with the rolling direction (RD) and transverse direction (TD). Preliminary tests of monotonic uniaxial tension at a temperature of 293 K were carried out and the basic mechanical properties of Al–Cu bimetal were determined. The hardening process of the material was described by the three-parameter Swift’s equation. The essential creep tests were carried out at a temperature of 523 K in the range of stress 88.5–137.9 MPa. The relation between minimum creep rate and applied stress for the specimens cut from the RD and TD directions were determined. The relationships between the time to fracture, stress, and rupture elongation, obtained from the creep tests, were determined as well. Variations of the steady creep rate with time to fracture by using the Monkman–Grant’s model and its modifications were analysed. It was found that the rolling process strongly affected the short-time monotonic deformation at 293 K and the creep process at 523 K temperature.     

Failure analysis on the inner cylinder explosion of a gas producer water jacket

A large area of the inner cylinder wall in a gas producer turned inside while exploded in the trial running for 16 days. The failure analysis of the accident was presented in this paper. Firstly, on-site tests reveal that the installation location and type of the steam outlet caused 115 mm high steam at the top of the water jacket. Then, finite element simulation indicates that the maximum equivalent stress of the water jacket was higher than the yield stress of the material, even if the gas producer was in normal operating conditions. Furthermore, it was found that there were non-metallic inclusions and banded structure near the initial cracking area, which reduced the mechanical strength of the steel plate. It is concluded that the cracking started from the weakest parts of the material where were under high stress. The inner cylinder of the water jacket burst under the internal pressure, and turned inside due to the shock of the steamed water in the water jacket. Finally, some suggestions were proposed to improve the stress state of the water jacket and prevent similar accidents.     

Study of a liquid insulation for the solid rocket motor

Formula of a liquid insulation for a solid rocket motor (SRM) was studied in this paper. By analyzing vulcanization mechanism and comparing mechanical and thermal properties of the rubber, composition of the insulation with liquid raw material was determined. Ignition tests of motors with liquid insulation were successful. Results indicated that the tensile strength, elongation at break, shear strength, specific heat and thermal conductivity of the liquid insulation were 6.25 MPa, 590%, 3.61 MPa, 3.4174 × 10³ J/(kg K) and 0.421 W/(m K) respectively. The success of various ground ignition tests and flight test proved that this new liquid insulation can fulfill requirements of the SRM.     

Modelization of the thermal coupling between the ITER TF coil conductor and the structure cooling circuit

The ITER Toroidal Field (TF) coils are required not to quench during the most demanding event: a plasma disruption followed by a fast discharge of the Central Solenoid (CS), the Poloidal Field (PF) coils and the Correction Coils (CC). This event creates large heat deposition in the ITER magnet stainless steel structures in addition to the conductor AC losses. In order to prevent quench occurring in the TF conductor, cooling channels, implemented in the TF coil structure (TFCS), have to remove a large fraction of the heat deposited. The first integrated TF and structure mock-up has been manufactured and then tested in the HELIOS cryogenic test facility (CEA Grenoble) to determine the thermal coupling between the TFCS and the TF conductor, both actively cooled by supercritical helium at 4.4 K and 5 bar. It consists in a stainless steel casing, a cooling pipe glued with resin in the casing groove, winding pack (WP) ground insulation, a radial plate and a copper dummy cable-in-conduit-conductor (CICC). Steady state as well as transient thermal characterizations have been completed in May 2015. Simulation results by thermal hydraulic codes (VENECIA/SuperMagnet) and some of the experimental data are presented and discussed. The thermal coupling between the helium in the cooling tube and the TF coil structure is then modelled as an equivalent heat transfer coefficient in order to simplify the thermal hydraulic (TH) models. Comparison between simplified coupling and detailed coupling is presented.     

Research into mechanical properties of reaction-bonded SiC composites at cryogenic temperatures

The mechanical properties of reaction-bonded silicon carbide (RBSC) composites at cryogenic temperatures have been reported for the first time. The results show that the flexural strength and fracture toughness increase from 277.93 ± 23.21 MPa to 396.74 ± 52.74 MPa and from 3.69 ± 0.45 MPa·m^1/2 to 4.98 ± 0.53 MPa·m^1/2 as the temperature decreases from 293 K to 77 K, respectively. The XRD analysis of the phase composition reveals that there is no phase transformation in the composites at cryogenic temperatures, indicating cryogenic mechanical properties are independent of phase composition. The enhancement of mechanical properties at 77 K over room temperature could be explained by the transition of fracture mode from predominant transgranular fracture to intergranular fracture and stronger resistance to crack propagation resulting from higher residual stress at 77 K. The above results demonstrate that such composites do not undergo similar deteriorations in the fracture toughness as other materials (some kinds of metals and polymers), so it is believed that such composites could be a potential material applied in cryogenic field.     

The superplasticity and microstructure evolution of TC11 titanium alloy

The superplasticity is the capability of some metallic materials to exhibit very highly tensile elongation before failure. The superplastic tensile tests were carried out at various deformation conditions in this paper to investigate the superplastic behaviors and microstructure evolution of TC11 titanium alloy. The results indicate that the smaller the grain size, the better the superplasticity is, and the wider the superplastic temperature and strain rate is, in which the superplastic temperature is ranging from 1023 to 1223 K and the strain rate is ranging from 4.4 × 10^?5 to 1.1 × 10^?2 s^?1. The maximum tensile elongation is 1260% at the optimum deformation conditions (1173 K and 2.2 × 10^?4 s^?1). For further enhancing the superplasticity of TC11 titanium alloy, the novel tensile method of maximum m superplastic deformation is adopted in the paper. Compared with the conventional tensile methods, the excellent superplasticity of TC11 titanium alloy has been found with its maximum elongation of 2300%.     

An investigation of the synthesis,consolidation and mechanical behaviour of Al 6061 nanocomposites reinforced by TiC via mechanical alloying

Nanostructured Al 6061–x wt.% TiC (x = 0.5, 1.0, 1.5 and 2.0 wt.%) composites were synthesised by mechanical alloying with a milling time of 30 h. The milled powders were consolidated by cold uniaxial compaction followed by sintering at various temperatures (723, 798 and 873 K). The uniform distribution and dispersion of TiC particles in the Al 6061 matrix was confirmed by characterising these nanocomposite powders by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), differential thermal analysis (DTA) and transmission electron microscopy (TEM). The mechanical properties, specifically the green compressive strength and hardness, were tested. A maximum hardness of 1180 MPa was obtained for the Al 6061–2 wt.% TiC nanocomposite sintered at 873 K, which was approximately four times higher than that of the Al 6061 microcrystalline material. A maximum green compressive strength of 233 MPa was obtained when 2 wt.% TiC was added. The effect of reinforcement on the densification was studied and reported in terms of the relative density, sinterability, green compressive strength, compressibility and Vickers hardness of the nanocomposites. The compressibility curves of the developed nanocomposite powders were also plotted and investigated using the Heckel, Panelli and Ambrosio Filho and Ge equations.     

High-temperature low cycle fatigue behavior of a gray cast iron

The strain controlled low cycle fatigue properties of the studied gray cast iron for engine cylinder blocks were investigated. At the same total strain amplitude, the low cycle fatigue life of the studied material at 523 K was higher than that at 423 K. The fatigue behavior of the studied material was characterized as cyclic softening at any given total strain amplitude (0.12%–0.24%), which was attributed to fatigue crack initiation and propagation. Moreover, this material exhibited asymmetric hysteresis loops due to the presence of the graphite lamellas. Transmission electron microscopy analysis suggested that cyclic softening was also caused by the interactions of dislocations at 423 K, such as cell structure in ferrite, whereas cyclic softening was related to subgrain boundaries and dislocation climbing at 523 K. Micro-analysis of specimen fracture appearance was conducted in order to obtain the fracture characteristics and crack paths for different strain amplitudes. It showed that the higher the temperature, the rougher the crack face of the examined gray cast iron at the same total strain amplitude. Additionally, the microcracks were readily blunted during growth inside the pearlite matrix at 423 K, whereas the microcracks could easily pass through pearlite matrix along with deflection at 523 K. The results of fatigue experiments consistently showed that fatigue damage for the studied material at 423 K was lower than that at 523 K under any given total strain amplitude.     

A novel approach to develop aluminum matrix nano-composite employing friction stir welding technique

The main object of the present study is to investigate the effect of nano-sized SiC particle on the mechanical properties of the friction stir welding (FSW) joints. Prior to FSW, nano-sized SiC particles were incorporated into the joint line. A combination of three rotational speeds and three traveling speeds were applied. Microstructural evaluation using optical microscopy (OM) and scanning electron microscopy (SEM) revealed a banded structure consisting of particle-rich and particle-free regions in stir zone (SZ). The joints fabricated with rotational speed of 1250 rpm and traveling speeds of 40 and 50 mm/min, exhibited the highest mechanical properties. Owing to the presence of SiC nano-particles, at 1250 rpm and 40 mm/min, ultimate tensile strength (UTS) and percentage of elongation were improved by 31% and 76.1%, respectively. Significant increase in UTS and percentage of elongation were attributed to the pinning effect and increased nucleation sites associated with SiC nano-particles. Moreover, reinforcement particles resulted in breaking of primary grains. On the other hand, at 1250 rpm and 40 mm/min, SiC-included specimen showed superior ductility to SiC-free specimen. The fracture morphologies were in good agreement with corresponding ductility results.     

Effect of post weld heat treatment on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel weldments

The effect of postweld heat treatment (PWHT) on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel joints with ER316LMn filler material was investigated. PWHT aging was performed for 1 h at four different temperatures of 600 °C, 760 °C, 870 °C and 920 °C, respectively. The microstructure revealed the sigma phase precipitation occurred in the weld metals heat-treated at the temperature of 870 °C and 920 °C. The PWHT temperatures have the less effect on the tensile strength, and the maximum tensile strength of the joints is about 630 MPa, reaching the 95% of the base metal, whereas the elongation is enhanced with the rise of PWHT temperatures. Meanwhile, the sigma phase precipitation in the weld metals reduces the impact toughness.     

Strengthening of AISI 2205 duplex stainless steel by strain ageing

In this study, static strain ageing behavior of commercially available and solution heat treated duplex stainless steel was investigated and the effect of static strain ageing on the mechanical properties was also determined in detail. Some of as-received duplex stainless steel test specimens were pre-strained in tension by 5% and then aged at 100 °C, 200 °C, 300 °C, 400 °C, 500 °C and 600 °C for 30 min in furnace. Some of duplex stainless steel test specimens were solution heat treated at 1050 °C for 30 min, water quenched and then pre-strained for 5% in tension shortly after the solution heat treatment.In order to identify the effect of static strain ageing on the mechanical properties, the tensile strength, the change in the strength due to ageing (ΔY), elongation fracture and hardness were determined. The test results showed that the mechanical properties were affected by static strain ageing mechanism which was applied at different temperatures for same time interval.     

Design of a new Ni-free austenitic stainless steel with unique ultrahigh strength-high ductility synergy

A conceptual approach was used to design a new Ni-free austenitic stainless steel with a unique combination of ultrahigh strength and ductility. The concept was based on the alloying of the 0.05C–18Cr–12Mn (wt.%) steel by 0.39%N and heavy warm rolling (84% reduction) at 1173 K (900 °C) to achieve the yield strength of minimum 1 GPa and high tensile strength and elongation due to a proper stability of the austenite as a result of the optimized stacking fault energy (SFE). The yield strength of 1010 MPa, tensile strength of 1150 MPa and high fracture strain of 70% were measured for the steel designed. Dislocation and solid solution hardening mechanisms are introduced as the main contributors for the ultrahigh yield strength of the steel. The strain hardening is gradual and the hardening rate reaches a high level of ∼2400 MPa at a high true strain of 40% due to slow α′-martensitic transformation and mechanical twinning. Consequently, the ductility of the designed steel is excellent.     

Low-temperature superplasticity of Al2O3(p)/Ni–Co nanocomposites

Nanocomposites of Al₂O₃/Ni–Co prepared using Al₂O₃ of various particle sizes were fabricated by pulse current electrodeposition. Their superplastic tensile deformation was investigated at strain rates of 8.33 × 10⁻⁴ s⁻¹ and 1.67 × 10⁻³ s⁻¹ and temperatures of 723–823 K. The Al₂O₃ particle sizes and the deformation temperature had significant influence on the elongation of the deposited materials. The optimal superplastic condition and the maximum elongation were determined. A low temperature superplasticity with elongation of 632% was achieved at a strain rate of 1.67 × 10⁻³ s⁻¹ and 823 K. Scanning electron microscopy and transmission electron microscopy were used to examine the microstructures of the deposited and deformed samples. The grains grew to a micrometer dimensions and were elongated along the tensile direction after superplastic deformation. Superplasticity in electrodeposited nanocomposites is related to the presence of S at grain boundaries and to deformation twinning.