Seung Hong

Influence of deformation induced ferrite transformation on grain refinement of dual phase steel

Abstract A controlled rolling process was simulated by thermomechanical simulator (Gleeble1500) to produce as-hot-rolled dual phase steel. The microstructure of as-hot-rolled dual phase steel using deformation induced ferrite (DIF) transformation could be refined to as fine as that of Intermediate Quenching (martensite as a starting microstructure). DIF transformation was influenced by austenite grain size, amount of strain and strain rate, and the grain of about 2 μm could be produced by heavy deformation of 80%. However, the ferrite growth occurred during the intercritical isothermal holding after deformation. Microalloying such as Nb, V was effective for suppressing the ferrite growth. Thus, microalloying elements that could restrain ferrite grain coarsening were required to produce fine grained dual phase steel.

Effects of Nb on strain induced ferrite transformation in C–Mn steel

Abstract Effects of the Nb addition on the strain induced ferrite transformation just above Ar 3 temperature were investigated. Hot compression tests were performed with varying the true strain up to 1.6 (80% reduction) using Gleeble 1500. After the hot deformation, samples were immediately water-quenched to examine ferrite formation characteristics. The grain boundary misorientation angles were measured by electron backscatter diffraction in order to observe evolution of the ferrite grains. For reheating temperatures such as 900 and 1000xa0°C, where Nb was mostly precipitated as NbC, strain induced ferrite grains of 1–2 μm were formed homogeneously within the austenite grain in Nb steel. In the cases of higher reheating temperatures 1100 and 1250xa0°C, where most of Nb was dissolved, the strain induced ferrite transformation was remarkably reduced and the ferrite morphology was changed to elongated grains. It was considered that the ferrite transformation during deformation was retarded by both the solute drag effect of Nb and the consumption of strain energy for the dynamic precipitation of NbC.

Internal Tin Nb3Sn Conductors Engineered for Fusion and Particle Accelerator Applications

The critical current density (J_c) of Nb₃Sn strand has been significantly improved over the last several years. For most magnet applications, high J_c internal tin has displaced bronze process strand. The highest J_c values are obtained from distributed barrier strands. We have continued development of strands made with Nb-47 wt%Ti rods to supply the dopant, and have achieved J_c values of 3000 A/mm² (12 T, 4.2 K). Such wires have very good higher field performance as well, reaching 1700 A/mm² at 15 T. To reduce the effective filament diameter in these high J_c strands, the number of subelement rods incorporated into the final restack billet has been increased to 127 in routine production, and results are presented on experimental 217 stacks. A new re-extrusion technique for improving the monofilament shape is also described. For fusion applications such as ITER, we have developed single-barrier internal tin strands having non-Cu J_c values over 1100 A/mm² (12 T, 4.2 K) with hysteresis losses less than 700 mJ/cm³ over non-Cu volume. The J_c-strain behavior of such composites is also presented.

Recent Advances in Bi-2212 Round Wire Performance for High Field Applications

There has been sustained interest in the development of Bi-2212/Ag round wire because of its unique potential for application in ultra-high-field magnets (>; 25 T). Our development activity with this material has been focused on improving the engineering current density. Filament densification by swaging and isostatic pressing processes have been evaluated, as has further optimization of the melt heat treatment conditions. These improvements lead to an increased mass density of the filament in the final wire, and are essential to reduce filament porosity and obtain high performance over long wire length. Engineering current density values exceeding 480 A/mm2 at 4.2 K, 15 T have been achieved on 1-m-long barrel samples.

Present and Future Applications for Advanced Superconducting Materials in High Field Magnets

Advances in high field magnets are driven primarily by the availability of high current density conductors. The restack rod process (RRP), internal Sn superconductors have achieved engineering current densities nearly five times that of bronze route conductors at high fields. Careful utilization of this low temperature superconductor (LTS) enables the production of magnets beyond the previous benchmark of 21 Tesla without an associated increase in magnet and cryostat volume. Steps to realize extremely compact high field magnets for a variety of applications are described. The next significant challenge is to produce magnetic fields beyond 25 Tesla solely using superconducting solenoids. High temperature superconductors (HTS) will be required and, to this end, Bi-2212/Ag matrix wires are at an advanced stage of development. The tangible objective is a new generation of compact, ultra-high field magnets.

Bi-2212 Round Wire Development for High Field Applications

Oxford Superconducting Technology continuously improves Bi-2212 round wire performance because this product has a unique property for ultra high-field magnet ( 25 T) applications. Our recent results on improving the engineering current density by filament densification and reducing trace carbon and hydrogen contamination are presented. The swaging, isostatic pressing, and over-pressure heat treatment processes have been demonstrated to effectively increase the Bi-2212 filament mass density in the final wire and results in high performance over long wire length. Engineering current density values exceeding 550 A/mm2 at 4.2 K, 15 T have been achieved on 1-m-long barrel samples. Several configurations have been developed to meet different operating current requirements with optimum filament size ( ~ 15 micron) on critical current density, Jc. The round wire has also been twisted down to 12 mm in twist pitch length without performance degradation.

Effect of Nb on grain growth of ferrite in C-Mn steel during isothermal holding after severe deformation

Abstract The effect of Nb on grain growth of ferrite during isothermal holding after severe deformation was investigated. For Nb and C-Mn steels, the deformation was carried out in the temperature range 645-772°C, encompassing ferrite transformation temperature Ar 3, after reheating at 900°C and 1250°C. After reheating at 900°C, the strain induced ferrite grains with an average grain size of 2 μm readily formed up to the equilibrium fraction at all deformation temperatures. Grain growth of the strain induced ferrite in the Nb steel was significantly inhibited during the following isothermal holding because of the grain boundary pinning by NbC precipitates. After reheating at 1250°C, the strain induced ferrite transformation was apparently reduced by solute drag of Nb so that strain free ferrite grains were formed during the subsequent isothermal holding. When strain free ferrite formed near the deformed ferrite grains, abnormal grain growth by strain induced boundary migration (SIBM) occurred in the Nb steel, as well as in the C-Mn steel. The SIBM occurred owing to the stored energy difference between the deformed ferrite and the strain free ferrite regardless of the presence of fine NbC precipitates or partially recrystallised grains.

Round Multifilament Bi-2212/Ag Wire Development for High Field Magnet Applications

Development efforts at Oxford Superconducting Technology (OST) have recently been aimed at manufacturing long length round wires with improvement of transport properties for high field magnet applications. Recently, significant improvements in the JE and Jc performance have been achieved by optimizing the starting precursor composition, the deformation processes, and the heat treatments. The highest JE of 1580 A/mm2 (Jc of 6140 A/mm2) at 4.2 K, 0 T, JE of 420 A/mm2 at 4.2 K, 31 T and JE of 312 A/mm2 at 20 K, 2 T were obtained in 0.81 mm wire with the optimized condition. In addition, significant progress on braided insulation has been made for enabling a robust procedure for wind-and-react Bi-2212 solenoid coils. Ic and generated field have been measured in a series of such coils of increasing dimensions. In this paper the progress on the development of Bi-2212 round wires and coils will be reported.