Yutaka Kasamatsu

Mechanical Properties of High Manganese Steels at Cryogenic Temperatures

Structural steels for fusion magnet applications at any ambient temperatures have hitherto been austenitic stainless steels, such as SUS 304L and 316L. Although these steels have good fracture toughness even at liquid helium temperature, they also have several drawbacks: low yield strength and istability of the austenitic phase against deformation.1

THE DEVELOPMENT OF A NITROGEN-STRENGTHENED HIGH-MANGANESE AUSTENITIC STAINLESS STEEL FOR A LARGE SUPERCONDUCTING MAGNET

Fusion Experimental Reactor, a big project following JT-60, needs a very large superconducting toroidal field magnet, of about 10-m diameter with a 12-T magnetic field. It is anticipated that the structural materials for the magnet should have a yield strength higher than 1,200 MPa and a fracture toughness exceeding 200 \( {\text{MPa}}\sqrt {\text{m}} \) at 4K in a very thick plate1.

Preliminary Study on Structural Material Selection for Large Superconducting Magnets

Structural materials for superconducting toroidal field (TF) coils of tokamak fusion reactors are required to have high yield strength as well as sufficient toughness at liquid helium temperature. SUS304LN austenitic stainless steel, of which yield strength is higher than 700 MFa at 4K, has been successfully used for the Japanese LCT coil at JAERI.1 The next project of JAERI is the Fusion Engineering Reactor (FER). The superconducting TF coils for the FER will be very large ones, of which diameter is about 10 meters, and furthermore its magnetic field will exceed 10 T. Consequently, structural materials of the coils undergo extremely high structural and magnetic stresses at 4K. So heavy thickness plates, which have high yield strength of about twice as high as that of SUS304LN with reasonably high toughness at 4K, are indispensable to construct the magnets.

A high manganese non-magnetic structural steel plate for the construction of fusion reactors

A new type of non-magnetic steel plate, NONMAGNE 30, has been developed by Kobe Steel, Ltd. and is being processed as a structural non-magnetic material in the fabrication of a plasma feasibility experimental device, JT-60, which is installed at the Japan Atomic Energy Research Institute. NONMAGNE 30 is a high manganese austenitic steel plate with a composition of 0.6%C, 14%Mn, 2%Ni and 2%Cr. This plate has a very low magnetic permeability of less than 1.02 and good mechanical properties, such as a high yield strength of over 294 Mpa (30 kgf/mm2) and an excellent notch toughness. Furthermore, NONMAGNE 30 has excellent weldability and the welded structures show a high resistance to brittle fractures and fatigue.