The structural materials of the coils of superconducting magnets utilized in thermonuclear fusion reactors are used at liquid helium (4.2 K) temperatures and are subjected to repeated thermal stresses and electromagnetic forces. A high strength, high toughness austenitic stainless steel (12Cr-12Ni-10Mn-5Mo-0.2N) has recently been developed for large, thick-walled components used in such environments. This material is non-magnetic even when subjected to processing and, because it is a forging material, it is advantageous as a structural material for large components. In the current research, a large forging of 12Cr-12Ni-10Mn-5Mo-0.2N austenitic stainless steel, was fabricated to a thickness of 250 mm, which is typical of section thicknesses encountered in actual equipment. The tensile fatigue crack growth properties of the forging were examined at liquid helium temperature as function of specimen location across the thickness of the forging. There was virtually no evidence of variation in tensile strength or fatigue crack growth properties attributable to different sampling locations in the thickness direction and no effect of thickness due to the forging or solution treatment associated with large forgings was observed. It has been clarified that there are cases in which small scale yielding (SSY) conditions are not fulfilled when stress ratios are large. ΔJ was introduced in order to achieve unified expression inclusive of these regions and, by expressing crack growth rate accordingly, the following formula was obtained at the second stage (middle range). da/dN = CJ ΔJmJ, CJ = AJ/(ΔJ0)mJ, where, AJ = 1.47 × 10−5 mm/cycle, ΔJ0 = 2.42 × 103N/m.

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Many cases exist where material supporting superconducting magnets at liquid helium temperature (4.2 K) are subjected to repeated applications of thermal and electromagnetic stresses. Additionally, since welding cannot be avoided, it is highly desirable that the materials used for supporting superconducting magnets possess excellent strength and toughness in addition to superior mechanical properties after heat treatment. In the current research, an austenitic stainless steel was subjected to a sensitizing heat treatment whose effects, at 4.2 K, on the tensile and the fatigue crack growth properties were clarified. Tensile ductility of the sensitized material was markedly reduced to about one-half that of the solution treated material because of an intergranular form of fracturing in such material. Fatigue crack growth rate of the sensitized material was faster than that of the solution treated material because of the tendency for cracks to develop intergranularly.