FATIGUE-CRACK GROWTH-PROPERTIES OF A CRYOGENIC STRUCTURAL-STEEL AT LIQUID-HELIUM TEMPERATURE

The structural materials of the coils of superconducting magnets utili zed in thermonuclear fusion reactors are used at liquid helium (4.2 K) temperatures and are subjected to repeated thermal stresses and elect romagnetic forces. A high strength, high toughness austenitic stainles s 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 fo rging 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 loc ation across the thickness of the forging. There was virtually no evid ence of variation in tensile strength or fatigue crack growth properti es attributable to different sampling locations in the thickness direc tion and no effect of thickness due to the forging or solution treatme nt associated with large forgings was observed It has been clarified t hat there are cases in which small scale yielding (SSY) conditions are not fulfilled when stress ratios ave large. Delta J was introduced in order to achieve unified expression inclusive of these legions and, b y expressing crack growth rate accordingly, the following formula was obtained at the second stage (middle range). da/dN = C-J Delta J(mI), C-J = A(J)/(Delta J(0))(mJ) where, A(J) = 1.47 X 10(-5) mm/cycle, Delt a J(0) = 2.42 x 10(3)N/m.