Effect of sensitizing heat treatment on fatigue crack growth properties at liquid helium temperature

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 clarifed. 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.