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 applicatio ns of thermal and electromagnetic stresses. Additionally, since weldin g cannot be avoided, it is highly desirable that the materials used fo r supporting superconducting magnets possess excellent strength and to ughness in addition to superior mechanical properties after heat treat ment. In the current research, an austenitic stainless steel was subje cted to a sensitizing heat treatment whose effects, at 4.2 K, on the t ensile and the fatigue crack growth properties were clarified. Tensile ductility of the sensitized material was markedly reduced to about on e-half that of the solution treated material because of an intergranul ar form of fracturing in such material. Fatigue crack growth rate of t he sensitized material was faster than that of the solution treated ma terial because of the tendency for cracks to develop intergranularly.