By using forging of a newly developed austenitic stainless steel (12Cr-12Ni-10Mn-5Mo-0.2N), a candidate material for the support structure of superconducting magnets for thermonuclear fusion reactors, the effects of a sensitizing heat treatment (24 hours at 650°C, followed by water quenching) on tensile and fatigue crack growth properties (crack closure behavior) at room and liquid helium temperatures were clarified. As a result, it has been found that although the sensitizing heat treatment has virtually no effect on both tensile and fatigue crack growth properties at room temperature, a decrease in tensile total elongation and an increase in fatigue crack growth rate relative to the effective stress intensity factor range due to such heat treatment are evident at liquid helium temperature. This is believed to be due to the difference in microscopic cracking path. Fracture occurs transgranularly in a solution treated material, whereas fracture occurs mainly along the grain boundaries in a sensitizing heat treated material. Furthermore, it has been ascertained that, at liquid helium temperature, the fracture surface profiles of fatigue cracks are much more serrated and the crack-opening load increases, due to sensitizing heat treatment.
