Effect of the thermal history on tensile and fatigue crack growth properties of a cryogenic austenitic stainless steel forged at room and liquid helium temperatures

By using forging of a newly developed austenitie stainless steel (12C-12Ni- 10Mm 5Mu 0.2N1. # candidate material for the support structure of superconducting magnets for thermonuclear fusion renetors, the eflects 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 clarifed. As a result, it has been found that although the sensitizing heat treatment has virtually nu effect on both tensile and fatigue erack 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 faetor 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 oecurs transgranularly in a solution treated material, wherens fracture pecurs mainly along the grain boundaries in a sensitizing heat treated material. Farthermore, it has been ascertane 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.