Effect of thermal history on tensile and fatigue crack growth properties of a cryogenic austenitic stainless steel forged at room and liquid helium temperatures
Konosu Shinji; Kishiro Tomohiro; Ivano Ogi; Nunoya Yoshihiko; Nakajima Hideo; Tsuji Hiroshi
Abstract: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 degree, 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. Key Words:fatigue, crack growth, crack closure, liquid helium temperature, sensitizing heat treatment, austenitic stainless steel forging