Fracture mechanisms and properties for irradiated austenitic chromium-nickel steel over elevated temperature range and formulation of intergranular fracture criterion

Abstract

The fracture mechanisms of austenitic 18Cr-9Ni steel have been investigated over the temperature range from 200° to 600 °C using the following states: (1) after neutron irradiation at temperature of 400 °C up to damage dose of 30 dpa; (2) after neutron irradiation and subsequent aging at 550 °C for 3000 h. The fracture properties and mechanisms have been determined under uniaxial tension of smooth round and notched round bars, i.e. for various stress triaxialities. Sharp decrease in the fracture strain and transition to intergranular fracture have been revealed for smooth round bars over high temperature range that indicates high temperature radiation embrittlement. Over the same temperature range the fracture strain is larger for notched round bars than for smooth round bars, moreover a portion of intergranular fracture is also larger. This finding is rather abnormal as the fracture strain usually decreases when stress triaxiality and intergranular fracture portion increase. On the basis of the obtained experimental results, for the first time, the fracture stress-controlled criterion and fracture model have been developed over temperature range of high temperature radiation embrittlement. The proposed model allows one to explain larger value of the fracture strain for notched round bars as compared with smooth round bars and also to predict fracture toughness over range of high temperature radiation embrittlement. The formulated criterion and model have been verified by comparison of the calculated and experimental values of fracture toughness for 18Cr-9Ni steel irradiated up to damage dose of (24–30) dpa. Experimental values of fracture toughness have been obtained from compact tension CT-0.5 specimens tested at 200 °C and 600 °C.