Kinetics and mechanisms of creep crack growth in a creep-resisting steel

Abstract

1. In EI415 steel, rapid subcritical creep crack extension Δa=0.5–2 mm at 103 h at a temperature of 560°C is detected at KI=0.15KIc, at T=510°C it is detected at KI=0.25KIc, and at T=460°C at KI=0.3KIc. 2. At comparatively high values of SIF, creep crack propagation takes place by means of the development of intergranular microcracks propagating in the opposite direction from the region positioned at a distance of 2–6 grain diameters from the crack tip, and is of jumplike nature. 3. At comparatively low values of SIF, the creep crack propagates by means of disintegration of the material of the crack tip. Voids appear both at the grain boundaries and in the body of the grain. It is evident that crack growth is almost continuous. 4. In intergranular failure, the incubation period of crack propagation, i.e., the time to the first jump, represents a large part of the total life, whereas at low crack propagation rates, the incubation period has only conventional meaning and amounts only to a small part of the total life. 5. Running for 137, 000 h at 520°C causes no marked reduction of the resistance of EI415 steel to the propagation of creep cracks.