Creep crack propagation at elevated temperature in a heat-resistant steel

Abstract

The creep crack propagation behaviour of a 25 Cr-20 Ni heat-resistant steel at 1103 to 1163 K has been studied using a CT-specimen with a thickness of 3 to 9 mm. With increasing specimen thickness, the crack growth rates increase in the thickness range 6 to 9 mm but remain almost constant in the range 3 to 6 mm. The temperature dependence of crack growth rates can be related to a thermally activated process of creep crack propagation. A creep mechanism is suggested to be the rate controlling process of creep crack propagation. The activation energy of creep crack propagation increases with increasing stress intensity factor. The effect of microstructure on crack growth rates shows that the as-cast specimen has a much higher crack growth rate than specimens pre-aged for 1500 to 8000 h and the specimen aged for 5000 h has the optimum crack propagation resistance. The characteristics of creep crack propagation are explained by the variation of microstructure with ageing, especially the size, distribution and stability of secondary carbides and the morphology of eutectic carbides.