Creep crack growth of a 12%CrMoV steel has been investigated with 1/2CT specimens at 823K in the constant load condition. Three kinds of specimens with different hardness levels were obtained from varying the tempering conditions in order to investigate the effects of hardness on the creep crack growth rate.
After load is applied, creep crack growth process is divided into following several steps. Initially, the crack tip is blunted due to extensive plastic deformation, and high stress concentration ahead of the crack tip is relaxed during relatively short period. This seems to have brought about relatively narrow K-controlled region represented in the load parameter map. Then, creep crack growth which is accompanied with the crack tip blunting is continued in minimum crack growth velocity and finally, as a crack length is extended catastrophic crack propagation occurs due to the high stress concentration.
From the correlations between the creep crack growth rate and load parameters it is shown that the most appropriate load parameter which is applicable to this material is $C_t$ parameter, and the relationship between them was
$a-[C_t]^{0.98}$
Effects of hardness on the creep crack growth rate are related to the crack tip blunting, and the creep crack growth rate has been increased at the same K and $C^*$ level with hardness increase.
Fracture surfaces of the specimens were examined in the scanning electron microscope. The creep crack is characterized by a terraced structure at low magnification and the transition from the rather smooth fatigue crack to the creep crack was marked by a step (blunting of the crack tip).
