The fracture mechanics approach was used to investigate creep crack growth phenomenon in 2.9Ni - 1.1Cr steel. The tests were conducted in air at 550℃ using compact tension specimen with two different hardness levels. The crack growth rates were investigated in terms of the stress intensity factor K, effective stress σeff, and the strain energy rate parameter $C^{\ast}$ - integral.
In general, the $C^{\ast}$ - integral turns out to be a better loading parameter than K or σeff even though uncertainties remain with the specimen with lower hardness level. At a given $C^{\ast}$ value, crack growth rate tends to increase with matrix hardness.
Microscopic observation of crack tip region by SEM revealed the presence of near-coalesced grain boundary cavities, and this is consistent with the model that the macroscopic crack advances by the linkage of coalesced cavities with the main crack. The amount of accumulated damage, measured in terms of cavities area fraction, tended to increase with the applied $C^{\ast}$, and the same was true with the size of the damage zone. The size of cavities ahead of the main crack was found to be much smaller than that out of the smooth bar creep test. This point was discussed by stress and stress state ahead of a comparing the sharp crack and smooth bar under creep conditions.