Creep crack growth in 2.9 Ni-1.1 Cr steel has been studied by using compact tension specimens under constant load. Attempts to correlate the crack growth rate $\dot{a}$ with several macroscopic parameter such as stress intensity factor K, effective stress $\sigma_{eff}$ and J-integral, $C^\star$-integral have been made under different testing condition.
The $C^\star$-integral has been found to be the most correlating load parameter. Experimental data indicate that minor differences in microstructure do not significantly influence $\dot{a}$ versus $C^\star$ curve. Also, in the range of 500℃ to 600℃, the $\dot{a}$ versus $C^\star$ curve is not significantly affected by temperature. The implications of these results are discussed and applied to current models which describe the relationship between the crack growth rate and macroscopic load parameter $C^\star$ under different cavitation mechanism. Results show that creep crack growth in 2.9 Ni-1.1 Cr steel is quantitatively described by a model based on creep constrained cavitation of grain boundaries ahead of the crack tip.
Microscopic observation of crack tip region by SEM revealed the presence of near-coalesced grain boundary cavities, which is consistent with the well known theory that the macroscopic crack advances by the linkage of coalesced cavities with the main crack. Damage profile indicates that the cavities area fraction, the cavities radius and the fully coalesence zone size ahead of the crack tip tend to decrease as $C^\star$-integral increase. The cavity size of the Rc 35 specimen was found to be smaller than the Rc 25 specimen, the result of which is analyzed in terms of the stresses near the crack tip.
