The fracture mechanics approach was used to investigate creep crack growth phenomena in 3.5 Ni-Cr-Mo-V steels. Creep crack growth tests were performed in air at 550℃. Four kinds of compact tension specimens with different hardness levels and specimen geometries were prepared.
In present investigation, the experimental results were analyzed in macroscopic and microscopic aspects, namely, continuum-mechanical and micromechanical view points. Continuum-mechanical analysis could determine the optimum load parameter to describe creep crack growth macroscopically under given conditions. Micromechanical analysis of crack growth could suggest the realation of cavity distribution and mechanism of crack growth during the test.
It was demonstrated that the $C_t$ parameter was better loading parameter than K or $C^*$-integral in the studied experimental conditions. Attempts to correlate creep crack growth rate to stress intensity factor K and $C^*$-integral were unsuccessful. Microscopic observation of crack tip region in CT specimen revealed the presence of coalesced grain boundary cavities. This observation was consistent with the model of the macroscopic crack advances by the linkage of coalesced cavities with main crack. As the creep crack growth rate increased with applied load, the cavity size and the number of cavities at the crack tip decreased.
