A PHENOMENOLOGICAL MODEL FOR CREEP CRACK GROWTH BEHAVIOR IN FERRITIC STEELS
A micro-mechanics model is described that rationalizes the effects of test temperature and microstructural variables such as grain boundary particles on the creep crack growth rate (CCGR) behavior of ferritic steels. The model predicts that as the average spacing between particles that initiate creep cavities on grain boundaries decreases, the CCG rates are expected to increase. CCGR data at several temperatures can be collapsed into a single trend using the newly proposed temperature compensated creep crack growth rate plot derived from the proposed model. The applicability of the model is demonstrated for Grade 22 and Grade 91 steels using extensive amounts of data available in the literature on new and service degraded conditions. The trends from the model are compared to CCGR data trends noted in fitness for service codes. It is shown that differences between the CCGR behavior of Grade 22 steel in new and ex-service are negligible in the base metal region but not near weldments. Significant differences were observed between new and ex-service materials in the CCGR behavior of Grade 91 materials even in the base metal region.