A Phenomenological Model for Creep and Creep-Fatigue Crack Growth Rate Behavior in Ferritic Steels

A model to rationalize the effects of test temperature and microstructural variables on the creep crack growth (CCG) and creep-fatigue crack growth (CFCG) rates in ferritic steels is described. The model predicts that as the average spacing between grain boundary particles that initiate creep cavities decrease, the CCG and CFCG rates increase. Further, the CCG data at several temperatures collapse into a single trend when a temperature compensated CCG rate derived from the model is used. The CCG and CFCG behavior measured at different temperatures is used to assess the effects of variables such as the differences between the base metal (BM), weld metal (WM), and heat-affected zone (HAZ) regions. The model is demonstrated for Grade 22 and Grade 91 steels using data from literature. It is shown that differences between the CCG behavior of the Grade 22 steel in new and ex-service conditions are negligible in the BM and WM regions but not in the HAZ region. The CCG behavior of the Grade 91 steels can be separated into creep-ductile and creep-brittle regions. The creep-brittle tendency is linked to the presence of excess trace element concentrations in the material chemistry. Significant differences found in the CCG rates between the BM, WM, and HAZ regions of the Grade 91 steel are explained.