An Improved Continuum Damage Mechanics Model for the Lifetime Prediction of HP40Nb Alloy in the Creep Environment

The reformer furnace tube serves as the crucial component in the steam reformer furnace. During the service exposure, the gradual accumulation of creep damage is the primary cause of the final failure of the furnace tube. To evaluate the creep damage evolution and predict the service time, a continuum damage mechanics (CDM) model is established based on the Kowalewski–Dyson model. This model integrates creep stress into the precipitate coarsening damage equation, accounting for its influence on activation energy in Ostwald ripening kinetics. Additionally, the stress‐related uniaxial creep ductility is incorporated into the cavitation damage equation. The material parameter optimization procedure and the associated ABAQUS user subroutine UMAT of the proposed model are also developed. Different from the Kowalewski–Dyson model, the present model can predict the stress dependence of fracture strain and rupture time. The precipitate coarsening damage is predominant in the initial stage, while the cavitation damage accelerates the growth of total damage in the latter stage, ultimately resulting in creep fracture. Eventually, the total damage of the furnace tube made of HP40Nb alloy reaches its critical threshold at 101,450 h.