Dislocation-based modeling of long-term creep behaviors of Grade 91 steels

To meet the 30 years (∼263,000 h) design lifetime of a typical thermal power plant, understanding and modeling of the long-term creep behaviors of the power plant structural steels are critical to prevent premature structural failure due to creep. The extremely long exposure to high operation temperature results in evolving microstructure during service, which cannot be observed in standard short-term (<1 year) creep tests. In fact, creep rupture life predictions based on short-term creep testing data often overestimate the creep rupture times for long period of time, and are therefore insufficient to provide a reliable failure time prediction. In this article, a microstructure-sensitive, long-term creep model is developed and validated against existing long-term creep experimental data (∼80,000 h) for ferritic steel Grade 91 (Fe-8.7Cr-0.9Mo-0.22V-0.072Vb-0.28Ni in wt.%). The mechanistic creep model is based on fundamental dislocation creep mechanisms – the particle bypass model based on dislocation climb from Arzt and Rösler [1] - that describe the steady state creep strain rate as a function of stress, temperature, and microstructure. In particular, the model incorporates the evolution of the particle size via coarsening into the dislocation climb theory, dislocation detachment mechanism and back-stress generated by subgrain dislocation structures. Model inputs were obtained based on the microstructure information obtained from published literature from the National Institute of Material Science (NIMS, Japan) creep database. The model, which can be used on many types of alloys, shows excellent agreement with existing long-term creep experimental data (∼80,000 h) of Grade 91 ferritic steel.