A simulation of softening during creep exposure of grade 91 steel in a time range over 100,000 h around 600?C

Hardness reduction is a useful indicator of microstructural degradation progressing in Gr. 91 steel during its creep exposure. Grade 91 steel is strengthened by its fine and elongated subgrain microstructure, and its hardness decreases with increase in its subgrain width. Subgrain width of the steel is stabilized by pinning force from M23C6 particles. Aggregation of the particles due to thermal aging (without creep) brings about coarsening of subgrain width (thermal recovery). Creep deformation assists the aggregation, accelerating the thermal recovery during creep. It also directly induces coarsening of subgrain width. These three mechanisms cause hardness reduction during creep exposure of the steel. On the basis of this model, hardness reduction during creep exposure of Gr. 91 steel in a time range over 100,000 h around 600 degrees C is simulated, and results of the simulation are compared with hardness of steam pipes measured after their service exposures. The simulation can well explain the hardness reduction of normalized and tempered Gr. 91 steel. The model is also verified with hardness measurement during thermal aging up to 233,000 h.