Change of precipitate free zone during long-term creep in 2.25Cr-1Mo steel

This work presents a residual life assessment method based on the quantitative analysis of changes in the fraction and width of a precipitate free zone (PFZ) around grain boundaries during long-term creep. Such changes resulted from precipitation and coarsening of carbides on grain boundaries and within the ferrite matrix during long-term creep exposure. Testing was conducted at creep temperatures ranging from 450 to 650 °C, for a maximum time of 142415.1 h. A relationship between the fraction of grain boundary with PFZ, PFZ width, and the Larson-Miller Parameter (LMP) was established, and the mechanism of PFZ formation was discussed. The increase in width, governed by the diffusion of alloy elements to grain boundaries, was mathematically modeled. The relationship between the width and creep exposure time was fitted to a square root law, and the PFZ broadening rate constant and activation energy for width growth were calculated. The proposed model will be applicable to determine the operating parameters of 2.25Cr-1Mo steel components in high-temperature equipment.