The stress exponent n1 of the minimum creep rate \(\dot{\varepsilon }\) min for Gr.91 steel with tempered martensitic microstructure is evaluated to be n1 ≳ 10 and n1 ≈ 4 to 7 at high and low stresses, respectively, at 823 (550) to 923 K (650 °C). The time to minimum creep rate tm increases with decreasing stress, while the strain to minimum creep rate εm is constant of 2.2 pct at high stresses of 160 to 140 MPa and decreases with decreasing stress below 140 MPa at 873 K (600 °C). The constant εm suggests homogeneous creep deformation, while the decrease in εm with decreasing stress suggests localized creep deformation, presumably in the vicinity of prior austenite grain boundaries (PAGBs). The n1 is evaluated to be 11 and 4 in the high and low stress regions, respectively, at 823 K (550 °C) for 2.25Cr-1Mo steel specified as ASTM A542/A542M with tempered martensitic microstructure. There is no sign of reduction of n1 in the low stress region in 2.25Cr-1Mo steel specified as JIS STBA 24 with ferrite/pearlite microstructure containing low dislocation density. The localized creep deformation is responsible for the reduction of n1 and apparent activation energy Q for the \(\dot{\varepsilon }\) min in the low stress region, although the creep deformation is due to the mechanism of dislocation creep in both the high and low stress regions. High density of dislocations in the tempered martensitic microstructure promotes the recovery of dislocations in the vicinity of PAGBs during creep, resulting in localized creep deformation.
