Creep Behavior, Deformation Mechanisms, and Creep Life of Mod.9Cr-1Mo Steel

Abstract

The creep behavior, deformation mechanisms, and the correlation between creep deformation parameters and creep life have been investigated for Mod.9Cr-1Mo steel (Gr.91, 9Cr-1Mo-VNb) by analyzing creep strain data at 723 K to 998 K (450 °C to 725 °C), 40 to 450 MPa, and t _r = 11.4 to 68,755 hours in NIMS Creep Data Sheet. The time to rupture t _r is reasonably correlated with the minimum creep rate ${\dot{\varepsilon }}_{ \hbox{min} }$ and the acceleration of creep rate by strain in the acceleration region dln${\dot{\varepsilon }}$ /d ε, as t _r = 1.5/[${\dot{\varepsilon }}_{ \hbox{min} }$ (dln${\dot{\varepsilon }}$ /d ε)], where ${\dot{\varepsilon }}_{ \hbox{min} }$ and dln${\dot{\varepsilon }}$ /d ε reflect the creep behavior in the transient and acceleration regions, respectively. The ${\dot{\varepsilon }}_{ \hbox{min} }$ is inversely proportional to the time to minimum creep rate t _m, while it is proportional to the strain to minimum creep rate ε _m, as ${\dot{\varepsilon }}_{ \hbox{min} }$ = 0.54 (ε _m/t _m). The ε _m decreases with decreasing stress, suggesting that the creep deformation in the transient region becomes localized in the vicinity of prior austenite grain boundaries with decreasing stress. The duration of acceleration region is proportional to the duration of transient region, while the dln${\dot{\varepsilon }}$ /d ε is inversely proportional to the ε _m. The t _r is also correlated with the t _m, as t _r = g t _m, where g is a constant. The present creep life equations reasonably predict the degradation in creep rupture strength at long times. The downward deviation takes place in the t _r vs ${\dot{\varepsilon }}_{ \hbox{min} }$ curves (Monkman–Grant plot). At the same ${\dot{\varepsilon }}_{ \hbox{min} }$, both the ε _m and t _m change upon the condition of t _m ∝ ε _m. The decrease in ε _m with decreasing stress, corresponding to decreasing ${\dot{\varepsilon }}_{ \hbox{min} }$, causes a decrease in t _m, indicating the downward deviation of the t _r vs ${\dot{\varepsilon }}_{ \hbox{min} }$ curves.