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

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 degrees C to 725 degrees C), 40 to 450 MPa, and t(r) = 11.4 to 68,755 hours in NIMS Creep Data Sheet. The time to rupture tr is reasonably correlated with the minimum creep rate (epsilon)over dot(min) and the acceleration of creep rate by strain in the acceleration region dln(epsilon) over dot/d epsilon, as t(r) = 1.5/[(epsilon)over dot(min) (dln(epsilon) over dot/d epsilon)], where (epsilon) over dot(min) and dln(epsilon) over dot/d epsilon reflect the creep behavior in the transient and acceleration regions, respectively. The (epsilon) over dot(min) is inversely proportional to the time to minimum creep rate t(m), while it is proportional to the strain to minimum creep rate epsilon(m), as (epsilon) over dot(min) = 0.54 (epsilon(m)/t(m)). The em 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(epsilon) over dot/d epsilon is inversely proportional to the epsilon(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 (epsilon) over dotmin curves (Monkman-Grant plot). At the same (epsilon) over dotmin, both the epsilon(m) and t(m) change upon the condition of t(m) proportional to epsilon(m). The decrease in em with decreasing stress, corresponding to decreasing (epsilon) over dot(min), causes a decrease in t(m), indicating the downward deviation of the t(r) vs (epsilon) over dotmin curves.