Controlling factors for creep rates and relating parameters of Gr.91 steel in transient region

The controlling factors for the creep rates in the transient region and for the relating parameters, such as the time to minimum creep rate t_m, strain to minimum creep rate ε_m and minimum creep rate $\dot{\epsilon }$_min, have been investigated for Gr.91 steel at 550, 600 and 650 °C. The creep rates at 0.2 h, an early stage of transient region, decrease with increasing 0.2 % proof stress. The increase in t_m with decreasing stress becomes less significant at low stresses, similar as the time to rupture. The ε_m is approximately a constant of 2.2 % at high stresses above a critical stress, while it decreases with decreasing stress below the critical stress by the localization of creep deformation, presumably near prior austenite grain boundaries. The strong heats exhibit large t_m but small ε_m. At high stress level above the critical stress, the $\dot{\epsilon }$_min is inversely proportional to the t_m as $\dot{\epsilon }$_min = 1.2 x 10⁻²/t_m. At stresses below the critical stress, both the t_m and ε_m change with stress and the $\dot{\epsilon }$_min is given by $\dot{\epsilon }$_min = 0.54 (ε_m/t_m).