The Role of Thermomechanical Processing in Creep Deformation Behavior of Modified 9Cr-1Mo Steel

In this study, to refine the microstructure and enhance the mechanical properties, thermomechanical treatment (TMT) was performed on modified 9Cr-1Mo steel. The creep deformation behavior of TMT processed steel and the steel in its normalized and tempered (NT) state were studied for different stress levels at 923 K (650 °C). Transient, secondary, and tertiary creep regimes were analyzed for both conditions of the steel based on the empirical equation \( \varepsilon = \varepsilon_{0} + \varepsilon_{\text{t}} \left( {1 - e^{ - rt} } \right) + \varepsilon_{\text{s}}^{ \cdot } t + \varepsilon_{\text{L}} e^{{p\left( {t_{\text{t}} - t_{\text{r}} } \right)}} \). The rate of exhaustion of primary creep (r), r with time to reach the minimum creep rate, minimum creep rate\( \left( {\varepsilon_{ \hbox{min} }^{ \cdot } } \right) \), \( \varepsilon_{ \hbox{min} }^{ \cdot } \) with time spent in the secondary creep regime, rate of acceleration of tertiary creep (p), p with time to reach the onset of tertiary creep, and creep rate with applied stress exhibited a proportional relationship in both conditions of the steel. This proportionality existence in the transient and tertiary creep deformation obeyed the first-order reaction rate kinetic theory. The enhanced MX (M = V, Nb; X = C, N) precipitation in the TMT steel significantly decreased the creep deformation rate and extended the secondary stages of deformation. TMT processing of modified 9Cr-1Mo steel led to a significant increase in the creep rupture strength through the stable microstructure.