On enhancing the creep performance of modified 9Cr-1Mo steel by employing a secondary short-term heat treatment

Creep behavior of an as-received, normalized, and tempered 9Cr-1Mo (P 91) steel and its secondary short-term normalized and short-term tempered counterpart at temperatures of 550 °C, 600 °C, and 650 °C, at stresses in the range of 80–260 MPa, has been studied. The secondary heat treatment leads to insignificant coarsening of the prior austenite grains but refines the martensitic lath sub-grains and the grain boundary M₂₃C₆ carbides and enhances the dislocation density compared to that of the as-received alloy. Tensile creep tests reveal that the creep life of the secondary heat-treated alloy is several folds higher than that of the as-received alloy. For both alloys, considering the effect of threshold stresses at different temperatures, an effective stress exponent n_eff of 4.5–5.5, and activation energies of 316.4 and 258.3 KJ mol⁻¹, it indicates that the steady-state creep relaxation mechanism involves dislocation climb and annihilation. Microstructural characterization, before and after the creep tests, reveals that lath sub-grains and M₂₃C₆ precipitates in both alloys undergo significant coarsening, with the former leading to microstructural instability at the onset of the tertiary stage of creep. Deformation mechanisms in the steady-state regime are described as a balance between processes that impede and facilitate dislocation motion in the sub-grain interior. The interplay between energy minimization-induced sub-grain coarsening and Zener pinning of the boundaries by M₂₃C₆ is discussed in the context of microstructural instability-induced failure.