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 degrees C, 600 degrees C, and 650 degrees 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 neff 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.