Interpretation of creep behaviour of a 9Cr-Mo-Nb-V-N (T91) steel using threshold stress concept

The creep behaviour and the microstructural evolution of a 9Cr-Mo-Nb-V(T91) steel were extensively evaluated by means of short term constant load cree p tests and TEM analysis. Statistical analysis of the microstructural data revealed that the precipitated phases M23C6 (where M is a metal, mainly Cr or Fe) and MX (where M is Nb or V, and X is C and/or N) were subject to coa rsening during creep exposure. The coarsening law and its dependence on app lied stress were identified, and the model was used to predict the magnitud e of the Orowan stress at the time corresponding to the minimum creep rate. The minimum creep rate dependence on applied stress at 873 K was described by incorporating the threshold stress concept in a power law with stress e xponent n = 5. In the resulting phenomenological model, the strengthening e ffect of the dispersed phases was thus expressed by a threshold stress prop ortional to the Orowan stress. MST/4310.