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In present work, the creep rupture life of 9% Cr heat-resistant steel (P91) was evaluated based on the creep cavitation modelling. At 873 K, a series of creep rupture tests were carried out under the stresses of 140–190 MPa to achieve several sets of ruptured specimens. The constrained and unconstrained models of cavity growth were introduced to investigate the creep behaviour of P91 steel. The obtained results showed that the rupture mechanism of these specimens was mainly attributed to the ductile rupture under the present creep conditions. Besides, reasonable extrapolations were calculated to predict the creep rupture life of P91 steel by combining different constitutive equations with the cavity growth model. It suggested that the model predictions with constraints were better than those with unconstrained conditions. In particular, the Sinh equation based constrained cavity model shows greater potential in predicting the long-term creep life of P91 steel. [ABSTRACT FROM AUTHOR]

The present work deals with the microstructure evolution during creep of P91 steel. In particular, the changes of prior austenite grain (PAG) and low-angle grain boundary (LAGB) during creep were studied using the electron backscatter diffraction (EBSD). Firstly, a series of crept specimens of P91 steel were prepared at 893 K under a constant stress of 145 MPa. Then, the EBSD measurements of these specimens were carried out. Subsequently, the PAGs were reconstructed successfully with the measured EBSD data, and the resultant distribution of LAGBs was calculated. Consequently, the obtained results suggested that the size of PAGs has not changed significantly during the whole creep. For the distribution of LAGBs, however, the peak shifted obviously to high angles during creep. In addition, it demonstrated that the EBSD method offers an effective and quantitative tool to characterise the microstructure evolution during creep of P91 steel. [ABSTRACT FROM AUTHOR]

The creep rupture behaviours and microstructural changes of a modified 9Cr-1Mo heat-resistant steel were investigated at 853 K. Analysis of creep results suggests that dislocation climb is the dominant deformation mechanism with true stress exponent of 5 under the present conditions. Based on the microstructural analysis, strengthening contributions from M23C6 carbides and MX carbonitrides were clarified. The M23C6 carbides can promote grain boundary strengthening by exerting Zener pinning forces, whereas MX carbonitrides can enhance the creep strength by interacting with mobile dislocations to induce threshold stress. Besides, softening of the steel is related not only to the decrease of dislocations, but also the coarsening of precipitates and substructures. The value of creep damage tolerance factor is close to 6.6, which further confirms that the creep damage is mainly attributed to the microstructural degradations, such as the coarsening of precipitates and substructures and decrease of dislocations. [ABSTRACT FROM AUTHOR]