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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]

In order to predict the long-term creep life of P91 steel, this work proposed a coupled model, combining the error-trained back-propagation artificial neural network (BP-ANN) and an improved θ model. Both the short-term rupture life obtained by creep experiments and the related creep data of National Institute for Materials Science (NIMS) database were used to validate the above model. In this model, creep parameters (temperature, stress) were used as the input and relative error of 5 θ model extrapolations as the output. Then, the prediction errors of BP-ANN method were introduced to correct the 5 θ model extrapolation results to achieve a more accurate prediction. Consequently, the long-term creep life of P91 steel was predicted. It demonstrated that with the proposed model, the prediction capability for long-term creep life of P91 steel with 200,000 h is much better than those of other θ methods, which has a prediction uncertainty less than 5%. • A model coupled the improved 5 θ model with BP-ANN was proposed. • The proposed model based on error-training predicted the long-term creep life of P91 steel successfully. • The prediction accuracy of proposed model was much better than those of the 4 θ and 5 θ models. [ABSTRACT FROM AUTHOR]

In this study, the change of geometrically necessary dislocation (GND) has been investigated during the elevated temperature creep of P91 steel. Firstly, the crept microstructures of P91 steel at 873 K and 165 MPa were characterized by electron backscatter diffraction (EBSD). Then, the resultant GND distributions were analyzed in detail. Meanwhile, the GND density was quantitatively estimated by two methods of kernel average misorientation (KAM) and Nye's dislocation density tensor, respectively. The obtained results indicated that during creep of P91 steel, the values of GND density calculated by the KAM method are highly consistent with those of Nye's dislocation density tensor method. Besides, the GND density increases significantly during the initial creep stage and attains the maximum approximately at the transition from initial creep to steady creep. During the subsequent creep, it gradually decreases. In addition, it suggested that the internal stress predicted by the obtained GND density is consistent with previous experimental data. Accordingly, it further demonstrated that the present characterization of GND evolution offers a convenient tool for the microstructure degradation analysis of P91 steel during creep. • The microstructure of P91 steel during creep has been investigated by EBSD method. • The change of GND density was quantitatively characterized by KAM and Ney's dislocation density tensor, respectively. • The GND densities obtained by the above methods are highly consistent. • The GND densities vary closely with the creep stage, resulting from the interaction between dislocation and precipitates. [ABSTRACT FROM AUTHOR]

The reliable prediction of long-term creep life is of great importance in order to ensure the service safety of high-temperature components. In this work, the novel Wilshire-Cano-Stewart (WCS) relationships were introduced to model the creep behavior and predict the long-term creep life of P91 steel based on the short-term creep data. The accelerated creep experiments were firstly carried out at the temperature range of 580–620 °C, while the stresses varied from 135 to 200 MPa. According to the analyses of normalized-mean-squared error (NMSE) and mean logarithmic error (MLE), it demonstrated that the WCS model can express all the obtained creep data very well due to the average relative errors lower than 10%. Meanwhile, these values are much lower than those of traditional Time-Temperature-Parameter (TTP) models. In addition, the WCS model can provide the reasonable estimates of 200,000 h creep life with accelerated creep measurements lasting up to only 2000 h. • WCS model made full use of Wilshire and Sinh model respective advantages and eliminated known weaknesses. • Results of WCS model are better than the traditional L-M model in life prediction. • Results of WCS model are accurate enough in life prediction. [ABSTRACT FROM AUTHOR]