Kinetics And Characterization Of Microstructure Evolution During Creep Of P91 Steel

Due to the increasingly prominent problems of environmental pollution and energy shortage,ultra-supercritical power plants have become the inevitable requirement of thermal power in China.To ensure reliable operation of the related infrastructure,it is necessary to accurately evaluate the service states of the components in time.Based on the systematic investigation of the evolution behaviors of P91 steel during creep,a modified Continuum Damage Mechanics-CDM model was proposed by combining with evolution kinetics.Then,the short-time creep behavior of P91 steel at 600? was analyzed with this model in detail.Subsequently,two non-destructive evaluation methods of X-ray diffraction and ultrasonic testing were employed to characterize the change of dislocation density.Consequently,the mechanisms of related microstructure evolution during creep of P91 steel were discussed in detail.Besides,the beneficial exploration was provided to evaluate the creep state and predict the remaining life of components under elevated temperature.The specific research contents and main achievements are as follows.(1)The elevated-temperature mechanical properties of P91 steel at 600? were measured by elevated-temperature uniaxial tensile test,for instance,yield strength ?0.2,tensile strength ?b and elastic modulus E.Then,the complete creep curves,creep rupture time and damage specimens with different creep states of P91 steel under given conditions were obtained by the continuous creep tests and interrupted creep tests.(2)The change of microstructure of P91 steel under different creep states was investigated by means of optical microscope and scanning electron microscope.The results indicated that creep is closely related to the evolution of various microstructures.Meanwhile,degeneration of martensitic structure,condensation and coarsening of precipitated phase,nucleation and growth of void were the major reasons for the creep rupture of P91 steel.(3)On the basis of analyzing the evolution behaviors of creep,the modified CDM model was proposed combining with kinetics analysis,and the short-time creep behavior of P91 steel at 600? was also simulated.Then,the effectiveness of the modified CDM model was verified by experimental data.The result showed that the modified CDM model based on microstructure evolution can be used to simulate the creep behavior of P91 steel at 600?fairly well.(4)The relationships between different characterization parameters and creep time were obtained using X-ray diffraction analysis and traditional ultrasonic testing.The results showed that the FWHM increased firstly and then decreased during creep.Besides,the ultrasonic attenuation coefficient fluctuated with the increase of creep time.(5)Combining with the related theories,the relationships between the change of FWHM and ultrasonic attenuation coefficient and microstructure were analyzed.The mobile dislocation density firstly decreases and then increases.The pinning dislocation density peaks at the beginning of the second stage and the valley appears at the beginning of the tertiary stage.Finally,the physical mechanisms of related microstructure evolution behaviors during creep were analyzed using the simulated results by modified CDM model.