| In fossil power plant, P91 steel is widely used under high temperature and pressure condition, organizational structure of material would change due to creep, resulting in reducing the mechanical property deterioration. Most of the equipment have already exceeded its original design life. Meanwhile, in the attempt to improve efficiency, energy saving and reduction of carbon dioxide emissions, operating temperature and load conditions of high temperature components have improved, exacerbating the creep process. Thus it can be seen, in order to ensure the thermal power important equipment of safe and reliable operation, to prevent major disaster accidents. There is an urgent need to study non-destructive evaluation techniques which can be used for both assessment of the current state and prediction of creep life.In this paper, P91 steel is taken as the object of study, creep data is obtained by short-term uniaxial creep rupture test at 600? with different stress. Norton stress exponent n, creep activation energy Qc, Monkman-Grant constant CMG creep variable damage tolerance? were fitted using creep data, and fracture morphology were analysis by scanning electron microscope (SEM), Based on above results, the elevated-temperature creep deformation mechanism and fracture mechanism were discussed. And Larson-Miller parameter method is used to analyze endurance strength with different CLM.Then creep interrupted test were carried out at 600?,175MPa, and electromagnetic acoustic resonance technology (EMAR) were used to characterize the microstructure evolution for different samples in order to extracting characterization parameter. The main contents and conclusions are as follows.(1) High temperature tensile properties of P91 steel at 600 ?:Yield strength ?0.2 is equal to 237.66M Pa, elastic modulus E is equal to 120.884GPa, tensile strength ?b is equal to 320.96MPa, elongation is equal to 0.252%. Creep and creep rate curve at 600? for four stresses of 150,160,165 and 175MPa have been analysed, analysis results showed the creep rupture time tr increased significantly, the minimum creep rate decreased and the fracture strain also decreased with the decreasing of applied stress. And obey the Norton power law constitutive equation.(2) Creep stress exponent n is equal to 9.7, which indicated that creep is dominated by a dislocation movement. Creep activation energy Qc is equal to 670.95kJ/mol, which is much larger than the self-diffusion activation energy, thus it can be inferred that cross-slip is the creep rate control mechanism, and the abundant precipitated particles which impeded effective dislocation movement in the grain boundaries and crystal may be the possible cause of high creep activation energy.(3) P91 steel conformed the relationship of Monkman-Grant and calculated creep damage tolerance ?> 5, which showed creep voids and precipitate coarsening played an important role in the creep rupture, and the fracture belongs to intergranular dimple ductile fracture, which is consistent with the creep fracture morphology observation.(4) The influence of different CLM (calculated 37.86 and empirical values 20,25 and 30) on ? - P curve have been analysed, the results showed that empirical values would lead to large errors for short-term creep life prediction, while calculated value would significantly over-estimated for long-term creep endurance strength extrapolation, thus the present paper suggested the CLM for long-term creep endurance strength extrapolation should be ranging between 25 to 30.(5) Compared with the relative velocity, the attenuation coefficient is more sensitive to creep damage. And there is a clear relationship between the attenuation and the life fraction. In the interval,10 to 20% of the creep life, attenuation experiences a minimum value for the first time,35 to 50% of the creep life, attenuation experiences a peak, and 65 to 80% of the creep life, attenuation experiences a minimum value once more. Attenuation peak shows that creep is at the end of steady-state phase, namely creep is about to enter the acceleration phase. And the minimum of attenuation is parallel to the interim of tertiary creep... |
PDF Full Text Request