| P91 steel has been widely used in main steam pipes and other structures of thermal power plants due to its excellent creep resistance at high temperature and good weldability.However,under the long-term coupling effect of temperature field and stress field,creep damage inevitably occurs in P91 steel,which becomes an important factor restricting its structural integrity and operational reliability.Therefore,it is necessary to investigate the creep behaviors of P91 steel in order to ensure the safe operation of these components.In this paper,the high temperature creep behavior of P91 steel has been studied by means of finite element numerical calculation on the basis of experimental and theoretical analysis.Firstly,creep rupture tests and interrupted tests were carried out under different conditions of temperatures and stresses.Then,the creep constitutive equation at different temperatures was constructed by further analysis of experimental data.Finally,the uniaxial and multiaxial creep behaviors of P91 steel were analyzed systematically by using the secondary development function of ABAQUS finite element software.Specific research contents and main conclusions are as follows:1)By the elevated temperature uniaxial tensile tests,the mechanical properties of P91steel can be derived at different temperatures.At 580?,the yield strength,ultimate tensile strength,and elastic modulus E are obtained to be 281 MPa,365 MPa and170.065 GPa.At 620?,the yield strength,ultimate tensile strength,and elastic modulus E are obtained to be 215 MPa,307 MPa and 107.541 GPa.Then,the applied stresses were selected at 2/3±10MPa at each temperature.By the creep rupture tests,creep life can be obtained at various temperatures and stresses.At 580?,the creep rupture time values were 631.9 h,285.8 h,139.2 h under the stresses of 180 MPa,190 MPa,200MPa.At 620?,the creep rupture time values were 305.1 h,110.4 h and 46.5 h,respectively.It is noted that both temperature and stress have significant effects on creep life.2)The microstructures of crept specimens and fractography of ruptured specimen were investigated at 620?,145 MPa.It reveals that the coarsening of precipitates and formation of voids are dominant creep damage mechanism under the investigated conditions.Afterward,the micromechanics based creep damage constitutive equation of hyperbolic sinusoidal model is established by fitting the complete creep curves.And a creep damage constitutive equation under multiaxial stress is constructed by generalized processing.3)Based on the above constitutive model of hyperbolic sinusoidal creep damage,the uniaxial creep behavior of P91 steel under different conditions was simulated by means of the secondary development function of ABAQUS finite element software-UMAT subroutine.The results show that the calculated results are in good agreement with the experimental data.Meanwhile,the extrapolated results of long-term creep behavior under low stress condition are compared with ECCC data,which further indicates that the creep life of 10?5 h can be predicted accurately by this method.4)The creep behavior of pipelines with multiaxial stress state in practical engineering application is simulated and calculated by ABAQUS finite element software combined with the above UMAT subroutine.The results show that,the creep behavior of such components is different in radial and circumferential directions due to the multiaxial and uneven distributions of stress.Meanwhile,the wall thickness decreases with time,which further accelerates the creep damage process.In summary,the creep behavior of P91 steel was systematically studied by constructing creep damage constitutive equation on the basis of creep experiments and simulation with ABAQUS finite element software.It is also a useful attempt to further study the creep damage behavior of pipeline components. |
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