Research On Creep Damage And Microstructual Evolution Of Power Station Components Under Complex Stress States

With the global energy shortage,environmental degradation,climate warming and other issues become increasingly prominent,the development of low energy consumption,low emissions,low pollution,low-carbon technology has become a consensus.Super critical and ultra-super critical units have the advantages of high power generation efficiency and low pollutant emission,and have become one of the vital part for national power industry to develop low-carbon technology.Large-capacity,high-parameter units mean that the service conditions of the power unit components are even worse.Due to the geometric dimension,the type of load,the environment and so on,the actual components are mostly subjected to complex stress states.According to the statistics,creep is one of the main failure reasons for the superheaters,reheaters,their headers and the like.With the excellent weldability,high-temperature strength and creep performance,P92 steel becomes one of the commonly used steels in super and ultrasuper critical coal-fired units.It is of great significance for units' safe and economic operation to investigate the creep damage propagation and microstructural evolution of P92 steel under complex stress states,which is the core part of residual life prediction of high-temperature components of coal-fired generating set.Based on the verification among high-temperature creep experiment,creep constitutive model and finite element numerical simulation,the high-temperature creep damage growth and microstructural evolution under complex stress states were studied.The specific work of the paper is as follows:Firstly,the standard uniaxial creep experiments were conducted on smooth specimens under different temperatures and stresses,and the creep rupture mechanism of the smooth specimens was analyzed from the microscopic point of view.Then the Norton-Bailey and Kachanov-Robotnov creep constitutive model was established and verified.The uniaxial creep experimental results provided the basic database for the follow-up research.Secondly,the multiaxial creep experiments and constitutive models were studied.The creep experiments were conducted on double-notched specimens under different stresses and notch acuity ratios,and the creep experimental results were analyzed from both macroscopic and microscopic points of view.The modified Kachanov-Robotnov model was used to predict the creep damage development of P92 notched specimens,and the stress and damage distributions and their evolutions were obtained.Based on the theory of strain depletion,the concept of ductile depletion was introduced.A ductile depletion model which was able to describe creep behaviors of P92 steel was established and verified,which made it possible to connect the continuous damage mechanics with the cavity growth theory.The effects of multiaxiality on fracture ductility,micro-cavity and hardness were concluded.Thirdly,based on the creep damage mechanism of P92 steel,the damage variables with actual physical meanings were introduced,and a creep constitutive model with multiple damage variables was established.The model parameters were determined,and the validation of the model was verified by uniaxial creep experimental results.The interrupted creep experiments were carried out on smooth and notched specimens.The precipitatiosn and cavities were quantified.The model was applied to predict notched creep experiments,which showed that the model predicted result agreed well with the experimental ones.The validity of the model was further proved.Finally,the full-scale creep experiment on pipe bend was designed,with most of the preparations completed,such as the measurements of stain,temperature and the like.The influences of unequal wallthickness and initial ellipticity on creep damage evolution were investigated by use of the finite element numerical software,which would provide data base for the subsequent full-size creep test.