Analysis Of Creep Behavior Of P91 Steel At High Temperature Based On Deformation Mechanism

Nowadays,under the background of energy inadequacy and heavy environmental pollution,thermal power generation will still occupy a dominant position for a period of time in China.The working parameters of thermal power units have been continuously improved,and have developed from medium pressure units in the past to the current ultra-supercritical units.Due to its excellent high temperature endurance,corrosion resistance,high thermal conductivity and excellent economic performance,P91 steel has been widely used in key components of ultra-supercritical units,such as turbine rotor,main steam pipe and intermediate heat exchanger.However,these components are susceptible to different degrees of creep damage and performance degradation due to long-term exposure to high temperature and pressure,which brings potential problems to the operation of the unit.Therefore,it is particularly important to carry out creep damage analysis and life prediction for equipment.In this paper,P91 tempered martensitic heat-resistant steel was used as the research object.Firstly,high-temperature tensile tests and creep endurance tests were conducted under different conditions.Next,the true stress creep model at different temperatures was established according to the test data,and the creep behavior under different conditions was studied systematically.Finally,the creep acceleration stage of P91 steel was further analyzed by coupled the nucleation and growth of cavities in the above model.The main research contents and conclusions are as follows:The mechanical property curves and parameters(such as modulus of elasticity E,yield strength ?0.2 and tensile strength ?b)of P91 steel at different temperatures were obtained by high-temperature tensile tests.Then high-temperature endurance creep tests were conducted on P91 steel with stresses selected around 2/3 ?0.2,and complete creep endurance curves and creep rate curves were obtained.On this basis,the creep damage mechanism of P91 steel under the conditions of this paper was preliminarily elucidated by introducing the damage tolerance factor parameter-due to the deterioration of the internal microstructure of the material.According to the change of true stress in creep process,the creep model of true stress and strain was established by combining the creep deformation mechanism of grain boundary sliding,intragranular dislocation climbing and dislocation gliding.Then,the creep behavior under different conditions was analyzed by solving the model with the above creep endurance test results,and the creep fracture life was predicted by using the model.The results show that true stress-strain creep model is in good agreement with the test result;the calculated life at creep fracture is in good agreement with the test measurements.The dominant mechanism in the creep process was analyzed by combining the true stress-strain creep model and fracture morphology observation.The results show that grain boundary sliding contributed very little to creep fracture strain in the creep process under test conditions,which indicated that the creep is mainly dominated by intragranular dislocation movement.The analysis of the specimen fracture morphology by Scanning Electron Microscope also shows that there were dimples clustered near the cavity,which can be judged as a typical ductile fracture.And the fracture surface obviously shows a through crystal fracture,which is consistent with the calculation results.The rapid accumulation of strain in the third stage during creep was caused by micro structure deterioration and the nucleation and growth of cavity.Therefore,the true stress-strain creep model was improved by coupled the cavity nucleation and growth in the creep process.The results show that compared with the above true stress-strain creep model,the model coupled cavity is nearer to the particular creep curve,particularly for the creep acceleration stage.