Research On Creep Behavior And Model Of Internally Pressurized P92 Steel Under Multi-axial Stress State

With the gradual deterioration of the environment and energy structure changes,the power generation enterprises are required to save energy and reduce the emission.The development of ultra-supercritical units with high parameters steam becomes the mean target of the construction and transformation of thermal power plants.With the increasing of steam parameters,high temperature heating surface of power plants’ boiler requires high material properties.The creep performance is one of the most important factors that affect the service life of high temperature and pressure pipelines.At present,many scholars at home and abroad are doing lots of research on P92 steel and other heat resistant steel.These studies are mostly based on uniaxial creep test.However taking actual operation conditions of the power plant into account,the components are of multiaxial stress state.Then a large number of notched specimen multiaxial creep tests are conducted.Although the notched specimens can form a certain range of multiaxiality of stress state at the notch portion,the multiaxiality formed is not well distributed and is of poor controllability to explore the creep life of materials under multiaxial stress state.On the basis of experimental results of uniaxial and notch specimens tests,we design a creep experiment bench wtih P92 steel under the condition of 650℃ temperature and internal pressure.And the axial and circumferential creep development were compared with the uniaxial and notched test results.The fracture surface morphology and microstructure of the specimens were observed and compared with the results of uniaxial and notched tests.Based on the experimental results,the K-R creep model coupled with damage parameters was established.The relevant parameters were obtained by uniaxial creep test.The results of internal pressure experiment were analyzed by the ANSYS software.The results of equivalent stress,first principal stress,and damage distribution and evolution over time are discussed.The influence of multiaxial factor and internal pressure pipe geometry on the multiaxial test results under internal pressure is analyzed by finite element method.The life prediction of thick-walled pipe under internal pressure is proposed based on finite element analysis.The results show that the stress multiaxiality is in the range from 1/3 to 2/3 under the internal pressure condition which is less than that of the and notched specimens under the similar stress level.Due to the constraining effect of the multi-axial stress on void growth,the strain value of the internal pressure specimen is between the uniaxial and notched specimens.The fracture surface of the specimen under internal pressure shows quasi-dissociation characteristics with less dimples.The holes and cracks observed in the microscopic morphology are mostly distributed in the grain boundary and strip boundary perpendicular to the direction of axial stress.On the basis of the damage model,the fracture stress obtained by the finite element analysis can be used to predict the life of the inner pressure pipeline within a certain range.