Research On Evaluation Criteria For Creep Of Key Structure Of Steam Reformer Based On Strain Classification

With the continuous improvement of the operating parameters of modern industrial equipment,industrial equipment has been working under high temperature and high pressure environment for a long time,which will cause creep damage.The traditional structural strength checking method is to check the stress of the equipment,but long-term creep damage will cause a large amount of deformation of the component,consume the ductility of the component,and cause the structural function to fail.In view of this,the high-temperature design standard puts forward the idea of restricting structural strain,and gives relevant restrictive clauses.The American ASME-NH subsection and the French RCC-MR standard both limit the inelastic film strain and inelastic film+bending strain of high-temperature components,but they do not distinguish between the performance differences between different materials and do not consider them.The impact on the structure when there is stress multiaxial effect in welded structure and non-welded structure.In view of this,to carry out the research on the assessment criteria of the critical structure of the steam reformer gas collector,which has industrial production application value.In this paper,the collecting pipe structure made of 20Cr32Ni1Nb alloy material and its welding structure are the research objects.The microstructure and fracture specimens of the material are analyzed.The method of combining continuum damage mechanics and finite element analysis is adopted,from the stress linearization Theoretical derivation,experimental research and finite element analysis and calculation are carried out in three aspects.The main work and conclusions are as follows:(1)Based on the stress linearization method in the finite element software,the selection of the stress classification line,the conversion of the local coordinate system and the formula for calculating the classification stress are studied.Compile the classified strain calculation formula into MATLAB program;compare the classified stress results calculated by ABAQUS and MATLAB.The results show that the difference between the film stress and the bending stress in the two calculation methods is no more than 5%,which solves the problem that ABAQUS cannot output the classified strain.(2)The creep performance of 20Cr32Ni1Nb alloy under different stress levels at 890?was studied,the creep deformation characteristics were analyzed,and the microstructure observation and fracture analysis of the creep samples were carried out using a metallurgical microscope and a scanning electron microscope.The results show that the creep behavior is composed of three stages with different creep rates,and the deterioration of the microstructure is the main cause of creep damage,which is manifested in the coarsening and growth of NbC and M₂₃C₆carbides.(3)The material parameters in the creep damage constitutive equation are optimized based on MATLAB;the UMAT subroutine is developed based on FORTRAN language;the results show that the optimized model material parameters predict the creep curve and the experimental curve fit well.When the creep ends,the strain of the film reaches the limit first,and the damage does not reach the critical value.Therefore,from the perspectives of damage mechanics and strain check,in the presence of stress multiaxial effects,the strain of 1%film in ASME NH is related to 2 The limit of%film+bending strain is conservative and can be used for strength verification under multi-axial conditions.(4)Based on the 20Cr32Ni1Nb alloy material,a finite element model was established with different welding forms as variables,and the effect of different welding angles and heat-affected zone widths on the classification strain results was studied.The results show that the maximum damage value always remains in the heat-affected zone during the creep process,and the weld groove form,groove angle and heat-affected zone width have little effect on the classified strain results.From the perspective of damage and failure,when the design life is reached,the damage value is far less close to the limit than the strain of the film.From the perspective of the classification strain assessment,the strain of the film has exceeded 50%of the limit.Therefore,when assessing the strength of high-temperature welded joints,First look at the size of the classification strain value.