| R26 alloy bolts are mainly used in high and medium pressure inner cylinders,high pressure side balance piston steam seals,high pressure stationary blade rings and other parts of thermal power turbine,and the service temperature is up to 650 ℃.In addition to complex environmental media,bolts are often subjected to impact load during service.The severe service environment requires high creep strength,excellent corrosion resistance and stress relaxation resistance of bolt materials.At present,the research data on the microstructure evolution of R26 alloy is very limited,especially the in-depth study on the microstructure change,morphology and distribution of precipitates after long-term service.In this paper,R26 nickel base superalloy was used as the research object.The microstructure changes of bolts after long-term service and their effects on mechanical properties were studied by means of SEM,EDS,TEM and XRD.JMat Pro thermodynamic simulation software is used to analyze the phase transformation law of the material in the heat treatment process.Based on this,a recovery heat treatment process for the bolt after service is designed to improve the microstructure and properties of the bolt.According to the service environment of the bolt,the equivalent stress of the bolt in service is calculated and compared with the simulation results of ABAQUS finite element simulation software.According to miner model,the residual life of the bolt is predicted and suggestions for prolonging life are put forward.The main results are as follows:(1)In the service process of R26 alloy bolt,the fine grain part has a tendency of coarsening,which weakens the effect of fine grain strengthening and reduces the room temperature properties of the material.The number of annealing twin boundaries in grains decreases obviously,and the number of abnormal banded γ′ precipitates at grain boundaries increases,which destroys the continuity of the material,loses the effect of aging strengthening,and reduces the plasticity and toughness of the material.Ti element diffuses from γ′ phase to grain boundary to form segregation,which weakens the aging strengthening effect of the alloy,and the existence of massive Ti as hardening phase at grain boundary will lead to the increase of hardness at grain boundary.Compared with the factory bolts,the hardness of the bolts after 45000 h service increased by about 9%,exceeding the standard range,and the elongation and impact toughness decreased by 7.6% and 30% respectively,which could not meet the standard of safe service.(2)After 45000 h service,the bolts were treated with 1031℃ solution for 1 h,oil cooling;820℃ intermediate treatment for 8 h,furnace cooling;730℃ aging treatment for 4 h,air cooling recovery.After heat treatment,the banded γ′ precipitates and massive Ti segregated at the grain boundary were eliminated,and the aging strengthening phase precipitated uniformly and dispersedly in the grain again,making the hardness of the material return to normal level,close to the factory structure,The elongation and impact toughness are increased by 8.7% and 140% respectively,which have good mechanical properties and meet the requirements of continuous service.(3)The cold tightening stress,hot tightening stress,thermal stress and equivalent stress of the bolt in service are 232.5MPa,94.7MPa,117.4MPa and 444.6MPa respectively,which are lower than the yield strength but higher than the fatigue strength and creep limit of the bolt.Creep and fatigue damage will occur during the service of the bolt.According to miner’s theory,the creep damage and fatigue damage of R26 alloy bolt are 0.098 in one loading cycle(20000 h),and the remaining life of the bolt is159000 h,corresponding to 8 overhaul periods.The creep damage of bolts can be reduced by applying the pre tightening force strictly according to the specifications;the thermal stress and fatigue damage of bolts can be effectively reduced by properly reducing the starting speed of the unit. |
PDF Full Text Request