Research On Fatigue Properties Of T91/TP347H Dissimilar Steel Welded Joints

With the proposal and continuous advancement of our country’s"dual carbon"goal,the process of low-carbon transformation of domestic energy has accelerated.Vigorously developing ultra critical thermal power units with deep peak shaving capability can improve power generation efficiency,reduce CO₂ emissions,reduce energy consumption,and improve the ability to absorb new energy.Superheaters and reheaters are important components of supercritical power plant boilers.The metal materials used by the two are different.Due to the different positions and operating temperatures of each component in the boiler,different materials are selected.Therefore,a large number of dissimilar steel welded joints appear.Due to the different chemical composition and microstructure properties of the materials on both sides of the dissimilar steel welded joints,early failure is likely to be caused in the operation of deep peak shaving and variable load.This paper simulates the actual operating conditions of dissimilar steel welded joints,and conducts low-cycle fatigue performance experiments and simulation studies of T91/TP347H dissimilar steel welded joints.The evolution law of the microstructure of the material after the fatigue test was analyzed,and the microscopic mechanism of fatigue fracture was revealed.In terms of fatigue behavior research,high temperature and low cycle fatigue tests of T91/TP347 welded joints with different strain amplitudes at 650℃were carried out,and the results of high temperature and low cycle fatigue were analyzed.The results show that under different strain amplitude tests,the stress amplitude first increases and then decreases with the increase of the number of cycles.T91/TP347H dissimilar steel welded joints show the characteristics of first cycle hardening and then gradually cycle softening.With the increase of the test strain amplitude,the elastic strain amplitude is basically unchanged,the plastic strain amplitude gradually dominates,and the fatigue life decreases rapidly.In terms of microstructure evolution,the fracture morphology,metallographic structure and composition of precipitates were analyzed for dissimilar steel welded joints with different strain amplitudes.The results show that the main fracture mode of high temperature and low cycle fatigue is ductile fracture.The microstructure of the base metal and HAZ of the original welded joint is tempered martensite,the microstructure of the weld is dendritic austenite,and the microstructure of the base metal and HAZ of TP347H is austenite.Under low strain,the structure of T91 BM is still lath martensite,and TP347H BM is still austenite,the structure aging is not obvious,and the fracture is located near the fusion line of T91.With the increase of the strain amplitude,the aging of the T91 BM continued to intensify,the martensitic lath continued to disappear,the size and number of the precipitates increased,and the fatigue fracture was located on the T91 BM side.The precipitation phases in the BM on both sides are mainly carbides M₂₃C₆ and MX containing Cr,Ni,Nb,etc.The precipitation phases near the fusion line on the TP347H side are mainly strip-shaped Nb C and granular M₂₃C₆,and the precipitation phases near the fusion line on the T91 side are mainly Cr and Ni carbides.For the actual conditions of depth peak,the stress distribution of different variation conditions and the stress distribution of the T91/TP347H heterogeneous steel weld joints under different grooved forms were used to explore the influence of stress distribution for joint failure.The results show that under depth peak conditions,the tensile stress subject to the total joint will change with the increase of the cycle,and during the peak time,the process is subjected to a small process,and it has experienced a large change in the trough.The process,the relatively stable state is reached after 50 weeks;the maximum tensile stress in the joint is located outside the outer side of the T91 BM is close to the fusion line;change the groove type,increase the welding slope angle of the T91 side,in two conditions,because the tensile stress value from the outside of the T91 side pipe is reduced,it effectively reduces the fracture failure risk of the joint at the low alloy steel side.