Investigation On Microstructure And Performance Stability Of N263 Weld Joint At High Temperature

Nimonic 263 superalloy has been an important candidate material for components in advanced ultra-supercritical steam turbine because of its excellent weldability and high temperature performance,and welding is the main production method of turbine rotor.When N263 weld joint is in service for long time at high temperature,the performance will change due to the nonuniformity of microstructure and the evolution of microstructure at high temperature.Therefore,it is of great significance to investigate the microstructure stability and performance reliability of N263 weld joint during long-term thermal exposure at high temperature,summarize the law of microstructure evolution,analyze the mechanism of fracture and failure,and reveal the internal relationship between microstructure evolution and performance transformation.In this paper,the microstructure and properties of precipitation-strengthening N263 superalloy weld joint after long-term thermal exposure were studied.Results showed that BM was composed of?austenite grains and annealing twins with Ti(C,N)carbonitride,MC carbide,M₂₃C₆ carbide and??phase.In HAZ,austenite grains grew,the number of twins decreased,and a lot of precipitates appeared in the grains.WM was composed of coarse columnar grains with dendritic structure.Due to the segregation of Ti and Mo,a large number of precipitates were found at grain boundaries and interdendritic regions,mainly including MC and M₂₃C₆ carbides.There were also??particles highly coherent with the matrix in WM.Secondly,long-term thermal exposure at 750?was performed.The sorts of microstructure evolution included??coarsening,?precipitation and MC carbide decomposition.During thermal exposure,??particles coarsened and grew.The coarsening behavior of??phase in BM conformed to the model of linear correlation between particle size and the three-power of time in classic Ostwald ripening theory.The coarsening behavior of??phase in WM could be described with a non-integer exponential model,whose index was larger than 3,representing an unusually slow coarsening rate.After thermal exposure for 1000 hours,acicular?phase precipitated both in BM and WM.?laths precipitated as bands along the rolling direction in BM,and they precipitated around MC carbides in WM.The precipitation of?phase was explained by??transformation and MC carbide decomposition.The reaction of MC carbide decomposition was:MC+??M₂₃C₆+??+?.Finally,the tensile,impact and creep rupture properties of weld joint were tested.The fracture locations of tensile specimens were in WM,and slight dynamic strain aging phenomenon was observed at 800?.With the increasing of time,impact toughness of BM and WM both decreased.Small?laths precipitated after 3000 hours could hinder impact crack propagation.After 5000 hours,the severe coarsening of??particles and massive precipitation of acicular?laths caused a sharp drop for impact toughness.The creep behavior of weld joint at 750?conformed to power law creep.The life equation was lg t_r=32.14-12.42 lg?,and the extrapolated strength was?₁₀₅⁷⁵⁰=153 MPa.During the creep,acicular M₂₃C₆ carbides precipitated at grain boundaries in BM.MC carbides decomposed and small?phase precipitated in WM.The creep rupture location of weld joint at 750?and 220 MPa was in WM,the failure mechanism was cavities gathering intergranular fracture caused by large carbides at grain boundaries.