Study Of Precipitation Behavior Of Grain Boundary Carbides In HR3C Steel Based On Structural Unit Model

With the development of ultra-supercritical?USC?coal-fired power generation technology and the increasingly serious environmental problems faced by mankind,improving power generation efficiency and reducing CO₂ emissions have become two major themes in the field of thermal power generation.The service environment of high temperature and high pressure in USC coal-fired generator units sets more stringent requirements on the performance of structural materials.HR3C steel has been widely used as superheater and reheater materials in ultra-supercritical boilers due to its high creep strength,thermal fatigue strength,weldability,resistance to fireside corrosion/erosion,and resistance to steam-side oxidation and spallation.However,in the long-term service,the carbides precipitated along the grain boundary as a continuous chain significantly increases the brittleness of HR3C steel,which brings safety risks to the safety service of ultra-supercritical power plant.Therefore,studying the brittleness of HR3C steel from the perspective of precipitation behavior of grain boundary carbides is of great significance for improving the service embrittlement of HR3C steel and related materials.In this paper,the correlation between grain boundary structure and carbides is theoretically analyzed based on the structural unit model,and a thermo-mechanical process is explored to change the grain boundary orientation of HR3C steel,so as to change the precipitation behavior of grain boundary carbides:The structural similarities between 8 kinds of symmetric tilt boundaries and 3 kinds of carbides are analyzed based on structural unit model and radial distribution function of atomic density,and the formation energies of single C atom on these grain boundaries were calculated by molecular dynamics simulation.The results show that the structure of Cr₂₃C₆ and Cr₇C₃carbides are more similar to that of the grain boundary.When the grain boundary angle is between 20-50°,the structural similarity between the grain boundary and the carbide is higher;when the one is close to 60°,the similarity is lower.Molecular dynamics results show that when the grain boundary angle is between 20-50°,C atom is more likely to be segregated to the grain boundaries;while the one is close to 60°,the tendency of C atom migrate to the grain boundary is weakened.Based on the theoretical research,the thermo-mechanical process was conducted to change the precipitation state of grain boundary carbides in HR3C steel.Then the effect of deformation,annealing temperature and annealing time on the microstructure of HR3C steel was analyzed,and the grain boundary orientation distribution of samples was characterized by EBSD.It is found that the grain boundary carbides precipitate discontinuously after the thermo-mechanical process.The proportion of grain boundaries which angles are less than 20°or more than 50°is increased;while the ones which angles are between 20°and 50°is significantly reduced.