| Nickel base superalloy pipes are widely used in the field of aviation, aerospace and military as a result of the good mechanical properties, oxidation and hot corrosion resistance, microstructure stability at high temperature. The properties of the nickel base superalloy pipes are extremely important, since the pipes are expected to serve under severe conditions for long period of time. The manufacture process of nickel base superalloy pipes is complicated and the microstructure control is difficult. Therefore, how to manufacture pipes with high quality and desired microstructure is a great problem. Hence, there is an urgent need to investigate the microstructure evolution process and mechanism during the manufacture of nickel base superalloy pipes.The present study is based on the manufacture process of alloy 617B pipe. The relationship between homogenization degree and dynamic recrystallization (DRX) is investigated in this work. As a result, the opinion of low-temperature homogenization is proposed. Based on this opinion, the left dendrites and element segregation after low-temperature homogenization can accelerate DRX process by providing more nucleation sites. Furthermore, oxidation and grain growth during homogenization can be controlled by low-temperature homogenization.The effect of hot deformation parameters on microstructure evolution during DRX is studied by isothermal simulated compression. Multiple microstructure observation methods are carried out for the investigation of DRX mechanism. The dominant nucleation mechanism of DRX in alloy 617B is discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) is an assistant nucleation mechanism. The results of present study shows that adiabatic heat is not the only reason for the accelerated DRX at high strain rate condition. The higher stored energy and dislocation density in combine with more twin boundaries all make the DRX nucleation mechanism more active, which makes up for the shortage of DRX time during high strain rate deformation.The processing maps and microstructure evolution models of alloy 617B are established based on the isothermal simulated compression data. The microstructure evolution during hot extrusion can be predicted by DEFORM-2D finite element simulation software in combine with the microstructure evolution model developed in present work. The manufacture parameters for hot extrusion can be optimized by finite element simulation results. The cold rolling and heat treatment parameters are investigated. The optimum solution treatment for alloy 617B tube is 1190～1210 ℃/1 h and the optimum annealing treatment is 1200℃ /30～60 min.The long term aging results show that there is no harmful phase in alloy 740H after 720 ℃/10000 h aging. However, element redistribution of MC carbide in alloy 740H has already taken place after 200 h aging and serves as the "spring" of elements during the evolution of other phases. Furthermore, μ, phase is detected in alloy 617B after aging at 720℃ for about 5000 h. The element redistribution relationships between different phases for alloy 617B and alloy 740H are established by microstructure observation and quantitative phase analysis.In conclusion, the critical steps during manufacture of alloy 617B pipe, such as homogenization, hot extrusion, cold rolling and annealing, are investigated in present study. The microstructure evolution principle and control method gotten in this work can provide experimental and theoretical guidance for the manufacture of nickel base superalloy pipes. |
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