Effects of long term thermal exposure on microstructure and mechanical property evolution of a new wrought Ni–Fe based superalloy

• The growth rate of γ' particles with spherical morphology is consistent with the diffusion controlled growth (LSW) model.
• The M23C6 carbides showed good thermal stability at high temperature along the grain boundary.
• The α-Cr phase appeared after 1000 h thermal exposure at 750 °C.
• The microhardness and yield strength at room temperature might be improved by the precipitation of α-Cr phase.

A new wrought Ni-Fe based superalloy was designed for advanced ultra-supercritical boiler tubes beyond 700 °C. In this study, the recrystallization treatment of the alloy following cold rolling and the microstructural evolution following 500 h, 1000 h and 3000 h at 750 °C were studied. The optimization of heat treatment was performed by thermodynamic calculation and the optimum combination was determined, i.e. annealing treatment at 1150 °C for 45 min, followed by a two-step aging treatment, at 810 °C for 1 h and at 770 °C for 16 h. Following this process, the M(C,N) precipitates and Cr-rich M23C6 carbides were formed at grain boundaries. Following 3000 h thermal exposure at 750 °C, the coarsening rate of the spherical γʹ precipitates was consistent with the LSW model and similar to that of the IN 740H alloy, while the variation in hardness values was relatively low, as the exposure time was increased. The α-Cr phase was observed after 1000 h at 750 °C and during the long term thermal exposure the coarsening of α-Cr phase occurred. The formation of α-Cr phase enhanced the intragranular yield strength and hardness values at room temperature and the stability of the mechanical properties was partially improved.

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