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

A new wrought Ni-Fe based superalloy was designed for advanced ultra-supercritical boiler tubes beyond 700 degrees 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 degrees 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 degrees C for 45 min, followed by a two-step aging treatment, at 810 degrees C for 1 h and at 770 degrees 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 degrees C, the coarsening rate of the spherical gamma' 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 alpha-Cr phase was observed after 1000 h at 750 degrees C and during the long term thermal exposure the coarsening of alpha-Cr phase occurred. The formation of alpha-Cr phase enhanced the intragranular yield strength and hardness values at room temperature and the stability of the mechanical properties was partially improved. (C) 2016 Elsevier Ltd. All rights reserved.