Abstract: The precipitation behavior of grain boundary carbides and the evolution of Cr-depletion region in Ni-based superalloy 690 after solid solution treatment (1 020 ℃/15 min) followed by heat treatment at different temperatures (650 ℃, 715 ℃, 800 ℃) for various times (1～40 h) are studied by means of transmission electron microscopy. Based on the analysis of microstructure, an optimal heat treatment is given. It is concluded from the present investigation that carbides precipitate on both sides of the grain boundary at 800 ℃. In addition, the distribution of Cr near grain boundary is most smooth at the treatment of 715 ℃/10～20 h. As a whole, the optimal heat treatment process is 751 ℃/10～20 h.

Key words: Cr-depletion region, Isothermal treatment, Microstructure, Ni-based superalloy