Creep behavior of INCOLOY alloy 617

The microstructural features of INCOLOY alloy 617 in the solution annealed condition and after long-term creep tests at 700 and 800 °C were characterized and correlated with hardness and creep strength. Major precipitates included (Cr,Mo,Fe)23C6 carbides and the δ-Ni3Mo phase. M6C and MC carbides were also detected within the austenitic grains. However, minor precipitates particularly γ′-Ni3(Al,Ti) was found to play an important role. At different exposure temperatures, the microstructural features of the Ni–22Cr–12Co–9Mo alloy changed compared with the as-received condition. The presence of discontinuously precipitated (Cr,Mo,Fe)23C6 carbides and their coarsening until the formation of an intergranular film morphology could be responsible both for a reduction in rupture strength and for enhanced intergranular embrittlement. The fraction and morphology of the γ′-phase, precipitated during exposure to high temperature, also changed after 700 or 800 °C exposure. At the latter test temperature, a lower volume fraction of coarsened and more cubic γ′ precipitates were observed. These microstructural modifications, together with the presence of the δ-phase, detected only in specimens exposed to 700 °C, were clearly responsible for the substantially good creep response observed at 700 °C, compared with that found at 800 °C.