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 degrees C were characterized and correlated with hardness and creep strength. Major precipitates included (Cr,Mo,Fe)(23)C-6 carbides and the delta-Ni3Mo phase. M6C and MC carbides were also detected within the austenitic grains. However, minor precipitates particularly gamma-Ni-3(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)(23)C-6 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 gamma-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 gamma' precipitates were observed. These microstructural modifications, together with the presence of the delta-phase, detected only in specimens exposed to 700 degrees C, were clearly responsible for the substantially good creep response observed at 700 degrees C, compared with that found at 800 degrees C.