Creep Behavior for Alloy 617 in Air at 950°C

Creep behavior for Alloy 617, which is considered as one of the major structural materials of a very high temperature reactor, was investigated in air at 950 degrees C. Creep experimental data was obtained by a series of creep tests with different stress levels at 950 degrees C. Alloy 617 revealed little primary creep strains and unclear secondary creep stages. A tertiary creep stage was initiated from a low strain level and was dominant in full creep curves. The creep constants of A, n, m, and C in Norton's power law and Monkman-Grant relationships were determined. In microstructure observations of crept specimens, it was found that a Cr2O3 oxidation layer was formed on the surface, and just beneath the Cr2O3 layer, an internal Al-oxide sub layer was formed with rod shapes. Also, below the internal sub layer, a thick carbide-depleted zone was developed due to reaction of the chromia and carbide precipitates. The thickness of the outer Cr-oxide layer increased with increasing creep rupture times. The increasing tendency showed a smooth slope like a parabolic curve.