Creep Rupture Strength of Several Nickel-Base Superalloys in Impure Helium

Abstract

In order to estimate long-term creep rupture strengths of pre-selected alloys for intermediate heatexchangers of a very high temperature gas cooled reactor, a creep rupture test in impure helium has been conducted with respect to three newly developed nickel-base alloys and Inconel 617 and the result have been discussed in comparison with those in air.
Solid solution strengthened 16% Cr-20% W-30%Co-Ni alloy and 23%Cr-18%W-Ni alloy indicate almost the same short-term creep rupture strength in helium as that in air. Long-term creep rupture strengths of these alloys in helium, however, seem to be higher than those in air because crack growth is caused only by mechanical stress in helium, while it is accelerated by oxidation following nitriding in air.
Inconel 617 indicates significantly lower creep rupture strength in helium than that in air. One of the causes is the difference of crack growth mechanism, but a more significant cause should be made clear.
Precipitation hardened 18%Cr-4%Mo-6%W-10%Co-2%A1-2.5%Ti-Ni alloy gives a slightly lower short-term creep rupture strength in helium than that in air, but the long-term creep rupture strength in helium seems to be higher than that in air.