Creep-fatigue interaction of inconel 617 at 950°C in simulated nuclear reactor helium

Abstract

Strain-controlled fatigue tests have been conducted in impure helium, simulating the primary-circuit coolant of a high temperature gas-cooled reactor to ascertain the influence of strain rate ( ε=4 × 10^-3 to 2 × 10^-5 s^-1), hold condition at peak strains (tension-only, compression-only and tension-plus-compression holds) and hold time (up to 120 min) on the low cycle fatigue behaviour of Inconel 617. A strain range of 0.6% and a temperature of 950°C were employed for all the tests. Microstructural changes which occurred during fatigue deformation were evaluated and damage mechanisms which influence fatigue life identified. A small reduction in fatigue life was found with decreasing ε. Irrespective of the position of hold at peak strain in a cycle, the hold time always reduced the fatigue life in comparison with continuously cycled tests at lower strain rates but of equal cycle duration. Tensile holds were found to be most damaging, followed by compression holds. Symmetrical tension-plus-compression holds led to fatigue lives which were very close to those of the continuously cycled tests. In the continuously cycled tests with strain rates down to ε=6.7 × 10^-5 s^-1, failure was always transgranular with no indication of creep damage. However, for tests with tensile holds, creep damage was evidenced by grain boundary cavitation and oxidation, which were responsible for a reduction in the fatigue life. The formation of thick oxide scales at the surface observed at longer hold times led to chromium-depleted surface zones in which the carbide precipitates were dissolved. The loss of carbides increased grain boundary sliding which enhanced the formation of grain boundary cracks. These cracks shortened the critical length of surface fatigue cracks which, during fracture of the specimen, linked with the intergranular creep cracks in tensile hold tests. The damaging effect of compression hold was attributed to increased inelastic strain and deformation ratcheting found in the cycle; failure occurred by local accumulation of tensile plastic strain which finally caused tensile necking.