The Role of Environment on High Temperature Creep-Fatigue Behavior of Alloy 617

Abstract

Alloy 617 is the leading candidate material for an intermediate heat exchanger (IHX) application of the Very High Temperature Nuclear Reactor (VHTR), expected to have an outlet temperature as high as 950°C. Acceptance of Alloy 617 in Section III of the ASME Code for nuclear construction requires a detailed understanding of the creep-fatigue behavior. Initial creep-fatigue work on Alloy 617 suggests a more dominant role of environment with increasing temperature and/or hold times evidenced through changes in creep-fatigue crack growth mechanism/s and failure life. Furthermore, previous work on corrosion of nickel base alloys in impure helium has suggested that this environment is far from inert with respect to Alloy 617. Continuous cycle fatigue and creepfatigue testing of Alloy 617 was conducted at 950°C and 0.3% and 0.6% total strain in air to simulate damage modes expected in a VHTR application. Continuous cycle and creep-fatigue specimens exhibited intergranular cracking, but did not show evidence of grain boundary cavitation. Despite the absence of grain boundary cavitation to accelerate crack propagation, the addition of a hold time at peak tensile strain was detrimental to cycle life. This suggests that creep-fatigue interaction may occur by a different mechanism or that the environment may be partially responsible for accelerating failure. INTRODUCTIONThe Next Generation Nuclear Plant (NGNP) being developed in the US is a Very High Temperature Nuclear Reactor (VHTR) with a gas as the primary coolant. Conceptual design requires an outlet temperature of greater than 850°C to efficiently generate hydrogen, with a maximum expected temperature of 950°C. A critical component in the VHTR system is the Intermediate Heat Exchanger (IHX), which will operate at the reactor outlet temperature of up to 950°C.

The development of microstructural damage during high temperature creep–fatigue of a nickel alloy

“…The continuous-cycle fatigue and creep-fatigue (with dwell times in tension of up to 1800 s) behavior of Alloy 617 at 950°C with 0.3% and 0.6% total strain in air and in a VHTR simulated impure helium has been reported elsewhere [11,12]. The main conclusions from these two previous studies were that the introduction of a hold time at the peak tensile strain decreased the number of cycles to failure while increasing the duration of the hold from 180 s to 1800 s did not further reduce the cycle life, indicating that the number of cycles to failure in creep-fatigue is independent of the overall test duration.…”

Characterization of elevated temperature properties of heat exchanger and steam generator alloys

“…Chromia was present on both crack and specimen surfaces. [4] Alumina was present at the crack tip and along intersecting grain boundaries.…”

“…[4] Specimen reduced sections were 7.5 mm diameter and 19.5 mm in length. Low stress grinding was used during the final machining.…”

Low Cycle Fatigue Behaviors of Alloy 617 (INCONEL 617) Weldments for High Temperature Applications

“…We applied triangular waveforms with equal push-pull mode to specimens. As in the literature review [3,9,10,15], we also defined the failure criteria as the number of cycles, which means a 20% level drop off from the initial level of the peak tensile stress. We used a personal computer through a high precision extensometer with 12.5 mm gage length that was attached to the gauge section of the specimen.…”

“…In addition, the results of hardness measurement on the WM showed a lower value than that of the HAZ, but still higher than the BM. In nickel based Alloy 617 [6][7][8][9][10][11][12], a reduction of fatigue life was attributed to creep, oxidation, and DSA at elevated temperature. In this study, however, the difference in fatigue resistance cannot be fully interpreted by an effect of fracture morphology.…”