Abstract
This paper presents the high-temperature creep-fatigue testing of a Ni-based superalloy of Alloy 617 base metal and weldments at 900 °C. Creep-fatigue tests were conducted with fully reversed axial strain control at a total strain range of 0.6%, 1.2%, and 1.5%, and peak tensile hold time of 60, 180, and 300 s. The effects of different constituents on the combined creep-fatigue endurance such as hold time, strain range, and stress relaxation behavior are discussed. Under all creep-fatigue tests, weldments’ creep-fatigue life was less than base metal. In comparison with the low-cycle fatigue condition, the introduction of hold time decreased the cycle number of both base metal and weldments. Creep-fatigue lifetime in the base metal was continually decreased by increasing the tension hold time, except for weldments under longer hold time (>180 s). In all creep-fatigue tests, intergranular brittle cracks near the crack tip and thick oxide scales at the surface were formed, which were linked to the mixed-mode creep and fatigue cracks. Creep-fatigue interaction in the damage-diagram (D-Diagram) (i.e., linear damage summation) was evaluated from the experimental results. The linear damage summation was found to be suitable for the current limited test conditions, and one can enclose all the data points within the proposed scatter band.
Highlights
The very-high-temperature reactor (VHTR) of Generation-IV type systems is being developed as a nuclear system with helium as the primary coolant, and is designed to produce an effective heat for hydrogen generation and electricity production [1,2]
It is necessary to conduct a laboratory simulation of the creep-fatigue phenomenon in Alloy 617 to supply the draft code case database. It will be used in the helium environment of the VHTR, the allowable design in the boiler and pressure vessel (B&PV) code refers to the time-dependent behavior in air [6]
Continuous low-cycle fatigue tests including a variety of hold times were completed at 900 ◦°C
Summary
The very-high-temperature reactor (VHTR) of Generation-IV type systems is being developed as a nuclear system with helium as the primary coolant, and is designed to produce an effective heat for hydrogen generation and electricity production [1,2]. The specifications of candidate materials with a combination of very-high-temperature operation and long duration of service requires structural materials with good thermal stability as well as high-temperature creep, fatigue (or combined creep-fatigue), and oxidation resistance Based on these qualifications, the nickel-base Alloy 617 is the most promising candidate material for the intermediate heat exchanger (IHX) and hot gas duct (HGD). It is necessary to conduct a laboratory simulation of the creep-fatigue phenomenon in Alloy 617 to supply the draft code case database. It will be used in the helium environment of the VHTR, the allowable design in the B&PV code refers to the time-dependent behavior in air [6]. Reports in studies in a helium environment showed that the growth rates of both
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