High Temperature Aging and Corrosion Study on Alloy 617 and Alloy 230

Abstract

The very high temperature gas-cooled reactor (VHTR), with dual capacities of highly efficient electricity generation and thermochemical production of hydrogen, is considered as one of the most promising Gen-IV nuclear systems. The primary candidate materials for construction of the intermediate heat exchanger (IHX) for the VHTR are alloy 617 and alloy 230. To have a better understanding of the degradation process during high temperature long-term service and to provide practical data for the engineering design of the IHX, aging experiments were performed on alloy 617 and alloy 230 at 900°C and 1000°C. Mechanical properties (hardness and tensile strength) and microstructure were analyzed on post-aging samples after different aging periods (up to 3000 h). Both alloys attained increased hardness during the early stages of aging and dramatically soften after extended aging times. Microstructural analysis including transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and electron backscatter diffraction was carried out to investigate the microstructure evolution during aging. A carbide particle precipitation, growth, and maturing process was observed for both alloys, which corresponds to the changes of the materials’ mechanical properties. Few changes in grain boundary character distribution and grain size distribution were observed after aging. In addition, high temperature corrosion studies were performed at 900°C and 1000°C for both alloys. Alloy 230 exhibits much better corrosion resistance at elevated temperature compared with alloy 617.

Methodology of Ni-base Superalloy Development for VHTR using Design of Experiments and Thermodynamic Calculation

“…[1][2][3][4][5] 특히 헬륨의 고온측과 저온측의 열교환이 일어나 는 중간열교환기(IHX, intermediate heat exchanger)와 고온가스관(hot gas duct) 등의 경우, 이러한 기계적 특성 과 더불어 열전달 효율과 제작 용이성을 고려하여 Inconel 사에서 개발한 Alloy 617, 일본에서 개발한 Hastelloy XR 및 Haynes 사에서 개발한 Alloy 230 등의 단련용 니켈계 초합금(Ni-base superalloy)이 후보재료로 연구되고 있 다. [1][2][3][4][5][6][7][8][9][10] Alloy 617은 Ni-Cr-Co-Mo계 합금으로서 우수한 고 온 크립 특성을 가지므로 VHTR의 IHX용 구조재로 많은 연구가 진행되었으나, 950 o C 정도의 초고온 헬륨 환경에서 장시간 노출될 경우 연신율이 낮아지고 고온 파단 시간이 짧아지는 한계가 보고되었다. [4][5][6][7] Hastelloy XR은 Ni-Cr -Fe계 합금으로서 870 o C까지 우수한 고온 기계적 특성을 가지고 산화저항성이 높지만, 재료의 수급이 용이하지 않고 초고온 헬륨 환경에서의 장기 수명 특성에 대한 자료가 많이 부족한 실정이며 고온 특성 또한 Alloy 617과 유사하다.…”

Corrosion of Nickel and Iron Based Superalloys in High Temperature Gas Environments

“…However, Inconel 617 also includes cobalt and molybdenum to reinforce mechanical properties. Thus, Inconel 617 has the greatest creep resistance of the three tested alloys (4).…”

Investigation of thermal aging effects on the tensile properties of Alloy 617 by in-situ synchrotron wide-angle X-ray scattering

“…This alloy was developed in the 1970s for high-temperature applications such as gas turbines, combustion cans, ducting, and structural components for power-generating plants [7]. Numerous studies on Alloy 617 have been performed to investigate creep properties, tensile behavior at various temperatures, corrosion resistance, and thermal aging effects [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. In 1974, Mankins et al studied the aging effects on the microstructure and phase stability of Alloy 617 at 649-1093°C and identified M 23 C 6 as the main stable precipitates at all temperatures and a small amount of gamma prime (L1 2 -structured Ni 3 Al) at 760°C [9].…”

“…Later in 1980, Kihara et al studied the creep properties at 1000°C and discussed the microstructural development, especially the evolution of M 23 C 6 precipitates during creep, and proposed the existence of the Ostwald ripening mechanism under loading [12]. Recently, Mo et al studied the aging effects at 900°C and 1000°C up to 3000 hours, using electron microscopy techniques to provide detailed information on the size distribution, morphology, and coherency of M 23 C 6 precipitates [13][14][15]. Ren et al have reviewed the aging effects on Alloy 617, and summarized most of the previous works and important experimental data [16,17].…”

“…To better understand their behavior and interaction mechanism with the metallic matrix during mechanical loading in different aging conditions, we conducted in-situ tensile tests combined with synchrotron wide-angle X-ray scattering (WAXS) to characterize the microstructural changes during loading. The experiments are based on long-term (up to 10,000 hours) thermal aging experiments on Alloy 617 at 1000°C [14,15,22]. From the high-energy X-ray measurements, both the macroscopic tensile behavior and the microstructural evolution involving the change in volume fraction of the carbide phase, the lattice strain development for both carbides and metallic phases, and dislocation densities during loading were determined.…”