상온과 800, 850 ℃ 고온에서 Alloy 617 확산용접재의 저사이클 피로 거동

Abstract

Very high temperature gas-cooled reactor is one of the promising advanced reactors that can produce hydrogen, generate electric power and utilize industrial heat using high temperature heat source. The key component of the very high temperature gas-cooled reactor is intermediate heat exchanger, which is manufactured by printed circuit heat exchanger. Service life of the component strongly relies on the quality of the diffusion welding technique. In this study, low cycle fatigue test was performed using a heat resistant Alloy 617 diffusion weldment. Low cycle fatigue test was performed with a strain-controlled method at 25, 800 and 850 ℃. Total strain ranges employed herein were 0.9 - 1.5% at 25 ℃ and 0.6 - 1.2% at 800 and 850 ℃. A fixed strain rate of 3 × 10-3 /s for 25 ℃ and 1 × 10-3 /s for 800 and 850 ℃ were employed. Diffusion weldment showed fatigue life of 57-112%, 46-58% and 97-102% compared to as-received specimens at test temperature of 25, 800 and 850 ℃, respectively. Diffusion-weldment and as-received both showed cyclic hardening up to about 100 cycles then continuously softened until failure only with relatively larger total strain ranges at room temperature. At 800 ℃, both material showed continuous cyclic hardening until failure. In contrast, both material showed few initial cycles of hardening and then saturation and softening occurred. Fatigue ductility coefficient of the diffusion weldment is –1.029 at 800 ℃, which is similar to that reported for Alloy 617 base metal in a literature. Fatigue failure mode was intergranular for both the as-received and diffusion weldment at test temperature of 25 and 850 ℃. In contrast, fatigue cracks were initiated and propagated along the interface of the diffusion weldment at test temperature of 800 ℃. Al-rich oxides, which were evolved during the preparation of the diffusion weldment, apparently decreased the interface quality.