900℃에서 Alloy 617의 크리프-피로 거동에 관한 실험적 연구

Abstract

The very high temperature reactor (VHTR) is one of the promising fourth generation (Gen-IV) reactor for the economic production of electricity and hydrogen. The conceptual design requires an outlet temperature of greater than 850℃ to provide for the efficient generation of hydrogen, with a maximum expected outlet temperature of 950℃. The important component in VHTR system is the Intermediate Heat Exchanger (IHX), which will be required to operate at the reactor outlet temperature of up to 950℃. And the components have a projected plant design service life of 40-60 year operation and 4-7 MPa in high pressures, the most important consideration is the creep-fatigue and fatigue behaviour for the materials. Material selection for the IHX is important, regardless of the design. Based on these material requirements, the nickel-based superalloy, Alloy 617 is the leading candidate material for the VHTR for the next nuclear reactor design consideration due to its superior material properties, oxidation resistance, and phase stability at high temperature compared with other potential superalloys. Therefore, it is important to understand the high temperature creep-fatigue and LCF information of the Alloy 617. However, there are few studies on LCF characteristics of this material in Korea. Especially, there is no research on creep-fatigue characteristics. the purpose of this work is to investigate the influences of temperature and hold time on the creep-fatigue behaviour of Alloy 617. In this study, the creep-fatigue properties of Alloy 617 were investigated at high temperature, 900℃, in air. Fully reversed (R=-1) strain controlled cyclic testing with a total strain range of 0.6%, 0.9% and 1.5% has been carried out with a triangular wave form to characterize fatigue properties with a tensile hold time to examine creep-fatigue behavior. A constant strain rate of 1.0×10-3/s was used. Weldment specimens were made from 25 mm think GTAW butt-welded plates such that the loading direction was oriented transverse to the welding direction. The hysteresis loops of creep-fatigue tests showed significant stress relaxation during tension hold time. With the increase of tension hold time, the creep-fatigue life was remarkably deceased which caused from the formation of crack by the creep fracture mechanism. Lastly, This work will be contributed for the basic data aquisition of the high temperature creep-fatigue properties for Alloy 617, and for the evaluation techniques of high temperature fatigue life prediction for Alloy 617.