850℃에서의 Alloy 617 용접재의 저사이클 피로 특성

Abstract

The nickel-based superalloy, the Alloy 617 is the prime candidate material for the very high temperature gas-cooled reactor (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. The fatigue deformation is expected to be an important damage mode for the intermediate heat exchanger (IHX) as a result of power transients and temperature gradient induced thermal strain during operation as well as startups and shut downs, each of which produces cyclic loading. It is important to characterize the fatigue properties of welding specimens over a strain amplitudes and temperature for ensuring the reliability of welded structures. A lot of research is carried out on the Alloy 617 material at the international level when compared to Korea, where only a handful of them is carried out. Especially the research on the weld joint seems to be rare in Korea. Therefore, in the present paper, the low cycle fatigue properties of Alloy 617 weldments were investigated at high temperature, 850℃, in air. Fully reversed (R=-1) total axial strain-controlled LCF tests were performed at four different total strain ranges of 0.6, 0.9, 1.2 and 1.5% under 1.0×10-3/s constant strain rate. Weldment specimens were made from 25 mm thick GTAW butt-welded plates such that the loading direction was oriented transverse to the welding direction. At all the testing conditions, the cyclic stress response showed a cyclic hardening in the early stage of fatigue life, and followed softening phase occurs till final fracture. LCF cracking in weldment specimens initiated in the weld metal zone. Lastly, This work will be contributed for the basic data aquisition of the high temperature low cycle fatigue properties for Alloy 617 weldments, and for the evaluation techniques of high temperature fatigue life prediction for Alloy 617 weldments.