Cyclic Stress Response and Fracture Behaviors of Alloy 617 Base Metal and Weld Joints under LCF Loading

Abstract

Cyclic stress response and fracture behaviors of Alloy 617 base metal (BM) and Alloy 617 weld joints (WJ) are investigated under strain controlled low cycle fatigue (LCF) loading. Axial fully reversed total-strain controlled tests have been conducted at room temperature with total strain ranges of 0.6, 0.9, 1.2, and 1.5%. At the all testing conditions, weld joint specimens have shown higher peak stresses than the base metal specimens, whereas the plastic strain accumulation of the base metal specimens is comparatively higher than those of the weld joint specimens. The cyclic stress response behavior of both base metal and weld joint specimens revealed initial cyclic hardening during first small number of cycles followed by progressive softening to failure. Higher strain amplitudes decreased the fatigue lives for both base metal and weld joint specimens; subsequently weld joint specimens had lower fatigue resistances relative to base metal specimens. Furthermore, the cracking in weld joint specimens initiated in the weld metal (WM) region. The crack initiation and propagation showed transgranular mode for both base metal and weld joint specimens; especially weld joint specimens showed a wedge type crack initiation about 45 degrees to the loading direction because of the dendritic structure.

Low Cycle Fatigue Behaviors of Alloy 617 (INCONEL 617) Weldments for High Temperature Applications

“…Therefore, Figure 7 shows the ratio of cyclic hardening measured by the ratio of the maximum stress amplitude (σ peak ) to the stress amplitude of the first cycle (σ initial ). From the figure, it is obvious that the ratio of cyclic hardening increases along with the transformation of LCF test environment from previous work [4,5] at room temperature to the present investigation at 800˝C. As in the literature survey [11], they also acknowledged that the DSA occurs in the temperature range of about 650-900˝C for this Alloy 617.…”

“…From previous work [4], the weldments showed a relatively wedge-type crack with some deviations oriented at 45˝to the loading direction. Figure 10 also shows the fractured specimens, which indicates the failure occurring inside of the gauge section in the WM region.…”

“…Nowadays, the Korea Atomic Energy Research Institute (KAERI) is developing a nuclear hydrogen development and demonstration plant with a capacity of 200 MW th with thermal and core outlet temperature of 950˝C [1,2,4,5]. The components have a projected plant design service life of 40-60 years operation and 3-8 MPa in He impurities.…”

“…The components have a projected plant design service life of 40-60 years operation and 3-8 MPa in He impurities. The most important consideration is the creep-fatigue and fatigue behavior of the materials, especially in welded structure [3][4][5]. The Alloy 617 for the IHX and other components is expected to operate at room temperature, which is assumed as a start-up condition, and at temperatures between 800-950˝C.…”

Effect of Strain Range on the Low Cycle Fatigue in Alloy 617 at High Temperature

“…The very high temperature gas cooled reactor (VHTR) is considered as a candidate design for Generation IV (Gen IV) reactors to authorize the eventual operation in the thermal and core outlet temperature of 950 • C. Materials of potential interest include nickel Alloy 800H, Alloy 617 and Hastelloy X of the VHTR main components, including the intermediate heat exchanger (IHX) and hot gas duct (HGD) [1,2]. The VHTR is designed for a life span of 60 years and will be exposed to a very high temperature environment.…”