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

Seon Jin Kim,Min Hwan Kim,Woo Gon Kim,Rando Tungga Dewa

doi:10.1155/2015/207497

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.

Highlights

The very high temperature gas-cooled reactor (VHTR) is currently the most promising reactor among the Generation4 reactors for producing electricity and hydrogen economically
Current research activities at Korea Atomic Energy Research Institute (KAERI) and Pukyong National University (PKNU) focus on the study of low cycle fatigue behavior of Alloy 617 base metal and the welded joint specimens were machined from gas tungsten arc welding (GTAW) butt-welded with temperature between passes under 177∘C and the postheat-treatment was not conducted because the Ni-based superalloy was not normally applied
This paper focuses on the comparative investigation on cyclic stress response and fracture behaviors of Alloy 617 base metal and Alloy 617 welded joints under low cycle fatigue (LCF) loading

Summary

Introduction

The very high temperature gas-cooled reactor (VHTR) is currently the most promising reactor among the Generation reactors for producing electricity and hydrogen economically. In addition to its high temperature strength, it has excellent resistance to wide range of corrosive environments and is readily formed and welded by conventional techniques For this reasons, Alloy 617 is considered as one of the leading candidates because of its highest allowable design stress compared to other superalloys for construction of the IHX associated with the reformer system [1,2,3,4]. Current research activities at Korea Atomic Energy Research Institute (KAERI) and Pukyong National University (PKNU) focus on the study of low cycle fatigue behavior of Alloy 617 base metal and the welded joint specimens were machined from GTAW butt-welded with temperature between passes under 177∘C and the postheat-treatment was not conducted because the Ni-based superalloy was not normally applied. This paper focuses on the comparative investigation on cyclic stress response and fracture behaviors of Alloy 617 base metal and Alloy 617 welded joints under LCF loading. To clearly compare the low cycle fatigue fracture modes of Alloy 617 base metal and the welded joint specimens, the microscopic investigations were examined for selected specimens

Experimental Procedures

Results and Discussion

Conclusions