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

Rando Dewa,Eung Kim,Seon Kim,Woo Kim

doi:10.3390/met7020054

Rando Dewa, Eung Kim + Show 2 more

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https://doi.org/10.3390/met7020054

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Abstract

The aim of this study is to investigate the fully‐reversed low cycle fatigue properties of Alloy 617 in the air at 950 °C; these tests were conducted at total strain ranges from 0.9% to 1.5% with a constant strain rate of 10−3/s. The result of the fatigue tests showed a decrease in fatigue resistance with an increasing total strain range. The reduction of fatigue resistance was due to the effect of the total strain range and microstructure evolution during high temperature, such as brittle oxides cracking. At all testing conditions, the cyclic softening mechanism was observed as a function of the total strain range in the current high temperature condition. An analysis of low cycle fatigue resistance was performed using the Coffin–Manson relationship and the total strain energy density; it was found that Alloy 617 followed these relationships well. In addition, this study compared well with previous work reported in the literature for a similar testing condition. Post‐fracture analysis on the fracture surfaces of failed specimens revealed a more severe damage cracking at the periphery of specimens due to the increase in the total strain range. The surface connected grain boundary cracks induced by oxidation were obvious at low strain range. Thus, the primary crack propagation occurred in transgranular mode from persistent slip bands.

Highlights

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
Reversed strain control tests were conducted at 950 °C in open air under different applied
Reversed strain control tests were conducted at 950 ◦ C in open air under different applied total strain ranges for Alloy 617

Summary

Introduction

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. Alloy 617, a Nickel-based superalloy, is a leading candidate material for IHX and HGD because of its excellent high-temperature mechanical properties, formability, and weld ability [3]. Alloy 617 is strengthened by solid solution hardening provided by the alloy chemical compositions of chromium, cobalt, and molybdenum. The strengthening process of this Alloy 617 is granted by the hardening precipitates of

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