High Temperature Low Cycle Fatigue and Lifetime Prediction of Alloy 617

Abstract

A very high temperature reactor (VHTR) is currently being developed in Republic of Korea as one of the promising candidates of Generation IV (Gen IV) reactors which embody a common goal of providing safe, longer lasting, proliferation-resistant, and economically produce the electricity and hydrogen. The VHTR merges the diversities of the baseline design to allow eventual operation at gas outlet temperatures up to 950oC. In the VHTR, some of the major components include the reactor internals, the reactor pressure vessel (RPV), the piping, the hot gas ducts (HGD), and the intermediate heat exchangers (IHX) are classified as key components, with helium as a primary and secondary coolant. The IHX performs the main purpose in the operation of the VHTR system, transferring heat from the primary reactor helium to an active working fluid at a lower temperature. These main components are designed for a design life of 60 years at 950oC and 3-8 MPa in He impurities. Although it will finally be used in a helium environment of the VHTR, the allowable design in the B&PV code are referred to the time-dependent behavior in air Leading materials of potential concern include nickel-base Alloy 800H, Alloy 617, Alloy 230, and Hastelloy X for the high temperature components. In the high temperature design, creep and fatigue resistance, oxidation resistance, and phase stability need to be satisfied. Alloy 617, a nickel-base super alloy, is a leading candidate material for a VHTR because of its excellent high-temperature mechanical properties, formability, and weldability. Alloy 617 is strengthened by solid solution hardening precipitates provided by the alloy chemical compositions of chromium, cobalt, and molybdenum, which are required for high temperature strength. In an actual high temperature design evaluation, however, fatigue and creep damage are usually more critical than other design parameters. In this circumstance, the low cycle fatigue (LCF) loadings represent a predominant failure mode from the temperature gradient induced thermal strain during operation as well as in the startups and shutdowns and in power transients or with temperature change of the flowing coolant having a low loading rate. Because of these shortcomings, significant consideration of LCF behavior is needed in the design and life assessment of such components working in high temperature conditions. The IHX have to be joined to piping or other components by welding technique, and thus, weldments used in its fabrication experience varying cyclic deformation and are a key element of all designs. The welded section material could be considerably affected by the welding process which is responsible for heterogeneities. As such, the weldments are critical considerations in the engineering design because they are the weakest links in the components and may have some original defects. A lot of data needs to be supplemented at very high temperatures due to the variability in the fatigue response of the weldments (i.e., weld, heat affected zone (HAZ), and base metal) to confirm the suitability of a baseline draft Code Case. In this work, the LCF behavior of Alloy 617 base metal and weldments, made from an automated gas tungsten arc welding (GTAW) process with Alloy 617 filler wire were comparatively investigated. LCF tests have been carried out through a series of fully reversed strain-controls (strain ratio, Rε = -1) regarding to the four different total strain ranges, i.e., 0.6%, 0.9%, 1.2% and 1.5% at the temperature range 900-950oC in an air environment. In addition, the effect of holding time was investigated for 60, 180, and 300 seconds in accordance with the ASTM Standard E606 and E2714. The fatigue life varies widely at high temperature and it is generally found that the weldment specimens have a lower fatigue life compared to the base metal, and also the fatigue life of both base metal and weldment specimens decreased with increase in the total strain range, temperature, and holding time. In Nickel-based Alloy 617, a reduction of fatigue life was attributed to creep, oxidation, and dynamic strain aging (DSA) at elevated temperature. However, the plastic deformation regarding stress response could be possibly reflected as damage accumulation in the structural material, and it could be correlated to the fatigue life of the material. At higher temperature condition (900-950oC), Alloy 617 both base metal and weldment specimens were deformed by the plastic flow mechanism and showed a cyclic hardening mechanism with respect to total strain range. However, the Alloy 617 both materials showed typical cyclic softening mechanism for each cycle under high temperature. It is noted that the base metal presented typical solute drag creep mechanism by means of the initial stress drop at first cycle for all tests. In addition, an assessment of LCF lifetime data was performed using the well-known relationship based on a Coffin-Manson-Basquin (C-M-B) relationship, also the other fatigue damage parameter using total strain energy density were comparatively described. For the case of LCF with hold time, linear damage summation (LDS) method and frequency-modified strain life method were adopted. The material constants are determined through the fatigue life models and its validity is assessed by comparing with the experimental data. The LCF fracture surface microstructures were characterized on selected fractured specimens, and thus, the microstructural changes under various conditions are also reported quantitatively using standard metallographic techniques.