Understanding Low Cycle Fatigue Behavior of Alloy 617 Base Metal and Weldments at 900 °C

Abstract

Abstract:In order to better understand the high temperature low cycle fatigue behavior of Alloy 617 weldments, this work focuses on the comparative study of the low cycle fatigue behavior of Alloy 617 base metal and weldments, made from automated gas tungsten arc welding with Alloy 617 filler wire. Low cycle fatigue tests were carried out by a series of fully reversed strain-controls (strain ratio, R ε =´1), i.e., 0.6%, 0.9%, 1.2% and 1.5% at a high temperature of 900˝C and a constant strain rate of 10´3/s. At all the testing conditions, the weldment specimens showed lower fatigue lives compared with the base metal due to their microstructural heterogeneities. The effect of very high temperature deformation behavior regarding cyclic stress response varied as a complex function of material property and total strain range. The Alloy 617 base weldments showed some cyclic hardening as a function of total strain range. However, the Alloy 617 base metal showed some cyclic softening induced by solute drag creep during low cycle fatigue. An analysis of the low cycle fatigue data based on a Coffin-Manson relationship was carried out. Fracture surface characterizations were performed on selected fractured specimens using standard metallographic techniques.

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

“…According to Wright et al [6], they confirmed that a decrease in peak flow stress followed by an increase in the strain range is characteristic of a solute drag creep deformation mechanism. Also, it has been reported in previous work that a consequence of the solute drag creep mechanism operating during LCF of Alloy 617 at high temperature ranges of 900-950 • C [6,7] was due to the amount of solute atoms motion, which reduces the flow stress.…”

“…According to Wright et al [6], they confirmed that a decrease in peak flow stress followed by an increase in the strain range is characteristic of a solute drag creep deformation mechanism. Also, it has been reported in previous work that a consequence of the solute drag creep mechanism operating during LCF of Alloy 617 at high temperature ranges of 900-950 °C [6,7] was due to the amount of solute atoms motion, which reduces the flow stress. Branco et al [10] reported that evaluation of the variation of the cyclic softening with the total strain range during cyclic loading can be done through the characterization of the degree of softening (DOS), as given in Equation (3):…”

“…Therefore, a thorough knowledge of the elastoplastic behavior is necessary in the design and life evaluation of such components that operate in the high temperature condition. Although Alloy 617 has many superior properties, many researchers have reported that the number of cycles to failure varies widely at high temperatures [5][6][7][8][9]. However, they did not provide sufficient data on the Alloy 617 in the LCF regime over a range of conditions.…”

“…Under this circumstance, the cyclic loadings in the low cycle fatigue (LCF) regime describe a main failure mechanism from a temperature-gradient-induced thermal strain during operation, as well as startups and shutdowns and power transients or temperature change of flowing coolant [5,6]. In the state-of-the-art, fatigue damage accumulation as a stress and strain concentration during cyclic loading is potentially causing the crack initiation site and propagation onwards [7]. Therefore, a thorough knowledge of the elastoplastic behavior is necessary in the design and life evaluation of such components that operate in the high temperature condition.…”

Low Cycle Fatigue Behaviour of DP Steels: Micromechanical Modelling vs. Validation

“…Several researchers have developed correlation models to predict the cyclic deformation behavior of steels from monotonic tensile properties and hardness [42][43][44][45]. Smith et al [46] discovered the correlation between the hardening or softening behavior and the ratio of ultimate tensile strength to yield strength (S u /S y ) from sixteen materials including steel, aluminium and titanium alloys.…”