Abstract
Operating conditions within industrial gas turbines are changing in response to pressures to reduce environmental impact and enable use of renewable sources. This is driving an increase in the operational temperatures and pressures of combustion in turbine systems. Additionally, diverse operating environments can result in higher sulphur and trace metal contaminant levels, exacerbating hot corrosion in GT systems. Low cycle fatigue (LCF) cycling can also be intensified as a result of increased start/stop shutdowns. The combined effects of hot corrosion and stress are experimentally studied on CMSX-4 single crystal (SC) γ/γ' system under both fatigue and static stress conditions, with either a multi-axial bending or uniaxial stress state. The associated stress intensity thresholds (KTH) under the various stress conditions were evaluated using finite element analysis (FEA). Cracking was observed both under static and fatigue stress conditions in a hot corrosion environment. Crack morphologies were analysed using SEM techniques. Bending stresses and fatigue cycles demonstrated increased crack propagation in the presence of hot corrosion with static uniaxial stresses showing the longest nucleation times and lowest propagation rates.
Highlights
Developments to improve the operational efficiencies of gas turbines (GT) in recent times have led to increases in the operational temperatures of turbine blade components
This paper proposes a combined mechanism for fracture under type II hot corrosion conditions, the mechanism is based upon the following fracture principles
Type II hot corrosion combined with stress, results in a form of stress corrosion cracking, the mechanism was exacerbated by stress gradient
Summary
Developments to improve the operational efficiencies of gas turbines (GT) in recent times have led to increases in the operational temperatures of turbine blade components. A study into the effects of stress state on the fatigue crack growth in polycrystalline Inconel 718 exposed to an oxidising environment, found that with increased oxidation cracks became less influenced by stress intensity (ΔK) and more by stress state [17] With many superalloys such as CMSX-4 demonstrating anisotropic behaviour [11,9], and the under-platform region of GT blades being under a multiaxial bending stress [18], it is important to develop an understanding of the materials behaviour under these combined conditions. The proposed mechanism works on the observation that hot corrosion had the impact of locally lowering both the Kcr critical fracture threshold and the fatigue threshold Kth at the crack tip. Stop start type environmental fatigue models have been proposed, these mechanisms are based on the time dependant nature of species diffusing ahead of the crack tip causing embrittlement [21]
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