Abstract
High-temperature corrosion fatigue, a combination of corrosion with a fatigue cycle, is an emerging generic issue affecting power generation and aero gas turbine engines and has the potential to limit component life. Historically, surface treatments, such as shot peening have been used to improve component life and have been optimised for fatigue response. Research into optimisation of shot peening techniques for hot corrosion and high-temperature corrosion fatigue has shown 6–8A 230H 200% coverage to provide overall optimum performance for nickel-based superalloy 720Li based on the limited data within this study. Utilisation of electron backscatter diffraction techniques, in combination with detailed assessment of corrosion products have been undertaken as part of this work. The resultant cold-work visualisation technique provides a novel method of determining the variation in material properties due to the shot peening process and the interaction with hot corrosion. Through this work it has been shown that all three shot peening outputs must be considered to minimise the effect of corrosion fatigue, the cold work, residual stress and surface roughness. Further opportunity for optimisation has also been identified based on this work.
Highlights
The drive towards more efficient methods of power generation and the associated reductions in greenhouse gas emissions require existing gas turbine engines to operate under ever-increasing service temperatures
It should be expected that an optimum condition may exist where the surface roughness is acceptably low and the strain-hardened depth (SHD) is sufficient to retard micro-cracking
Air fatigue, hot corrosion and high-temperature corrosion fatigue have all been assessed with regards to the interaction with shot peening on 720Li
Summary
The drive towards more efficient methods of power generation and the associated reductions in greenhouse gas emissions require existing gas turbine engines to operate under ever-increasing service temperatures. To meet these demands, future designs may utilise novel or improved alloy systems with an enhanced temperature capability and employ a range of thermal barrier coatings.[1] Irrespective of such advances, the safe and efficient operation of new and existing plant will, to an ever-increasing degree, rely on a fundamental understanding of rotor and blade materials. The mechanical response during prolonged exposure to hot corrosive conditions is an emerging generic issue facing the power generation industry.[2]. Nicholls et al have investigated the controlling parameters behind such damage, in particular, the roles played by surface condition, local in situ gaseous chemistry and surface residues.[3,4]
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