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A very common fretting environment is the dovetail root of turbomachinery blades. Shot peening is commonly used to prevent fretting failures of these roots. Blade roots have a characteristic fir tree shape. The tight mating fit coupled with demanding loading conditions require that the surfaces be shot peened to prevent failure associated with fretting. Many turbine and compressor blade roots are shot peened as OEM parts and re-shot peened upon overhaul to restore fatigue debits otherwise lost to fretting. The discs or wheels that support the blades should also be peened.
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Shot peening has proved its effectiveness in extending the service life and enhancing the performance of metal components by protecting them against fatigue, fretting fatigue, stress corrosion cracking and a variety of other failure mechanisms. Following are some of these success stories for turbomachinery / power generation applications:
Large thermal fatigue cracks were discovered in eight high pressure feedwater heaters used in a power generation application. These units operated in both an elevated temperature and thermal fatigue environment. Startups and shut downs caused thermal fatigue. Steady state operation was at 480-660 °F (250-350 °C). The cracks were circumferential in the weld heat affected zone between the water chamber and tube sheet. Fatigue cracking was attributed to years of service and 747 startups and shutdowns of the unit. This caused concern about the remaining life of the units. The cracked locations were machined and shot peened. Subsequent inspections showed that no additional fatigue cracks developed after five years of service and 150 startup and shutdown cycles.
Metal diaphragm couplings are often used in turbomachinery applications. These couplings accommodate system misalignment through flexing. This flexing, or cyclic loading, poses concerns for fatigue failures. Researchers concluded that the Electro-Chemical Machining (ECM) process produces parts that are geometrically near-perfect. However, they found under scanning electron microscope observation that small cavities sometimes developed on the surface of a part as a result of ECM. These cavities apparently generate stress concentrations that lead to premature failures. Shot peening after ECM was able to overcome this deficiency and has significantly improved the endurance limit of the diaphragm couplings.
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