Fatigue failure is the result of repeated fluctuations of stress, sometimes below the static design stress of the structure. Shot peening the finished part can significantly reduce the effect of the applied load/stress which can extend the life and strength of the component significantly.
Stress corrosion cracking (SCC)
SCC occurs when a susceptible alloy in contact with a corrosive environment is subjected to a sustained tensile stress which may be well below the elastic limit of the material. SCC can be delayed or avoided by using controlled shot and laser peening to convert these tensile stresses to beneficial residual compressive stresses.
This type of failure is apparent when localised corrosive attack occurs at the grain boundary areas at the surface of a metallic component. The corrodant reacts more readily with the grain boundary as this is a high energy site. Controlled shot peening disrupts the near surface microstructure, thus removing the pathway for the corrosive agent and so prolongs component life.
Components in corrosive environments can fail due to corrosion fatigue associated with cyclic applied loading. The failure mechanism is similar to that of SCC but is driven by the cyclic application of tensile stress rather than by a sustained tensile stress. The application of controlled shot or laser peening can extend component life considerably.
Water droplet erosion
Steam turbine blades in particular are affected by water droplet erosion which is caused by abrasive water droplets, carrying minute exfoliation/wear/corrosion pick-up materials, continually impacting and eroding the surface of the blade which can lead to crack initiation, propagation and growth. MIC can improve the life of the component by inducing residual beneficial compressive stresses either through the application of controlled shot peening or by laser peening.
Extending the life of welded components
The residual tensile stress from welding is created as the weld cools rapidly. It is unable to shrink because it has already bonded to the cooler, stronger base material resulting in a weld that is being “stretched” by the base material. Welded structures such as pressure and storage vessels, whether new or repaired are an ideal example of the success of controlled shot and laser peening where tensile stresses are converted to beneficial compressive stresses thus extending the life of the structure. SCC in these situations has been prevented in many harsh and corrosive environments.
MIC is a leading developer and applicator of engineered coatings to solve problems caused by the aggressive environments in which pumps and valves operate. Coatings can be applied that address service conditions, including wear, corrosion, erosion, seizing, steam and extreme temperatures (-420°F to 1800°F or -251°C to 982°C), as well as applicable aesthetic standards.
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