Fatigue is defined as: cumulative, localised and permanent damage caused by repeated fluctuations of stress, sometimes below the static design stress of the structure. Shot peening the finished part to replace the residual tensile stresses caused by manufacture 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 if tensile stresses resulting from manufacture or in-service use are converted to compressive stresses or reduced by controlled shot peening.
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 prolonging 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 peening to introduce compressive stresses can extend component life considerably.
Galling usually refers to adhesive wear and transfer of material between two surfaces as they slide or move together, initiating friction or plastic deformation between the surfaces resulting in cold welding between the peak points where they touch. The adhesion of the opposing surfaces when in contact can be minimised by a coating protection and/or changes in material properties in the near surface area.
Thermal cyclic failure
Components operating under fluctuating temperature conditions are subject to alternating cyclic thermal stresses which can result in plastic deformation and consequent failure. Shot peening has been shown to improve the life of these components.
Extending the life of welded components
Welded structures such as pressure and storage vessels, whether new or repaired are an ideal example of the success of controlled shot peening where tensile stresses are converted to beneficial compressive stresses thus extending the life of the structure. 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. The net result is a weld that is essentially being “stretched” by the base material and residual tensile stresses result. SCC in these situations has been prevented in many harsh and corrosive environments.
Engineered coatings for pumps and valves
MIC is the 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/-251°C to 982°C), as well as applicable aesthetic standards.
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