Shot Peening

Author:  Fuchs, Henry O.
Source:  Mechanical Engineers Handbook, pp 941-951 Chapter 32
Doc ID:  1986022
Year of Publication:  1986
Abstract:  
Shot peening is a process for treating metal parts by controlled high-speed impact of many balls (shot). It is used mainly to increase the resistance of metal parts gainst fatigue, fretting fatigue, stress corrosion, and corrosion fatigue. It is also used to form slender metal parts to desired shapes, to produce a desired surface topography, to harden surfaces, to close pores, and to test bonds. Practically all automobile valve springs and suspension springs are shot peened to prevent fatigue failures and most modern airplanes have wing skins formed by shot peening and shot peened to prevent stress corrosion. The process is extremely flexible: Parts of any shape and of any size can be treated. Control of the process is essential, but the process is more tolerant of minor deviations from the specified parameters than heat treating or precision machining are. Table 32.1 shows data on nine typical applications. Although each application must be considered individually, the following points can be made. In general, the process works by plastically deforming a shallow surface layer. The shot impacts must be strong enough to achieve this, and they must cover the surface well enough to subject all of it to the effects of the plastic deformation. Plastic deformation that extends too deeply into the part decreases the beneficial effect and may be detrimental. This also applies to the presence of sharp corners and to fins on thicker parts. Peening produces a small extension of the peened surfaces and an extension or curvature of the entire peened part. This must be considered when high precision of dimensions is required. When used to increase fatigue resistance, shot peening is more effective on harder materials than on softer materials (Example 6 of Table 32.1); is more effective on notched parts and on weakened surfaces than on smooth specimens (Examples 2 and 5 of Table 32.1); is more effective against applied tensile stresses and against shear stresses than against compressive stresses; and is more effective against applied stresses cycling from around zero to a maximum than against fully reversed stresses. To prevent stress corrosion, shot peening must completely cover the surface. When forming parts, shot peening is more effective for forming slender parts, for instance, aircraft skins. No dies are required, only checking fixtures. The reasons for these rules and the methods for predicting effects on fatigue will be discussed in Section 32.5.