The Process of Shot Peening and its Effects on the Fatigue Life of Steel Springs

Shot peening is a widely used surface treatment process in which small spherical particles, known as shot, are repeatedly impacted onto the surface of a material. This process is typically used to improve the fatigue life and strength of components, particularly steel springs. The correct answer to the given question is option 'D', which states that shot peening results in residual compression at the surface.

Explanation:
Shot peening induces compressive residual stresses on the surface of the material, which is one of the main reasons for the increased fatigue life of steel springs. This effect is achieved through the following mechanisms:

1. Plastic Deformation:
When the shot particles impact the surface of the material, they cause localized plastic deformation in the form of dimples or craters. This plastic deformation introduces compressive residual stresses in the affected regions. These compressive stresses counteract the tensile stresses that are typically present during the service life of the spring, thereby reducing the risk of crack initiation and propagation.

2. Cold Work:
The repeated impacts of the shot particles also induce cold work on the surface of the material. Cold work refers to the plastic deformation of a material at temperatures below its recrystallization temperature. This cold work increases the dislocation density in the material, leading to the formation of dislocation tangles and other structural changes. These changes contribute to the development of compressive residual stresses.

3. Grain Refinement:
Shot peening can also induce grain refinement in the material. The high strain rates and localized plastic deformation during the peening process promote the formation of smaller and more equiaxed grains. This grain refinement further enhances the strength and fatigue resistance of the material.

4. Surface Hardening:
Although not the main reason for the increased fatigue life of steel springs, shot peening can also result in some degree of surface hardening. The impact of the shot particles causes work hardening, which increases the surface hardness of the material. However, it is important to note that the residual compression at the surface is the primary factor contributing to the improved fatigue life.

In conclusion, shot peening increases the fatigue life of steel springs mainly because it introduces residual compression at the surface. This residual compression, along with other effects such as plastic deformation, cold work, grain refinement, and surface hardening, helps to reduce the risk of fatigue failure by counteracting the tensile stresses experienced during service.