Shot peening vs. deep rolling

When components fail prematurely under dynamic loads, it’s often due to a lack of compressive residual stresses in the subsurface area. Shot peening is considered a proven process for addressing this issue—but how does it work, what are its limitations, and what alternatives, such as deep rolling, offer even greater potential?

Targeted adjustment of surface and subsurface properties is becoming increasingly common. However, it is not uncommon for suitable post-treatment processes to be applied rather haphazardly and as a stopgap solution. In the field of mechanical surface treatment, the surface of a component is mechanically strengthened using various mechanisms of action. Thus, no additional heat treatments or coating methods are used [1]. 

Among the most commonly used processes are: 

• Shot peening
• Roller burnishing
• Diamond smoothing
• Deep rolling
• Machine hammer peening

In all of these processes, a “tool” is pressed against the surface of the component. This generates such high local mechanical stresses that the surface undergoes local plastic deformation.  The result is always a change in surface topography, cold work of the subsurface area, and the introduction of strong compressive residual stresses. 

In this series, shot peening is, so to speak, the geometrically undefined process, analogous to grinding in machining. In this process, the blasting medium—also known as blasting material—is accelerated and propelled onto the surface of the component. Blasting particles strike the surface in an undefined manner, causing plastic deformation and the desired strengthening of the material [2]. 

Shot peening always takes place in specialized shot peening systems. These systems are often very large, require a high degree of maintenance and operator expertise, and are therefore frequently relocated to specialized service providers where the process can be purchased as a service. 

Blasting media is accelerated, for example, by a centrifugal wheel or by a jet of air. In both cases, velocity can be controlled. This alone has a significant impact on the process outcome. A higher impact velocity also means greater momentum per individual grain and thus greater deformation.

In addition to velocity, the blasting media is, of course, another critical parameter. It can be made of various materials (steel, hardened steel, glass, or ceramic) and can have different shapes. The term “shot peening” implies that the blasting media are spheres. However, this is not necessarily the case. Depending on the quality of the media, a distinction can be made between wire shot and spheres. 

In the case of wire-shaped grit, a wire of a defined thickness is cut and not further processed. These particles tend to have sharp edges and bear little resemblance to a sphere. If the grit is further processed, it can be shaped into a nearly perfect sphere. The shape of the abrasive naturally also influences the result, just as the thickness of the grain does. Whether the abrasive is 2 mm thick or only 0.2 mm, the energy required for deformation will naturally differ. 

The third key parameter is coverage. If an area is moved very slowly beneath the media, then naturally a large number of grains will strike the surface. The same applies when the surface is passed through the stream multiple times. 

The result of the process is then measured in what is known as the “Almen intensity.” In this process, a defined sheet metal strip—the Almen strip—is blasted using specified parameters. As a result of the change in residual stresses, the sheet metal strip deforms, and this deformation can then be quantified using the “Almen intensity” parameter. 

The process result can therefore only ever be verified indirectly. The Almen strip is therefore only a makeshift solution, since the actual process on the real component can never be verified directly and a subsequent measurement of residual stress cannot be performed non-destructively. 

Plastic deformation of the surface produces three desired effects: 

1. Inducing compressive residual stresses
   Plastic deformations in the subsurface area ensure that compressive residual stresses form in that area. Magnitude and depth of the compressive residual stresses can be influenced by coverage, blasting velocity, and size of the blasting media.
2. Induction of cold work
   Plastic deformation in the subsurface area also causes a change in the microstructure. This results in an increase in dislocation density, which in turn increases hardness and strength in the subsurface area.
3. Increased component service life
   Both of these changes in properties ensure that service life and resistance to dynamic loading are increased. A shot-peened component can therefore withstand higher loads over a longer period of time, and its fatigue strength increases (Figure 3). 

The most commonly used alternative to shot peening is deep rolling, also known as burnishing. In this process, a roller or ball is pressed into the surface with a defined force, thereby creating the necessary plastic deformations. 

Unlike shot peening, burnishing tools can be directly integrated into machining centers (lathes, milling machines etc.). The required process force is so low that it can be easily applied by a machine tool. 

Continuous movement of the burnishing body enables highly precise machining and also significantly reduces surface roughness (improvement of >95% possible). 

Compared to shot peening, the roller or ball used in deep rolling is generally many times larger than the blasting media. For this reason, the achievable influence on the subsurface area is also greater in most cases. Although the maximum compressive residual stress value is comparable, the depth effect is simply greater in deep rolling due to contact geometry. 

This also results in the fatigue strength of 20MnCr5 being a further 80% higher compared to shot peening under otherwise identical conditions, as demonstrated by Lechleiter et al. [3]. Figure 4 shows the 50% Wöhler line for a 20MnCr5 material under cyclic bending loading for both shot peening (see Figure 3) and deep rolling.

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