Shot peening has long been recognized for its ability to enhance the mechanical performance of materials. Particularly in the automotive industry, it has become a standard surface treatment, valued for its significant impact on fatigue resistance and component durability. Today, shot peening is not just an auxiliary process but an integral part of design and engineering strategies to ensure optimal in-service strength and extended operational life.

The primary advantages of shot peening are attributed to the introduction of compressive residual stresses and surface work hardening in the treated material layer. These effects are critical in delaying crack initiation and growth, thereby boosting fatigue life. As such, shot peening is typically performed as a final treatment step. Any subsequent surface modification—mechanical or thermal—risks compromising the beneficial surface layer and diminishing the properties achieved through peening. For example, post-peening operations such as grinding may remove the hardened surface, while heat treatments can relax residual stresses and reduce dislocation density, undermining the treatment's effectiveness.

Research done in recent years has opened new frontiers for shot peening, and it has been demonstrated that by using high coverage or intensity, shot peening can induce severe plastic deformation, leading to significant microstructural refinement—including grain sizes reduced to the nanometer scale. This positions shot peening within the broader family of severe plastic deformation (SPD) techniques, which leverage high dislocation densities and strain accumulation to achieve grain fragmentation and microstructural transformation.

Implementing SSP effectively requires a shift in process design. Rather than optimizing parameters solely for residual stress induction, the focus must shift to maximizing plastic strain. This involves adjusting both the energy input (intensity) and treatment duration to reach the threshold necessary for nanostructuring.

But what makes this unusual way of shot peening remarkable is its potential to enhance high-temperature diffusion processes, a property directly linked to the presence of ultrafine grains. For instance, nanostructured surfaces exhibit improved atomic mobility, which can significantly accelerate diffusion-driven treatments such as nitriding or low-pressure carbonitriding.

There are two main strategies to exploit this synergy: maintaining conventional thermochemical parameters while leveraging the enhanced diffusivity of the nanostructured surface to increase case depth and mechanical performance (e.g., improved fatigue resistance and wear behavior), and optimizing treatment efficiency by reducing the duration or temperature of the thermochemical process without compromising results. For example, nitriding could be shortened in time while maintaining standard temperatures (500–550°C), yielding substantial cost and time savings—especially beneficial in high-volume production settings typical of the automotive sector.

Alternatively, reducing the treatment temperature can lead to significant energy savings, though this approach may be less attractive to commercial treatment providers due to lower profit margins associated with reduced thermal input.

That said, the integration of this way of shot peening into mainstream industrial practice is still in its early stages. Transitioning from laboratory validation to industrial implementation requires robust experimental evidence and a deep understanding of the interplay between microstructural evolution and final component performance.

Nevertheless, the potential of this treatment as both a surface-strengthening and a thermochemical-enhancing treatment is undeniable.

Shot Peening in the Automotive Industry

by Mario Guagliano

Contributing Editor MFN and 

Full Professor of Technical University of Milan

20156 Milan, Italy

E-mail: mario@mfn.li