In Part 1, we covered the core principles behind shot peening. Here, we take that foundation a step further—because achieving optimum fatigue performance isn’t just about setting standard parameters. It requires a more considered approach, taking into account how the spring is designed, the material it’s made from, and the conditions it will ultimately face in service.

While factors such as shot size, media type, and Almen intensity remain central to the process, the most effective specifications are always those tailored to the real-world application.

Duplex Peening

For more demanding applications, we often look beyond single-stage peening. Duplex peening is one such approach that can deliver measurable performance gains.

This involves two carefully controlled stages. The first uses a larger shot size to introduce deeper compressive stresses beneath the surface, creating the familiar dimpled profile. The second stage then uses finer media to refine the surface, smoothing those indentations and redistributing the compressive stresses closer to the surface where crack initiation typically begins.

In a standard single-stage process, peak compressive stress is usually found around 50 microns below the surface. With duplex peening, this can be brought closer—often to around 20 microns—offering improved resistance to crack initiation and early-stage fatigue failure. Between stages, we commonly apply a low-temperature heat treatment (typically 200–250°C) to stabilise the stress profile before refining it further.

Distortion

One of the realities of shot peening is that introducing compressive stress beneath the surface can lead to slight distortion. In most cases this is minimal, but where tight tolerances are critical, it’s something we need to factor in early.

There are several practical ways to manage this:

• Using smaller shot sizes
• Reducing Almen intensity
• Applying a light stress relief (around 225°C for 30 minutes)
• Introducing cold pre-stressing after peening
• Running sample trials to understand distortion and design accordingly

In many cases, a small adjustment at the design stage can avoid unnecessary rework later.

Spring Design Considerations

Shot peening is fundamentally a line-of-sight process, which means geometry matters more than you might expect.

Features like tight coil spacing, shadowed areas, or complex profiles can limit how effectively the media reaches the surface. When that happens, coverage—and therefore performance—can become inconsistent.

To address this, we may need to adapt the process or even the design itself:

• Selecting smaller shot sizes to improve access
• Extending processing time
• Reducing batch sizes to improve exposure
• Using pulse peening techniques
• Introducing fixtures to improve part movement
• Adjusting design features such as coil spacing

Early collaboration between design and processing can make a significant difference here.

Material Selection

Material choice has a direct impact on how shot peening should be applied.

For example:

• Stainless steels and non-ferrous alloys often benefit from non-metallic media, such as glass or ceramic, to prevent contamination
• Post-peening treatments like pickling or passivation may be required to restore corrosion resistance if steel shot has to be used

In reality, material details aren’t always fully defined at the processing stage, which is why close communication between customer and manufacturer is so important when finalising specifications.

In-Service Relaxation

It’s also worth considering what happens after the spring leaves production.

The compressive stresses introduced during peening can gradually relax over time—particularly in elevated temperature environments—which may lead to a loss of load.

To reduce this risk, we typically consider:

• Lowering shot intensity or shot size
• Applying stress relief after peening
• Pre-setting the spring (cold or hot methods)
• Choosing materials with better resistance to relaxation

These small decisions can significantly improve long-term performance.

Corrosion Considerations

Shot peening can also influence corrosion behaviour. By removing the oxide layer formed during heat treatment, the process can expose fresh material—particularly in carbon steels—making it more susceptible to corrosion if left untreated.

Where this is a concern, we would usually recommend:

• Post-peening stress relief
• Temporary corrosion protection
• Application of coatings (passive or sacrificial)
• Selecting inherently corrosion-resistant materials

Again, it’s about looking at the full lifecycle of the component, not just the immediate performance gain.

Relevant Standards

When developing or validating a shot peening process, several established standards can provide useful guidance:

• DEF STAN 03-21/2 (1995) – Mechanical Methods for Inducing Compressive Surface Residual Stress
• MIL-S-13165C (1991) – Shot Peening of Metal Parts
• SAE HS-84 (1992) – SAE Manual on Shot Peening

These serve as a strong reference point, particularly for critical or regulated applications.

While standard shot peening parameters give us a solid starting point, the best results always come from a more holistic view. By considering geometry, material behaviour, and real service conditions together, we can ensure the process delivers maximum fatigue performance—without compromising dimensional accuracy or long-term reliability. 

If you’re looking to explore shot peening options, feel free to get in touch with our Shot Peening Manager, Andrew Ruddiforth, who will be happy to help with advice or a quotation.