302 Stainless Wire - Shot Peening and Residual Stress
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302 Stainless Wire - Shot Peening and Residual Stress
A spring almost always fails by fatigue, and fatigue almost always starts at the surface. That single fact explains why shot peening, a process that does not change the material or the dimensions at all, can multiply the life of a part. Peening puts a thin layer of the surface into compression, and a crack cannot open while the material around it is being pushed together. It is a well established process with equally well established ways of going wrong. This page covers what peening does, how it is specified, and the four process mistakes that quietly remove the benefit.
Product Specifications
| Item | Specification |
|---|---|
| Product Name | 302 Stainless Steel Wire |
| Material | AISI 302 / SUS302 / UNS S30200 |
| Diameter Range | 0.15-12 mm |
| Diameter Tolerance | ±0.01 mm (fine wire), ±0.02 mm (0.5-3 mm), ±0.03 mm (above 3 mm) |
| Tensile Strength | 1400-1800 MPa, spring temper (size dependent) |
| Peening Media | Ceramic bead or stainless cut wire shot; no iron based media on stainless |
| Intensity | Specified by Almen strip, typically within a range rather than a single value |
| Coverage | Normally specified as full coverage, sometimes 200% where the duty is severe |
| Surface | Bright drawn, soap coated; peened where specified |
| Coil Weight | 1-100 kg |
| Applicable Standards | ASTM A313, SAE J442, SAE J443, EN 10270-3 |
What Peening Does to the Surface
Compression Instead of Tension
Shot striking the surface deform a thin layer plastically while the metal beneath stays elastic. Because the underlying material resists the expansion of that layer, the surface ends up in compression and the interior in balancing tension. A fatigue crack needs the surface to be in tension to open, and a compressive layer simply closes it. This is why peening is more effective against surface initiated fatigue than almost any other treatment, and why it does nothing at all for a part that fails from an internal defect.
Depth and Magnitude
The compressive layer is typically a small fraction of the wire diameter. On a 1 mm wire the affected depth is measured in tens of microns, so the benefit is real but thin: anything that removes material from the surface, such as grinding, polishing or aggressive pickling afterwards, removes the very layer that was created. On very fine wire the layer becomes a large proportion of the section, and the process has to be controlled more carefully to avoid distorting the part.
How Peening Is Specified
Intensity by Almen Strip
Intensity is not set by shot velocity or by pressure, it is measured. A standard steel strip is peened under the same conditions and its curvature is read on a gauge, giving an arc height in thousandths of an inch or in millimetres. The specification names the strip type and the target arc height range. This makes the process reproducible between machines and between suppliers, which is the whole point of using an indirect measure.
Coverage
Coverage describes how much of the surface has been struck, expressed as a percentage of a fully dimpled surface. Below full coverage, the unpeened areas are exactly where fatigue will start, so a partially peened part can be worse than an unpeened one because it concentrates the damage. Coverage is verified visually against a reference or by the tracer method, and severe duty parts are sometimes specified at 200% coverage, meaning twice the time needed to reach full coverage.
Media Choice
Ceramic bead and stainless cut wire shot are the usual media for stainless parts because they do not leave iron on the surface. Carbon steel shot transfers iron, and the transferred iron rusts, producing the familiar brown spotting that gets blamed on the wire. Media condition matters as much as media type: broken or worn shot changes the intensity, so the machine has a shot classification and replacement schedule.
The Four Ways It Goes Wrong
Peening Then Polishing
Polishing after peening removes the compressed layer and returns the surface to the unpeened condition, having paid for the process twice. The correct sequence is to finish the surface first and peen last.
Peening Then Plating
An electroplated coating deposited after peening introduces tensile stress and can also require a bake that relaxes part of the compressive layer. Where a coating is needed on a peened part, the sequence and the bake temperature have to be agreed as one process rather than two purchases.
Service Temperature Above the Relaxation Range
Residual stress is not permanent. Above roughly 200-250 °C the compressive layer begins to relax, and a part peened for a 20 °C application loses its advantage in a hot one. Where the service temperature is high, peening either has to be combined with a stabilising treatment or dropped from the specification.
Wrong Intensity for the Wire Size
An intensity that suits a 4 mm spring can fold a 0.3 mm wire. For fine wire the media size and the intensity are both reduced, and the part is often supported during peening to prevent distortion. Copying a specification from a larger part is a common source of scrap.
Applications
Valve and Clutch Springs
These parts cycle millions of times at a stress amplitude near the material limit, and peening is a standard step rather than an option. The specification normally includes intensity, coverage, media and the peening position on the spring.
Medical and Instrument Springs
Small springs in devices that must survive repeated actuation benefit from peening at low intensity, with ceramic media to keep the surface free of iron. The process is usually validated on the finished part rather than on the wire.
Suspension and Structural Springs
Automotive and equipment suspension springs are peened to a controlled intensity and often to 200% coverage, because the fatigue requirement is severe and the consequence of failure is not cosmetic.
Product Advantages
- Spring temper 302 with consistent surface condition, so a peening specification produces the same result from coil to coil.
- Surface free of seams and laps before peening, since a defect under the compressed layer still acts as a crack starter.
- Diameter tolerance of ±0.01 mm on fine sizes, keeping the intensity appropriate to the wire section consistent.
- Grade advice against the service temperature, because a compressive layer relaxes above the range where 302 is normally used.
- Traceability by heat number, supporting validation when a peened spring has to be qualified for a safety function.
Quality Control
Diameter, tensile strength and elongation are tested per lot, and the surface is inspected over the full coil length before any peening operation. Where peening is specified, the intensity is verified by Almen strip at the start and during the run, coverage is checked against a reference, and the media condition is recorded. The sequence of forming, peening and any coating is agreed as a single route, because a change of order can remove the benefit without changing any individual step.
Selection and Ordering Notes
- Specify intensity by Almen strip type and arc height range, together with coverage, rather than describing the process in words.
- State the media type, and exclude iron based shot for stainless parts.
- Fix the process sequence: finish, then peen, then any coating. Peening before polishing wastes the treatment.
- State the service temperature, because residual stress relaxes in hot applications and the specification may be pointless there.
- For fine wire, name the support arrangement during peening, as an intensity copied from a larger part can distort the spring.
Shot peening is a surface treatment that buys fatigue life without changing the material, and it is only as good as the sequence and the verification around it. Send the spring, the cycle requirement and the service temperature, and the peening specification can be set to match the duty rather than copied from a similar looking part.
