Spring stress relief and shot peening: what to specify

Why residual stress matters

Coiling and bending leave residual stress in a spring. The outside of the coil is stretched and the inside is compressed, and the balance of those stresses remains in the part after the tool is removed. Some residual stress is useful, because it helps the spring hold its shape. Too much, in the wrong place, can open a crack or move the free length after the part is packed.

Residual stress also interacts with the working stress. A tensile residual stress at the surface adds to the tensile stress from the load, which is where a fatigue crack starts. A compressive residual stress does the opposite. Stress relief and shot peening are two ways to change that surface balance.

Neither process is a substitute for a sound design. If the working stress is too high for the material, a post-forming treatment will not make the spring reliable. The treatment buys margin; it does not create it from nothing.

Stress relief: what it does

Stress relief is a controlled heat treatment at a temperature below the tempering temperature of the wire. It allows the residual stress to relax without softening the material enough to lose the spring temper. The result is a more stable free length and a lower risk of cracking at the bends.

The temperature and time depend on the wire grade, the amount of cold work and the strength the part needs to keep. A music wire spring and a stainless spring do not use the same schedule. The wire supplier or the heat treater should confirm the window for the actual lot, because an over-high temperature can reduce the load and an under-high temperature can leave the stress in place.

Stress relief is often confused with annealing. Annealing softens the material and changes the spring rate. Stress relief is a lower temperature process that keeps the temper. If a drawing says anneal when it means stress relieve, the supplier may deliver a spring that no longer meets its load.

Shot peening: what it does

Shot peening bombards the surface with small media at a controlled velocity. Each impact leaves a small dimple and a compressive residual stress in the surface layer. That layer opposes the tensile stress that starts a fatigue crack, so the fatigue life at a given stress range can rise.

The process is controlled by the Almen intensity and the coverage. Intensity describes the energy of the shot stream, measured with a standard Almen strip. Coverage describes the proportion of the surface that has been peened. A note that says shot peened without both numbers is not a specification.

The media, the nozzle distance, the angle and the time all affect the result. A part with a deep recess or a shielded area may need a fixture or a second pass to reach the surface. Ask how the supplier confirms coverage in the areas that matter, not only on the outside of the coil.

The sequence that works

The order of the operations changes the result. Forming comes first. Stress relief follows forming, before the final set, so the part can relax and then be set to the final length. Shot peening follows the set if the set would otherwise erase the compressive layer, or precedes it if the set is light and the process is qualified that way. The sequence has to be written down.

If the part is plated, the plating and the bake come after the mechanical processes. A bake before plating does nothing for the hydrogen introduced by plating. If the spring is peened and then plated, check that the plating does not need a bake that would soften the peened layer or the base wire.

Cleaning sits between the operations. A contaminated surface can trap media or resist the heat treatment. A part that is handled with bare hands between cleaning and peening can carry oil into the surface. The process route should say where the cleaning steps sit.

1. 01

   Form and cut

   Coil, bend and cut to length. Record the wire lot and the machine settings.

2. 02

   Clean

   Remove oil, drawing lubricant and loose particles before any thermal process.

3. 03

   Stress relieve

   Heat to the agreed window for the wire grade, hold, then cool in a controlled way.

4. 04

   Set or scragg

   Compress beyond the working length to stabilise the free length and the load.

5. 05

   Shot peen

   Apply the specified Almen intensity and coverage, including recessed areas.

6. 06

   Finish and verify

   Plate or coat if required, bake where needed, then measure load and free length.

What to specify and what to record

A process note is only useful if it can be checked. The table below sets out the minimum specification and the record that should come back with the parts.

What to put on the drawing

Put the process call in a note, not in a tolerance block. Name the standard where one applies, give the intensity and coverage for peening, and give the temperature window and hold time for stress relief. State the sequence if the set and the peen interact.

• Stress relief temperature window, hold time and atmosphere
• Set length and the sequence relative to peening
• Shot peening standard, Almen intensity and coverage
• Areas that must not be peened, with a mask or a plug
• Plating and bake requirement, with the delay before baking
• Load and free length check after all thermal and mechanical processes

Inspection and records

Inspection should prove the process, not only the dimension. For stress relief, ask for the furnace record and the batch identity. For shot peening, ask for the Almen strip result and a coverage check.

Measure the load and the free length after the final process. A load test before stress relief or before the set does not represent the delivered part. If the process changes the free length, the drawing tolerance has to allow for it, or the process has to be tuned to land inside the band.

Keep the records with the first article. If a batch later shows a load drift or a crack, the process record is the first place to look. A supplier who cannot produce the record is asking you to trust the note on the drawing.