Written by Andrew Ruddiforth, Shot Peening Manager IST
Shot Peening
Here at IST, we have accumulated over 50 years of experience in shot peening, specifically applied to springs. The aim of this guide is to help advise our customers while providing the background information required to assist spring manufacturers and their clients to develop an appropriate shot peening specification.
Shot peening is one of the most effective techniques for enhancing the fatigue performance of springs and is a critical manufacturing step for applications where springs are exposed to high cyclic stresses over long periods. However, achieving optimal performance depends heavily on adhering to a precise and well‑defined process.
Shot peening is a line-of-sight cold forming process relying on the impact of hard shot particles on the surface of the spring to generate a beneficial sub-surface residual stress profile in the spring. To achieve this, the springs are continuously tumbled in small batches beneath a curtain of high velocity shot.
Standard Processing
The main factors to be considered when developing a shot peening process specification are the shot size, shot type and the Almen arc rise.
Shot size must be appropriate for the section size (wire diameter for round wire springs) of the spring to be peened. Large shot sizes can develop a high Almen arc rise but can cause significant distortion problems within the springs, particularly in smaller springs. If the shot size is excessively large, then the fatigue performance may also be impaired.
We perform shot peening using two shot media, the first being a rounded steel shot of high hardness (i.e. conditioned cut wire, cast steel) and the second being glass beads.
Steel shot is only available down to a shot size of S070 (0.18 mm nominal diameter) this is not appropriate for springs made from wire smaller than .076 mm diameter.Â
The choice of peening media is dictated by the spring material as well as the wire diameter of the spring. With non-ferrous springs, it is recommended to peen using glass bead media due to issues with post-peening surface corrosion. There are also other circumstances which may require different conditions or additional processing (such as heat treatment or pickling and passivating).
Almen strips are used to measure an Almen arc rise, which is the main process control parameter. The arc rise is an indication of the intensity of the blast media. It is measured using either type ‘A’ or ‘N’ Almen strips, depending on the energy of the shot stream. The value specified should be the arc rise achieved when all the surface of the spring has been covered (known as 100% saturation). The intensity of the peening can be controlled by varying the air pressure or impeller speed, depending on which of our type of machines is used. We can also vary the peening time to achieve the required Almen arc rise.Â
The Almen strip dimensions are as follows:
- ‘A’ strips 76 X 19 X 1.30 mm
- ‘N’ strips 76 X 19 X 0.80 mmÂ
‘N’ strips are thinner than ‘A’ strips so arc (bend) more easily, therefore are more sensitive to the shot intensity than the thicker ‘A’ strips.
Although the above standard process conditions will generally provide the optimum fatigue performance.
IST have available for sale quality ‘C’, ’A’ & ‘N’ strips to the precise hardness & dimensional specifications.
The main factors which the spring designer should consider will be outlined in part 2 in our next edition of The Spring Industry Newsletter.