In this series of blog posts, we will look at how to reduce the net stress on springs through quality manufacturing. In this post, we delve into how shot peening can be used to achieve this.
Shot peening can be traced back to its use in the automobile industry in the late 1920s, with the process still widely used today in the production of high performance components in aerospace, automotive and other sectors.
What is Shot Peening?
Shot peening is the cold working process of firing spherical pellets at the surface of a spring material at high speed to induce plastic deformation which modifies the mechanical characteristics of the metal’s surface through inducing residual compressive stresses and work-hardening the surface of the spring.
Plastic deformation is the ‘permanent deformation or change in shape of a solid body without fracture under the action of a sustained force’.
The coverage of shot peening is impacted by the number, size and composition of the shots as well as the size of the exposure area, and is generally defined as the percentage of the surface area which is indented as a result of the process. This coverage (sometimes referred to as intensity) of shot peening is measured using Almen strips – made from SAE 1070 steel – and Almen Testing.
The Almen test is often conducted prior to the proposed shot peening process and involves a thin, test strip of the material undergoing the intended conditions. When the strip bends due to residual compressive stress (RCS) causing deflection, the largest amount of deflection (in other words, the shot peening intensity) undergone by the Almen strip is recorded as the value of the arc height.
How Does Shot Peening Act as Stress Relief?
The most important benefit of shot peening springs is the residual compressive stresses (RCS) imparted as a result. Net stress on a spring surface is the applied stress minus residual compressive stresses; therefore, shot peening is considered stress relieving as it results in reduced net stress.
The second benefit of shot peening springs is work hardening of the surface, which increases the time before the onset of fatigue cracks in the service life of a spring, and has the potential to increase a spring’s lifespan by millions of cycles. Though this is significantly less important than the gaining of RCS.
As an example, compression springs made from carbon steel (EN10270-1 DH) operating between corrected stresses of 104 and 727 MPa would be at risk of fatigue failure soon after 100,000 cycles. However, if the springs had been shot peened the predicted life would be more than 10 million cycles.
The Institute of Spring Technology’s (IST) Goodman Diagrams below illustrate the predicted performance of a compression spring with and without shot peening:
In summary, shot peening spring materials creates compressive residual stress which defers the occurrence of cracking, due to fatigue, within the spring surface.
To discover more about shot peening, visit Spring Expert, Mark Hayes’ in-depth article. For best practices when it comes to springs or to find out more about our work producing them, browse our other educational blogs and webpages. Alternatively, to request a quote or advice about a specific spring query, please complete the enquiry form on our Contact Page.