Spring Insights Series
Spring Metallurgy
Part 1 • Introduction
Exploring how spring alloys achieve the unique combination of strength, ductility and fatigue resistance required for demanding applications.
Within the spring industry, we use a wide range of alloys, from plain carbon steels to nickel-based superalloys. However, spring materials require a very specific combination of mechanical properties to perform correctly.
First and foremost, alloys need a relatively high yield strength so that the material can withstand significant stresses during operation without permanently deforming. Tensile strength also needs to be high, as this has a major influence on a spring’s fatigue life. If the same stress amplitude is applied to a weaker material and a stronger material, the stronger material will generally achieve a longer fatigue life, assuming everything else remains equal and there isn’t a particularly unfriendly inclusion lurking in the microstructure.
Figure 1. Simplified stress-strain curve showing yield strength, tensile strength, elastic deformation and plastic deformation.
At the same time, these alloys must be capable of withstanding the deformation required during cold coiling and forming. This creates a challenging balance, as increasing strength generally results in reduced ductility, which is the material’s ability to deform without fracturing.
Broadly speaking, there are three main ways to alter a material’s mechanical properties:
- Plastically deform it (cold working)
- Heat it and cool it in a controlled manner (heat treatment)
- Add alloying elements
This is, of course, a simplification, but it is fundamentally true. To achieve the properties required for spring applications, all three methods are often used in combination. In many cases, multiple processing steps and tightly controlled parameters are required. Patented drawn wire is a good example.
Deviation from these processes can produce a material that lacks the properties necessary for successful spring manufacture and operation.
This may seem obvious to those who have worked with spring materials for many years. But why does deforming a material make it stronger? Why can heating and cooling dramatically change its properties?
Over the coming months, I will explore the methods used to transform alloys from a simple raw material into highly engineered products with the precise properties required by the spring industry. Along the way, we will examine topics such as dislocation movement, phase transformations, and the microstructural changes that ultimately determine mechanical performance.
The aim is to keep these articles accessible while providing enough depth to explain why spring materials behave the way they do. Hopefully, by the end of the series, you’ll have a greater appreciation of everything that has happened to these alloys before they start bouncing around in service.
If there are any aspects of spring metallurgy that you would like covered in a future article, feel free to email me at conor@ist.org.uk. I’ll do my best to provide a clear and practical explanation.