What Is Spring Steel and How Is It Made?
What is spring steel? The definition might seem to describe itself, leading you to believe it's a simple descriptor of whatever steel is used to fabricate springs. However, that is not the case at all. In this brief survey, we will draw back the curtain on one of the most commonly used yet ill-defined spring design and manufacturing terms.
In broadest terminology, Spring Steel is an encompassing, descriptive name for a loosely defined category of steel commonly used to manufacture products such as helical coil springs. The term doesn’t refer to one specific steel composition or manufacturing process; it refers to a broad class of steels that have undergone specialized secondary processing operations and have similar mechanical properties, such as excellent elasticity and resilience, as well as high strength and hardness.
These properties and others make spring steel an excellent choice for many high-performance spring applications.
What Is Spring Steel?
Spring steel is one of many types of spring materials, which determines many properties that define how a spring functions in application.
Specifically, spring steels all have a high material Yield Strength. This property refers to the material's ability to elastically deform and return to the previous shape without any permanent deformation. This is a very important attribute for springs because springs of all types are designed to experience visible deformation of the helical coils under applied loads.
Spring steels also have excellent Resilience, allowing springs to absorb large amounts of energy and experience significant strain without permanent deformation. This property is a function of the spring steel’s elasticity and yield strength.
Spring steels have excellent Fatigue Resistance, meaning they can handle cyclic loading of large magnitudes without fracturing.
Spring steels also have excellent Hardness, which refers to the material's ability to resist highly localized plastic deformation in a surface indentation or scratch. This property makes springs resistant to wear and allows the spring to maintain its shape under deformation.
The above mentioned properties are all a function of the spring steel’s chemical composition and secondary manufacturing operations. Both of these are detailed in the following sections.
Composition and Properties of Spring Steel
Spring steel is formally defined as low alloy, high carbon steel; however, this definition requires careful explanation and qualification.
Content of Alloying Ingredients
First, what is commonly called “Alloy Steels” get their name simply because they are alloyed with significant quantities of alloying elements, not just carbon. Other modifying terms, such as “Low” or “High” alloy steels, refer to the amount of alloying ingredients contained in the steel. High-alloy steel contains total alloying ingredients that comprise more than 8% of the steel’s total composition, while Low-alloy steel contains small concentrations of total alloying elements- typically around 5%.
To that end, many spring steels rely on Manganese or Silicon as additional alloying ingredients.
Manganese adds hardness and strength to steel and is common in some volumetric percentages of spring steel. Silicon is present in most spring steels steel alloys and also adds hardness and strength to the steel as well; however, it also acts as a deoxidizer to remove impurities and improve the soundness of the steel, meaning it helps ensure the microstructure is free of lattice defects. Silicon and Manganese together significantly increase the yield strength of the steel as well, which is the most important mechanical trait that spring steel must have so that the springs can deform elastically under high loads.
While these represent two of the most common alloying ingredients in spring steel, other combinations of alloying elements are used as well, such as chromium, vanadium, or molybdenum.
Carbon Content
One last clarification related to the chemical composition of spring steels is critical to understand. High-carbon steel is defined as a total carbon content above 0.60%. Conversely, low-carbon steels contain approximately 0.30% carbon or less, while medium-carbon steels contain between 0.30% and 0.60%.
Therefore, putting all of this together, spring steels are low alloy, high carbon steels.
How Is Spring Steel Made?
However, the chemical composition of spring steel answers part of the “How is spring steel made?” question. The remainder of the story is told through the steel fabrication process. In general terms, the first three steps below describe the steel-making process using an electric arc furnace (EAF) and continuous casting method, which is the process by which most steel in the USA is produced today.
“Typical” Steel
- Raw Material Selection and Preparation: The base ingredients of steel are chosen before beginning any of the processing steps. The base low alloy steel is usually scrap steel, while additional ingredients are chosen as needed depending on the desired alloy.
- Alloying: The base metal and alloying ingredients are heated past their melting point to create molten steel within the EAF. Here, the base steel and alloying elements such as silicon, manganese, or chromium are combined at a chemical level.
- Continuous Casting and Hot Rolling into Strips or Rods: The steel is fed through continuous casting roller mills, which form the spring steel as it cools. This process draws the steel into long strips, rods, or plates, depending on the desired end product.
Spring Steels
One of the hallmarks of spring steel is that it has undergone an additional secondary process called Hardening, which involves undergoing one or both of the following processes.
- Work Hardening: Work hardening (or cold working) spring steel involves deforming the spring steel at room temperature using a high-energy physical process such as hammering, rolling, or drawing. Work hardening deforms the spring steel beyond its elastic limit, reworking the atomic grain structure of the spring steel to strengthen and harden the spring steel. This plastic deformation process reworks the grain structure of the spring steel to increase the material's strength and hardness.
- Heat Treatment Hardening: Heat treatment hardening spring steel involves heating the spring steel to high temperatures, approximately one-third of the melting temperature, in a furnace in a high-energy heating process. After heating, the steel is quenched in oil or water to quickly return to room temperature. This rapid cooling process is the hallmark of heat treatment hardening because it preserves the changes in the atomic grain structure, increasing its strength and hardness.
One or both secondary processes are performed depending on the specific spring steel. Every aspect of these processes, such as temperature, time held at temperature, and quenching, must be carefully controlled to ensure that the spring steel meets all desired mechanical strength, hardness, and ductility specifications.
As an additional note, Annealing or Tempering are additional processing steps that may be performed on the spring steel, depending on the specific grade. In simplest terms, both processes generally involve heating the spring steel to high temperatures that range from one-fifth to one-third of its melting point. However, annealing is done before the heat treatment and/or work hardening to restore some ductility to the metal. The spring steel is cooled at a slow, highly controlled rate during annealing.
Tempering is performed after the work hardening or final heat treatment and typically requires a long hold time at the specified temperature and a slow cooling of the metal at room temperature. Tempering is a final processing step that relieves internal strains in the metal and, like annealing, returns some ductility to the spring steel lost during the hardening process.
Types of Spring Steel
As detailed previously, there are many different types of spring steels. The most common steels are shown below by their SAE Grade designation, which uses 4-digit codes to determine the composition of steels. The first digit defines the general class of steel; for example, 1000-series steels are carbon steels, and 5000-series steels are Chromium steels. Furthermore, these first two digits further define the grade of steel. For example, 1300 represents carbon-manganese steels. The last two digits define the carbon content of the steel - denoted as a percentage below 1%.
With these descriptors, we can navigate the common grades of spring steels as follows. For example, shown in the table below, a very common spring steel grade is 9255. Using these SAE designations for the steels makes it easy to determine the composition of the steel because the first two digits of 92 mean the steel is silicon-manganese steel.
Below are a few of the most common spring steels.
Common Spring Steels | |
| SAE Steel Designation | Notes |
| 1070 |
|
| 1095 |
|
| 5160 |
|
| 9255 |
|
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