Selecting a spring material can be challenging and confusing. I have often misdirected by the depth of great insight to this subject. Sometimes too much information leads to indecision.
Music Wire Steel is the best choice in spring material. Only change to a different material if your spring is over 0.18″ in diameter, used in a highly corrosive environment or used medical, food, aeronautics or nuclear applications. There may also be better materials for used in extremely high or low temperatures.
First of all, this is a very broad topic and is not an in-depth resource by design. If your questions are not answered here, please reach out to spring manufacturers for your particular design needs. Whatever situation you are in, they have probably seen it before.
Creep and Magnetism
Before we get into the specifics of spring materials, I wanted to elaborate on two terms first. Creep and Magnetism.
Creep is a form of plastic deformation but it occurs under lower than yield loads and over long periods of time. Aluminum is notorious for this. The power distribution industry has longed to use more and more aluminum instead of copper due to price, availability and better electrical properties, but creep has prevented them. What happens is that when an aluminum wire is terminated with a screw clamp, the aluminum, over time, will conform to the shape of the screw clamp. This leads to a loose wire and the possibility of electrical sparks and potentially fires. There has been lots of research and testing done on new methods of termination that negate creep in aluminum wire. As a result, use of aluminum wires are now on the upswing.
Likewise, having a spring that deforms under load for a long time is not a good thing. This is the primary reason that aluminum is not used for springs. The next reason is low fatigue strength. Plastic springs are a great example of creep in a material. After activation, they then need long periods of rest to maintain their original shape.
In general purpose springs, magnetism is a neutral category. Yes, if you drop a bunch of small screws on the floor, you can use a magnet to pick them up. However, I would never select a screw based on that criteria. The inverse is true though. I would never want to put a steel spring in a Magnetic Resonance Imaging (MRI) machine. At best, the spring would interfere with the image and at worst destroy the machine.
What is unknown to most people is that even stainless steels can be magnetic. Some actually can become magnetic as they are cold worked. If this is a requirement in your design, you may want to stay away from stainless steel springs altogether.
Steel Springs
Steel is by far the most common spring material for the following reasons:
- High strength – exceeding 400 ksi (2.76 GPa) tensile strength.
- Low cost
- Magnetic
- Easy to form
- Excellent fatigue properties
- Resistance to creep
- Great surface finish
Music Wire
Music wire is a high carbon plain steel that boasts upwards of a 400 ksi (2.76 GPa) tensile strength. It is generally a material in the range of AISI 1070 to 1095. Due to its high carbon content, it is not weldable. (but why would you want to) These springs are readily available in a multitude of shapes and sizes as off the shelf parts.
The major down side is that music wire is only available in diameters up to 0.283” (7.2 mm). Generally speaking, as the diameter of a material increases the strength reduces. This is caused in the manufacturing process. If a wire is very small, you can filter out most of the impurities during the forming process. If that doesn’t happen, the wire will most likely break in the process or forming and a new wire is started. Either way, that material doesn’t have impurities in it. As you increase the diameter, it is harder to remove impurities and it is likely that you won’t break the material. This is the reason that filament fiberglass is super strong, but a plate of the same glass is much weaker.
If you need a strong spring in larger diameters, there are three main choices: chrome silicone wire, oil tempered carbon (MB) wire and hard drawn carbon (MB) wire. They are ordered in highest to lowest strength and as you can imagine, highest to lowest cost.
Chrome Silicon (Steel) Alloy Wire
This alloy wire leaves off where the music wire stops. The line of strength vs diameter doesn’t change much. This is however a much more expensive option costing roughly 1.7 times what music wire does.
Oil Tempered Carbon and Hard Drawn Carbon Wires are more popular with larger springs. Of course, the reduction in carbon leads to decreased strength, but the cost will be lower as well. Since the strength is reduced, the size and weight of the spring will increase. Many times, these factors can justify the use of the chrome silicon wire instead. Be sure to contact your local spring distributor if this is your concern.
Stainless Steels
Stainless steels contain at least 10% chromium. Chrome is very corrosion resistant which is why people love to put chrome accessories on everything; they stay shiny for a long, long time. When steel is infused with chrome it makes the steel less likely to corrode. It also increases strength, ductility, toughness, wear resistance and hardenability. However, there is a point of diminishing returns because the more chrome you put in, the more steel you take out. As a result, stainless steel springs are generally weaker than their pure steel counterparts.
There are two types of stainless steels used in spring design; austenitic and precipitation hardened. The austenitic springs are the 300 and 400 series alloys with 302 and 316 being the most popular for springs
Both of these alloys get their strength from cold working. Interestingly, as the spring is worked, the magnetism of the material will increase. The 302 has more magnetic properties than the 316 alloy even though both are categorized as non-magnetic. Although the material properties are very similar, SS316 is the better choice. It provides much better corrosion resistance with chemicals and seawater as well as able to be used in food applications. This is due to its additional molybdenum content. Neither material is hardenable.
Precipitation hardened steels are classified with their chrome and nickel content. Common examples are 17-7 PH and 18-8 PH. Precipitation hardened (PH) stainless steels offer higher strength, temperature ranges and corrosion resistance. The most popular PH spring material is 17-7 PH and is used primarily in wave springs. Another awesome benefit is this material can be used to temperatures of 650°F (343°C) without loss of strength.
Inconel X750 is another alloy used in spring design it is precipitation hardened like the 17-7 PH but also has aluminum and titanium added. This material is great for really hot (700°F / 371°C) as well as cold environments used in cryogenic applications. As a result, it used in very specialized applications such as nuclear reactors.
Elgiloy (yeah, it’s fun to say) is another stainless-steel alloy that has excellent corrosion resistance and strength properties that are close to music wire. These springs are also non-magnetic and can be used in some medical applications. The major drawback is that it is only available in very fine spring sizes, typically those less than 0.063 in (1.6 mm) outside diameter. This is a good thing because as consumer products like cell phones and computers are getting smaller, the springs inside them need to be small as well.
Copper Alloys
Copper, bronze and brass are not common spring materials. Their main use is for applications that the spring will conduct electricity such as battery terminal contacts. The one standout among the group is Beryllium Copper. It can withstand temperatures of 600°F (315°C) and is a great conductor of electricity. However, it is ridiculously expensive at 3 to 4 times the cost of stainless-steel springs.
Plastic Springs
As our world changes, plastics are taking over. They have been invading the spring market over the last few decades. There are many advantages and disadvantages in designing with plastic springs.
Advantages
- They are molded parts which allows for shapes and designs previously not possible when starting with a wire.
- The strength to weight ratio is great for those applications where mass is critical
- Highly resistant to corrosive chemicals such as strong acids or bases.
- The mechanical properties hold up in elevated temperatures
- Non-magnetic – allowing for use in Ferro-sensitive environments
- Electrically insulating
- Available in a wide variety of colors
- Recyclable and RoHS compliant
The main disadvantages are low tensile strength (compared to music wire), UV degradation and the propensity to suffer from creep
When designing a plastic spring, don’t think about coils. Think about clips. Plastic coils springs are not usually seen in the market, but once you train your brain to link about springs differently, you will see them everywhere.
Plastics are subject to UV degradation and lose their strength every year. This is why car seat manufactures have expiration dates on their product. (Also, it helps them sell more product.) The plastics manufacturer can provide you with data on the particular type of plastic and its strength vs UV exposure. There are also UV inhibitor additives that improve the plastic’s performance.
The other problem is creep. Plastics that are under constant load will creep and lose their mechanical properties. Most steel springs are applied so that that are always compressed at least a little bit. That is why coil springs should not plastic.
So, the best way to illustrate the idea plastic spring is with a life jacket clip. We have used this type of clip for decades and never really thought of it as a spring. Well, it is! By squeezing the clip, we compress the ‘spring’ and after inserting it we release one cycle. At this point, the load on the clip changes from bending (squeeze action) to an axial load as tension on the clip is applied. The creep issue is eliminated from the design because the spring is actuated for only a few seconds, but it rests for very long periods of time.
The super cool thing about plastic springs is that they are molded. This means that almost any shape can be designed and fabricated. You are not subject to forming a spring out of a round wire so the sky’s the limit. The only downside is paying for tooling and that can get expensive. Fortunately, with 3D printing, you can run through several design iterations before investing the money in tooling.
Material Selection based on application
If you skipped down to this section, we have already discussed different materials and what their strengths and weaknesses are. Now I will give some example applications and state the most common materials for that application in order from most to least common.
General Purpose Springs
- Music Wire / Chrome -Silicon Alloy
- Oil Tempered Carbon
- Hard Drawn Carbon
- Stainless Steel 316
- Stainless Steel 302
- Elgiloy
Highly corrosive environment
- Stainless Steel 316
- Stainless Steel 302
- Plastics
- Inconel X750
- Stainless Steel 17-7 PH
- Elgiloy
Electrical Conduction
- Beryllium Copper
- Copper / Brass / Bronze
- Any steel-based spring
Electrical Isolation
- Plastic
Medial (Ferro-Sensitive) Applications
- Plastics
- Elgiloy
- Stainless Steel 316
- Copper / Brass / Bronze
Nuclear Applications
- Inconel X750
- Some Plastics
Extreme high temperatures
- Stainless Steel 316
- Plastics
- Elgiloy
Extreme high temperatures
- Inconel X750* (700°F / 371°C)
- Stainless Steel 17-7 PH (650°F / 343°C)
- Elgiloy (650°F / 343°C)
- Beryllium Copper (600°F / 316 °C)
- Stainless Steel 316 (550°F / 288°C)
- Stainless Steel 302 (550°F / 288°C)
- Chrome -Silicon Alloy (425°F / 218°C)
- Oil Tempered Carbon (350°F / 177°C)
- Music Wire (250°F / 121°C)
- Hard Drawn Carbon (250°F / 121°C)
- Copper / Brass / Bronze (212°F / 100°C)
- Plastics (See manufacturer)
*some tempers can allow up to 1000°F (538°C)
Extreme low temperatures
- Inconel X750
Toys
- Plastics
- Music Wire
- Elgiloy
Cost
The one thing that has been sprinkled throughout the article is cost. Unfortunately, us engineers don’t live in a vacuum; we need to make decisions that deliver a good product but also one that people can afford. Springs are not usually one of the areas that I like to cut corners on, but there are certain things within you control than might just allow you to use a less expensive spring without sacrificing quality.
Change the environment: Often times a spring is visible to the eye when installed on the machine. This means that it is subject to corrosion from every day hazards like water (rain or dew) or road salt debris. These things can significantly reduce a springs lifespan so a more expensive material is used like stainless steel. However, if we give the spring a better protective coating (yellow zinc chromate for example) and shield if from debris and water coming in while allowing water to exit, it might just be protected enough to switch to a music wire spring.
For plastic springs subject to UV degradation, can the life of the spring be extended by hiding it from the sun? If so, you may be able to reduce the size and shape considerably because the material properties won’t change much over time.
Conclusion
As I mentioned before, it is hard to select the right material for a spring. There are so many criteria that effect the ultimate material selection. As we find in all engineering tasks, there is no ultimate material; so, we need to compromise. You may find that the best decision is to replace a less expensive spring on regular intervals to keep its corrosion protection in place while saving money.
This guide should help to steer you in the right direction for your spring application. Good Luck!