Material Selection in Spring Manufacturing Matters
Recently, we wrote about raw material classifications in the steel spring manufacturing industry. These materials are chosen, largely, by their properties and how they will impact the end use. Or think of it inversely: the end use dictates what alloys to choose.
Because steel spring design programs are plentiful and quite easy to use, most engineers know just what they want, and know how to plug the numbers into the computer program when designing steel springs. Many engineers, even though well-educated in metallurgy, are not quite so knowledgeable of each alloy’s suitability to a spring application. Choosing an alloy for steel spring use is not as automated a process as it is for raw metals. As a result, many alloy inquiries involve providing solutions for which alloy should be used for a particular spring purpose.
9 Key Properties that Influence Application Choice
When considering what (alloy) material to use, there are at least nine key properties that come into play.
- Constant modulus
- Corrosive resistance
- Heat hardenability
- Heating elements
- High stress
To get a broader understanding, let’s discuss them in more details.
A spring often serves double duty. It not only acts as a spring, but also carries current. One such application is in the needle of an instrument. The reaction of the needle is both a function of the spring and of the amount of current running through the spring.
- Constant Modulus
With some alloys, the spring will weaken if the temperature is too high or too low. But if the spring is in a device that is subject to wide temperature swings, a material with what is known as a constant modulus, must be chosen.
- Corrosive Resistance
Many springs are used in marine applications, or in even worse and very caustic atmospheres. In such cases, there are about seven different stainless steels that can be chosen. Some work in fresh water, but not salt water. Some work in salt water, but not more caustic environments. In such cases, a class of spring steels known as high nickel alloys can be employed.
- Heat Hardenability
Some spring steels get their “hardness” from heat treating, while others get their hardness from being forced through a reduction die that compresses the steel. However, many of the alloys that get their hardness from reduction dies cannot be hardened by heat treating them. When making a spring or clip that has a very sharp bend on it, if you use pre-hardened steel, the steel will crack or break. In such an instance, you must use a steel that is in the “soft” or annealed condition, and then harden it after the spring or clip is formed.
- Heating Elements
Some steels are themselves used for heating elements. In this instance, the alloy must resist electricity so that the electricity being forced through it heats up the wire to a red hot condition. The alloy must be able to resist this heating and cooling without either melting or deforming in any way.
- High Stress
When we were kids, we all made springs by twisting some copper wire around a pencil. But once we completed the spring, it was hard to get it off the pencil without deforming it! The spring’s ability to be deflected and then return to its original shape is governed by how much stress that particular alloy can handle. Sometimes a spring has to deflect a long way and then return to its original size. To be capable of this, the spring is said to be able to resist high stress.
Springs are commonly used in sensitive instrument environments. When used in these applications, the spring can have no magnetism. Since many spring-tempered wires get their temper by being forced through reduction dies, they often pick up magnetism. For example, in its natural state, 302 stainless is totally non-magnetic. As a matter of fact, many engineers question us about the authenticity of our stainless springs because of this magnetism. The truth of it is that the material begins non-magnetic, but when forced through the die, it picks up magnetism. If a spring is to be totally non-magnetic, it must be made from an alloy specifically designed to be both spring tempered and non-magnetic.
When a spring is in a mildly corrosive environment such as an outside application, the spring has to be corrosive resistant, yet inexpensive. This calls for a spring that is electro-plated after formation. However, some materials are more porous than others, and when being electro-plated, hydrogen (the smallest atom), can get into the porous surface of the material and cause micro-cracks. These later become bigger cracks, and necessitates that a less porous material is chosen when plating.
Sometimes a spring has to be made of an alloy that can withstand high temperatures. For example, the springs on an oven door have to be able to withstand the heat from the oven without “relaxing.” In fact, many ovens function to extremely high temperatures and require springs that can maintain their properties, and do so for an extended period of time.
In spring manufacturing, we have the capacity to create virtually any shape and type of spring with a wide variety of spring steel, possessing many properties, for an array of uses. Contact one of our spring geeks today to find out which spring steel will work best for your next project.