The Quick and Easy Material Review

Articles, Springs Magazine

When I was a coiling operator, I had no clue what the difference was in the materials I wound … and nobody told me much unless I asked. Whenever I needed to replace material, I had to consult engineering. I wish I’d had even a rudimentary understanding of spring steel in those days. Assuming some operators are in the same boat, let me break down some of the common materials.

Carbon Steels

These material types are by far the most common. They include hard drawn, oil tempered and music wire. They have some common traits:

  1. They are the most common and available in many stock sizes; therefore, economical.
  2. They will rust quickly if not protected by a secondary finish.
  3. They are not suitable for any extreme lower high-temperature application.

Also, they have some unique properties:

Hard drawn. This material is made for noncritical applications. It is a drawn wire, which means it gets its tensile strength from being drawn through dies and reduced in size. The smaller the wire, the higher the tensile. Drawn wires have what is called “cast” (see “Glossary”) and may require straightening before coiling or forming. The material color tends to be dull and grayish. Being a low-grade steel, it may have surface marks and defects, which is the best reason for not using it for high-fatigue designs. Hard drawn does, however, plate very well and is often zinc plated to fend off rusting.

Oil Tempered (TMB or OT). Tempered wires are cold drawn to size and then heated. This heating gives the material better properties than hard drawn, although it may still have some surface imperfections. Tempered wires are easily distinguished from drawn wires, as they are usually dark brown in color and do not have cast. Tempered wires, therefore, must be approached carefully when breaking the bands of a coil of material because the material wants to straighten out to its natural state and can be very dangerous if not contained. The advantage of tempered steel is that it does not need straightening, which is a time-saver for a spring maker, especially where torsion spring legs are involved.

Music. Music wire is considered a superior carbon steel. Although it will rust very quickly, as will all other carbon steels, it has a very good surface finish, making it the best choice for springs that require high life. Since it is a drawn wire, it has cast, the same as hard drawn, and a grayish-silver color. Unlike most other types of spring steels, music wire is not manufactured in large wire sizes.

Stainless Steels

Stainless steels come in many chemical compositions, but the most common types to spring makers are 302, 304, 316 and 17-7PH. The common traits are as listed below:
  1. Corrosive resistant, with 316 offering better corrosion resistance than 302, 304 or 17-7PH.
  2. Handle much higher temperatures than carbon steels.
  3. Lower magnetism properties than carbon steels.
  4. More costly than carbon steels.
  5. Can be “electro-polished,” which produces an extremely shiny surface and passivates the surface.

Unique properties include the following:

302,304,316. These types are not high-tensile materials. This makes them less robust, as far as stress handling is concerned, than carbon steels. However, if the application demands corrosive resistance at an economical price, 300 series stainless may be a good choice, with 316 offering the most corrosion resistance and consequently the highest price of the three grades.

Stainless 17-7 PH. This stainless steel is a bit different in chemistry than the 300 series. The composition of this material makes it necessary to heat treat it after coiling to obtain its tensile strength. However, after heat treatment, the tensile strength of 17-7 PH comes very close to that of music wire, making it the best choice for high-life designs.

Chrome Alloys

Two very popular alloys are chrome vanadium and chrome silicon. Chrome silicon has slightly better tensile strength than chrome vanadium, making it the best alloy for high life.

Both of these materials can be purchased in “valve quality,” which has a higher quality surface than non-valve-quality material. Automobile valve and transmission springs are commonly made of these two alloys to gain the high life and high temperature handling required.

Although not as good as stainless steels for handling high temperatures, chrome silicon can handle 475°F without an alteration of its mechanical properties. This is very close to stainless 302/304/316, which can handle 500°F.

Where rust and corrosion is considered, alloys do not fare any better than carbon steels; they will rust without protection.

Chrome vanadium is a popular steel for die springs because it can be rolled to a rectangular shape without breakage. Chrome silicon is often too hard for this process without special equipment.

Both alloys are tempered and have no cast.

One critical difference between chrome silicon and the other materials discussed in this article is its stress relieving. Because of chrome silicon’s high hardness, it needs to be stress relieved within four hours of coiling. The stresses induced from coiling can produce stress cracks in the material if it isn’t stress relieved, preferably with an in-line oven straight from the coiler.

Material Selection

Another little-known fact to the operator is that wire is a market. That means that it is not possible to define material cost by type or size. For instance, although 0.130′′ hard-drawn wire is inferior to music wire, it may not be as abundant. Therefore, the lower quality wire may actually cost more for a given amount.

In addition, some wire types are not as prevalent as they once were. Valve-quality carbon steel (a high-quality version of oil tempered) was a very popular steel in past decades. However, alloys have since replaced a lot of valve wire due to their superior surface finish and high temperature handling. A lot of blueprints call for valve material, but the cost would be extreme and alloys are a good, logical replacement. A blueprint produced in 1956, for instance, might still call out carbon valve. Rather than simply following the blueprint, a spring maker should realize that it is in everyone’s best interest, especially the customer’s, to find a more cost-effective material solution that can meet the need and, possibly, improve part performance at the same time.

One last note: Stainless steels do not have the same rate-producing elastic properties of carbon or chrome alloys. Therefore, stainless steels cannot be used in place of high-tensile steels, such as music or chrome silicon. They are entirely different materials and cannot be interchanged. However, chrome-alloy steels will produce the same spring rates as carbon steels and, therefore, the same loads at given heights. Therefore, chrome-alloy steels can serve as an allowable substitute for carbon steels.

By: Randy DeFord, Engineering Manager Mid-West Spring & Stamping