Over the years, one of the questions most frequently asked regarding the forming of elastic material is the difference between stress relieving and heat treatment. Although stress relieving is, in itself, a heat treatment in a broad sense, there is a generally accepted difference in the two processes.
When springs are formed, they are forced into shape by hard tooling. A tool is pushing the wire to a desired diameter, and a tool is pushing against the material to create pitch. All this forced shape-shifting creates stress in the wire. The wire does not, by nature, appreciate being forced into another shape, especially elastic material, whose sole purpose in life is to spring back to a given free position. If you stop and think about it, the very definition of spring steel is a material that is “elastic” enough to return after being bent, but is also “plastic” enough to be formed and hold its shape.
Most of the detrimental stresses created during coiling must be eliminated. To do this, springs are heated after they are formed. Carbon steels are typically heated at a temperature between 400°F 600°F.
The heating will also have other effects: It can tend to turn the material a given color or hue. A temperature of 420°F can turn carbon tool steel and spring wire a faint yellow, sometimes referred to as “straw.” The high stress relieving temperatures can turn it a purplish to bluish color, sometimes very pronounced and iridescent. This is the reason stress relieving may be called out as “bluing” on blueprints. Carbon steels also have a habit of changing diameter when heated. The change will result in springs “turning in” – they reduce in diameter. The spring maker can compensate for the diameter change by making a few pre-production test samples before production parts are made.
Alloy steels, such as chrome silicon or chrome vanadium, are stress relieved between 700°F and 800°F. Because chrome silicon is high tensile, it requires stress relieving within 4.0 hours of forming. Allowing alloys to sit in a barrel overnight can induce cracks in the material and ruin the parts.
Stainless steels in the 300 series, such as 302, 304, 316 and 321, also require stress relief. The standard stress relieving temperature for Stainless 302, for instance, is 700°F for 45 minutes. Unlike carbon steels, stainless tends to expand or “turn out” when heated, or not move at all.
All the previous examples are steels that already possess their tensile properties. The heating of the metal does not change the material structure or force the material into a different state – it merely relieves coiling stresses.
On the flip side, there are special materials that obtain their high tensile properties only after a high heat is applied. Stainless steel 17-7PH, for example, does not possess its full tensile properties until after it is formed and then heated. When purchased, the tensile strength will be lower than after heat treatment at 900° for one hour. Only then will its full tensile strength be realized; the heat treatment changes the material structure, making it more resistant to taking set. Some exotic materials, such as Inconel, may require heat treatment at 1200°F to 1350°F for many hours to have time to change properties. This also means that the springs must be held at the given heat for the required time for the full process to take place. Reducing time or temperature will result in lower tensile strength than needed. Consequently, the spring may not perform in the application and will be cause for rejection.
Stress relieving implies a lower heat applied for at least 30 minutes to relieve coiling stresses – sometimes called coiling “strains.” Whatever the description, the intent is to heat the spring and make it relax into a comfortable state with no true change in material structure, and to promote dimensional stability.
Heat treatment is a strengthening process that involves applying a very high heat for a long period of time that will change the material structure to allow it to reach its full tensile properties.