Heat Treatment vs. Stress Relieving for Springs

Springs are generally made of hardened steel. Pre-hardened steel is used before forming the spring.  Music wire, stainless steel, chrome silicon, oil tempered wire, and chrome vanadium are the most commonly used spring steels. The different materials are ideal for a variety of situations. Wire is cold drawn through various dies to attain the size of the wire wanted.

Qualities of Various Spring Materials

  • Music Wire is typically used in applications that require a great deal of strength. It creates a very high-quality spring.
  • Stainless steel works well in moist environments since it won’t rust.
  • Chrome Silicon is often used in higher temperatures as it has a higher strength and quality version of oil tempered wire.
  • Oil Tempered Wire can be found in many common applications but may not result in the strongest or most uniform product.
  • And Chrome Vanadium is ideal for high temperature applications.

A simple video showing an operator manually coiling a spring – cold winding

Manufacturing or coiling the spring can be done with either cold or heated wire. Cold winding starts with a wire that is at room temperature and involves winding the wire around a shaft. The process of hot winding is used for thicker wire or bar stock. First, the metal is heated. This helps to increase wire’s flexibility. Then the steel is coiled around a shaft while it’s still extremely hot. As soon as it’s been coiled, it’s taken off of the shaft and dipped into oil to cool it and quickly harden it. The coiling process causes stress in the wire, which is alleviated by stress relieving – heating the spring in an oven for a specific amount of time at a set temperature and then allowing it to cool slowly. Stress relieving is more effective because it minimizes residual stresses and reduces dimensional changes.

Benefits of Stress Relieving

There are many benefits to stress relieving, especially when it comes to parts that have tight dimensional tolerances.

“Essentially the real benefit to stress relieving is that it doesn’t change the material’s structure nor does it significantly affect its hardness. Without stress relieving, materials may give rise to unacceptable distortion leading to service problems, such as stress corrosion cracking. When we manufacture we always stress relieving our springs,” said David Ellner, Vice-President and co-owner of Ajax Springs.

The Process of Stress Relieving

For steel parts, the stress relieving temperature is normally between 350 and 500°F. Copper and brass components can also be stress relieved – depending on the alloy the temperature should be 350°C for brass springs.

The components should be cooled down slowly for one to two hours after stress relieving. The reason for the slow cooling process is to avoid tensions caused by temperature differences in the material. This is especially important to pay attention to when stress relieving larger springs.

Stress relieving is applied to both ferrous and non-ferrous alloys and is meant to remove internal residual stresses caused by previous manufacturing processes. Non-ferrous alloys are stress relieved at a wide variety of temperatures associated with alloy type and condition.

In the end stress relieving, normalizing and annealing all prepare metals and alloys for further processing. They control the ability of materials to be created with ease, to be formed without cracking, and be hardened with the less amount of distortion.

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