Heat treatment of springs, low-temperature annealing, bluing.

After spring forming, heat treatment must be performed to remove residual stress.

When processing spring materials, residual stress occurs within the material. If this residual stress is left untreated, the spring will attempt to return to its original shape, resulting in deformation over time. To avoid this phenomenon (springback), stress relief heat treatment is always performed after spring forming. Higher heat treatment temperatures are more effective at removing residual stress. However, spring materials have the characteristic that their strength decreases sharply when exposed to temperatures exceeding 450 degrees Celsius. Therefore, the stress relief heat treatment for springs is conducted at relatively low temperatures ranging from 200 to 450 degrees Celsius. This heat treatment is referred to as "low-temperature annealing." (In our spring manufacturing field, we call it tempering.) Materials such as piano wire, which are iron-based, develop a layer of iron oxide on the surface due to low-temperature annealing, and the color changes from brown to deep blue (as shown in the photo) as the heat treatment temperature increases. Because of this blue coloration, low-temperature annealing is sometimes referred to as bluing treatment. However, in the case of stainless steel, there is not much color change even after heat treatment, so the term "bluing" does not seem appropriate as a description for heat treatment of springs in general.

Contract manufacturing