A compression spring should be stress relieved to remove residual forming stresses produced by the coiling operation. Depending on design and space limitations, compression springs may be categorized according to stress level. Please view the categories below.
There are compression springs which can be compressed solid without permanent set, so that an extra operation for removing set is not needed. These springs are designed with torsional stress levels when compressed solid and they do not exceed about 40 percent of the minimum tensile strength of the material.
Another type of compression spring are those which can be compressed solid without further permanent set after set has initially been removed. These may be pre-set by the spring manufacturer as an added operation. These are springs designed with torsional stress levels when compressed solid that usually do not exceed 60 percent of the minimum tensile strength of the material.
Lastly there are springs which cannot be compressed solid without some further permanent set taking place because set cannot be completely removed in advance. These springs involve torsional stress levels which exceed 60 percent of the minimum, tensile strength of the material.