When components are processed thermally or mechanically, tensions arise in the material, for example when irregular cooling occurs after hardening. These tensions can be released during further processing of the material and lead to distortion. In order to prevent this, the stresses are significantly reduced during stress relief annealing. This is a special method of annealing, whereby the basic properties of the material are not changed.
The internal stresses in the material can only be reduced by plastic deformations in the micro range. Thus, during annealing, the yield point and the maximum tensile strength for the time of heating are reduced, whereby the stresses can then be reduced up to the set yield point. For steel the temperature of the process is between 500 and 650°C and the heating and cooling is conducted in a very slow manner. This is the only way to prevent cracks or new stresses in the material. The time at temperature for the material is between approximately 60 minutes and several hours. Another advantage of this method is that at these temperatures there is no decarburization or scale formation.
fields of application
As the name implies, the purpose of Stress Relieving is to remove internal residual stresses from steels. The stresses may have come from the steel making mills, machining, forming, welding or even former hardening processes with irregular cooling. If residual stresses are not removed prior to final hardening, distortion may occur, whilst if stresses are left in components before they go into service, then premature failures can result, as well as more subtle movement causing the parts to go out of tolerance.
Typical parts that benefit from stress relieving are large and complex welded assemblies, castings with a lot of machining, parts with tight dimensional tolerances and machined parts that have had a lot of stock removal or rapid stock removal.