A simple heat treatment process to do exactly what it says!
If a component has been cast, forged or heavily machined stresses have been introduced into the material. When final machining is performed and tight tolerances are expected to be held the secondary machining allows the stresses to break out and component accuracy is impossible to maintain.
Performing a stress relieving operation by heating to approximately 550°c to 580°c, allowing to equalize and soak followed by a slow cooling rate the residual stresses dissipate and component accuracy will be easier to achieve.
The stress relieving process also has advantages before finishing machining and final heat treatment as, due to its very nature, heat treatment and quench cycle can cause distortion within the component and once the item has been hardened it is very difficult to rectify.