Sometimes mechanical drawings state "stress relieve before machining". Stress relieving is done to relieve residual stresses and prevent parts from warping when getting machined.
Cold working, hot rolling, grinding, quenching treatments, welding, and thermal cutting all can induce residual stress into metal.
Welding, in particular, because of the rapid thermal expansion and contraction created along a very localized area, is a prime source of residual stress. A very high heat source is applied to a small area relative to the cooler surrounding area. That point where the arc is directed is rapidly heated from ambient temperature to temperatures that can be in excess of 3000°F. The metal expands as it is brought to a molten state. As the molten weld pool solidifies along the joint, there is resistance to its shrinkage by the already solidified weld metal and the unmelted base metal adjacent to the weld. This resistance creates a tensile strain in the longitudinal and transverse directions of the weld. Distortion is often the result, and if the stress is excessive, buckling, stress corrosion cracking, and shortened fatigue life are possible.
All welds will have some residual stress, and it will never be totally reduced to zero strain. But the level of stress can be very high depending on certain conditions. Heat input, base metal thickness, cooling rate, restraint of the weldment, and welding process play roles in the level of residual stress induced into a weldment.
There are two major approaches to stress relieving: thermal and mechanical. A major difference between the two is thermal treatment, which in addition to relieving stress, will also effect metallugical changes in the metal. A postweld heat treatment entails uniform heating of the weldment, holding at temperature, and then a controlled cooling.
As metal becomes hotter, it becomes weaker. Once a certain temperature is reached, there is a reduction in yield strength from the rated property of the steel. The residual stress decreases to that of the lower yield strength, and it is thereby relieved. The effect sometimes visibly manifests itself in the straightening of a distorted material.
For carbon and low-alloy steels, stress relieving is commonly performed in the range of 1100° to 1350°F. The time at which the weldment is held at temperature is dependent on the thickness of the workpiece and its chemical composition.
The most commonly used method of stress relieving weldments is by postweld heat treatment. Its effectiveness is dependent on the control exercised in bringing the component to temperature and then its subsequent cooling. Therefore, it should be performed by those knowledgeable in its application.
Shot Peening is a cold working method that reduces stress.
Another mechanical means of stress relief is by vibration. A mechanical vibrating device is attached to the weldment. The vibration's resonant frequency can be controlled by specially designed machines. The amount of time the weldment is subjected to the vibration is usually dependent on its weight. The vibration can be applied during or immediately after welding. The vibration seems to even the stress distribution within the weldment by means of plastic deformation of the metal's grains, reducing sharp peaks of stress.
This process is known to be effective in bringing geometric stability to the workpiece. Presently, the process is not endorsed by codes, but neither is it rejected.