Even minor stress can cause significant issues. It’s true in life – and in weld distortion. Stress can ruin a weld, slow down a project, or result in a costly error. Stress relief and pre-stressing are two techniques for mitigating unwanted distortion. Both methods — which are essentially mirrored opposites of each other — are designed to control the finished shape of parts.
The Difference Between Stress Relief and Pre-Stressing
When stress relief is used to minimize distortion, the stresses that accumulate during welding are removed afterward with a heat treatment.
Welding engineers anticipate distortion/welding stresses and compensate appropriately. If they predict distortion will bend a part upward by one inch, they will pre-stress the part downward by approximately one inch to counteract the distortion and finish with a flat surface. It’s important to note that residual stress will rarely amount to net zero, but it is useful when considering this example.
Appropriate Uses for Each
The decision of which method to use is made on a case-by-case basis. Some materials respond better to one technique than the other, as do some geometric shapes.
If the same results can be achieved through pre-stressing, that will usually be the first choice. Pre-stressing is relatively inexpensive and can be done in the shop with clamping and welding. It is critical to accurately predict distortion beforehand in order to compensate with the correct level of pre-stress. Since it usually requires subjecting the part to a thermal cycle in a furnace, stress relief can be costlier and more time consuming. Stress relief also often requires welders to create an external fixture to accompany the part into the furnace. The fixture is responsible for securing and supporting the part, so the frame must be able to retain its shape and rigidity through the high-heat cycle along with the part.
Once the stress relief process is complete, the fixture is removed to verify that the shape of the part is accurate and that the process was successful. If it was not successful, the team regroups and pursues other options. They may combine stress relief and pre-stressing in the second attempt. If the part bowed during stress relief, they may add pre-stress in the opposite direction and apply stress relief again. One method pre-applies distortion and relies on welding distortion to restore the desired shape, the other restricts distortion and allows stresses to build up until they can be removed after welding. Which method a team chooses relies of variables such as materials and shape. The end results it the same, but the path taken to get there is determined by a knowledgeable team.