After a pipe weld, you may need to engage in a post-weld heat-treating service known as pipe weld stress relief. The weld is heated to a specific temperature, kept at that temperature for a period of time and is then cooled. However, you might wonder why this is necessary.
Why Welded Pipes Become Stressed
Stress relief reduces distortion and regains dimensional tolerance. The distortion occurs when the molten metal weld cools and contracts in a manner that is different from the base metal that is around it. If the part is restrained, the part will have residual stresses.
This can lead to the pipe bending as it cools. The restraints are the result of all sorts of reasons, such as clamps, the opposite ends that are welded to members, and the sheer mass of the part. When the base metal does not have anywhere to move, an aspect of the metal part has to give. This leads to defects that could jeopardize the quality of the weld. The weld stays under a tensile load, which is residual stress. They can cause the pipe to fail early.
Properly Relieving Pipe Weld Stress
As the metal shrinks as it cools, it pulls the base metal toward it. The top of the joint is pulled more than the bottom of the joint. This can also cause the pipe to fail prematurely. Fortunately, these problems can be corrected by hiring pipe weld stress relief services. The pipe is heated to a temperature that is below the stress-relieving temperature. By being held at this temperature for long enough, the residual stress can then be lowered.
The temperature of the pipe must be allowed to fall to room temperature. Otherwise, you may find yourself with the same residual stresses that you were trying to avoid. The stress relief can be very slow because the pipe can only be heated a few hundred degrees Fahrenheit per hour to avoid residual stress.
The length of time that the pipe is kept at the higher temperature is based on the width of the material being welded. After surpassing the holding time, the pipe weld stress relief service allows the pipe to cool at a controlled rate that is based on codes. Because the methods used to relieve stress from a pipe are so exact, it's important to rely on a professional service rather than attempting the procedure on your own.