Welds look strong, but they actually create a stress problem as they connect pieces of metal. While the newly welded pieces may seem locked into place, the pieces are now experiencing additional stress as parts of the metal pull back from each other. This stress weakens the weld and the parts overall, so after welding, you have to treat the welds to relieve the stress on the metal. The stress is due to two main issues, and if you don't address them immediately, you could see that weld fails spectacularly.
Heat Stress and Metal Distortion
When you weld pieces together, you heat the two pieces so they essentially melt and join together. That seems simple enough. However, when you heat those two pieces, the heat creates thermal stress that can make the metal shrink, expand, and bend. It might not be visible to you at first, but as the weld cools, the metal then contracts and creates more bending. The metal near the weld may start to pull against metal farther away from the weld that is held in place, creating a weakened spot where the metal in between can become comparatively fragile due to this pulling stress. So, two perfectly strong pieces of metal welded together can become weaker, distorted versions of themselves.
Reheating for Stress Reduction
It sounds counterintuitive at first, but reheating the welded area and metal is how you relieve the stress. What you have to do first, however, is let the weld cool, and this is where a lot of welding goes wrong. Stress relief can't be done right after welding because heating a hot weld does nothing to change the situation; you're just prolonging the amount of time the weld is heated initially. And the longer the weld is heated, the worse the potential stress could be, although the actual amount of stress can vary.
Cooling the weld first allows it to set, and yes, this does mean allowing some stress to form. Once that happens, though, that's when you need to jump on the situation and start reheating to essentially let the now-welded metal re-adjust. So, the initial heating and cooling of the weld cause the metal to deform, but the second round of heating allows the metal to redistribute itself a bit, lessening the stress.
What Happens if You Don't Relieve Weld Stress
Two issues can occur if you don't address weld stress. One is the aforementioned weakness, where the metal is just not as strong as it could be because it's being pulled apart by both the weld and whatever the metal is attached to at its other end. The other issue is that, if the metal was already unsound -- maybe there was a microscopic crack -- the stress just makes that defect worse and more likely to create structural problems. The metal could bend when it shouldn't, or it could snap if the piece is thinner.
If you're a construction company owner who needs to get a project done as quickly as possible and want to know if there are ways to shorten welding time, do not skip pipe weld stress relief services. This is one process that absolutely has to be done.