Thermal Post-Treatment Stress Relief

You do stress-relief annealing after welding when the part has picked up too much tensile stress during cooling. You heat the part in a controlled manner, hold it at temperature for a while, and then let it cool slowly. This allows the structure to settle and the stresses to relieve. If you skip it, the part can distort or even crack later. Be careful not to get too hot, otherwise you'll take away the material's strength, and too cold does nothing.