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Residual stress is an internal stress that is not a result of externally applied loads. If stress buildup in the weldment is excessive, the fatigue life of the metal is reduced.
Importance of Heat Treatment Stress Relief
Cold working, hot rolling, grinding, quenching treatments, welding, and thermal cutting all can induce residual stress into metal. The nature of residual stress, its distribution, and prediction of the level within a metal is a complex and not completely understood phenomenon, but you can be sure it is present.
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, then 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.