Residual stress in welds is produced by localized metal tensions occurring immediately after welding, which are:
a) Contraction stress. This is the main source of residual stress. It takes place during the cooling of the welded areas, which have undergone non uniform heating.
b) Stress due to higher surface cooling. When a weld cools down the surface cools faster than the inside, even if this cooling occurs in still air. The greater the thickness, the more stress is generated.
c) Stress due to phase transformation. It occurs due to the transformation of austenite (face centered cube, fcc) to ferrite (body centered cube, bcc), that causes an increase in volume to which the base metal is opposed.
The three types of residual stress usually take place at the same time. Experience proves that the individual effects of each one can be linearly superposed.