Shrinkage stress
Shrinkage stress occurs when the weld shrinks during cooling but is prevented by the cold base material. It pulls the weld together and can create cracks in brittle materials.
Physicochemical mechanism of action
Hot gets big, cold gets small — that applies to every metal. The hot weld wants to shrink as it cools, but the cold material around it holds it in place. What cannot shrink builds up tension, and that goes up to the yield point and beyond. On thick parts, the weld is restrained from three directions at once. That is the worst case, because the material can no longer get out of the situation by deforming.
Metallurgical relevance & risks when welding
On top of the shrinkage stress comes the transformation, because needs more space than the structure it forms from. If that hits a hardened, brittle zone, it cracks, and you have a . If the component can give instead, you get . Same cause, two outcomes. How tightly it is clamped decides which one you get.
Workshop practice & instructions
Don't clamp everything down rock solid. Give the part one direction it can work into. Plan the weld sequence beforehand, weld in skip-weld or alternating sequence, and set enough tack welds. Do the root first and quickly so it doesn't open up under the load of the next layers. For critical components, afterwards.