Shrinkage Crack
Fracture generated by hindered thermal contraction during the cooling phase. It occurs when the local tensile strength of the material is exceeded.
When the weld metal cools, it contracts. If the weld cools too quickly or the part is clamped too rigidly, the material can't compensate and it cracks. These cracks usually appear only after the weld has already solidified, and they are often hard to see from the outside. To prevent shrinkage cracks, the main thing is to slow down the cooling so the stresses can relax. Make sure the weld isn't too narrow and the parts aren't too tightly clamped. For critical components, it helps to reheat the weld after welding to relieve stress. If you find cracks, you must grind them out completely and reweld; just going over them doesn't help.