Underbead cold crack
Forms in the heat-affected zone due to the coincidence of martensitic microstructure, diffusible hydrogen, and mechanical residual stresses. Often appears hours after the welding process.
Cold cracking occurs when three things come together: a hard, sensitive microstructure, hydrogen in the weld metal, and tensile stresses. The crack often appears only after cooling, sometimes hours later. You must therefore ensure that the hydrogen can escape from the weld and that the microstructure does not become too hard. This is achieved with controlled cooling and dry filler materials. For critical steels, it helps to keep the weld warm after welding or to perform a heat treatment. Pay special attention to the preheat temperature and adhere to the interpass temperature. If you cannot stress-relieve the parts immediately after welding, let them cool slowly and cover them if necessary.