Carbide network precipitation

If you cool the steel too quickly after welding, carbides will precipitate at the grain boundaries. This forms a network that makes the material hard and brittle – it will break quickly. You have to slow down the cooling so that the structure has time to transform cleanly. Adjust the heat input and cooling so that you don't enter the critical range for these precipitates. If the part is too thick or the environment is too cold, preheating or slow cooling after welding helps.