Diffusion-Controlled Grain Growth
The enlargement of crystallites at high temperatures via the transport of atoms across grain boundaries to lower total energy by minimizing surface area. Smaller grains dissolve in favor of larger ones.
If you heat metal strongly and for a long time during welding, the grains in the structure start to grow. This happens because the atoms rearrange at high temperature and form larger crystals. Grains that are too coarse make the material brittle and prone to cracking, especially when it is later subjected to load. So you have to keep an eye on the heat input and not unnecessarily delay the cooling. Especially with steels that tend to grain growth, you should not put in more heat than necessary. If you run the weld cleanly and with controlled heat, the structure remains fine and the component stays tough.