Widmannstatten ferrite needles

Widmanstätten structure forms when you cool the steel too fast after welding. Then the ferrite needles grow crisscross through the grain and the structure becomes brittle and prone to cracking. You don't want that in a weld. To avoid it, you have to slow down the cooling, i.e. preheat or increase the heat input. This gives the austenite enough time to transform cleanly, and the weld stays tough. If you see such a structure, only normalizing can help to neutralize the structure again.