Widmanstätten structure
A needle- or plate-like microstructure formed during rapid cooling from high temperatures along specific crystallographic planes. It usually reduces the overall toughness of the material.
Widmanstätten structure forms when you cool steel too quickly after welding or during heat treatment. The crystals then align in a needle-like pattern, and the material becomes hard and brittle. You don't want that because the part then breaks quickly under load or gets cracks. Especially with higher carbon steels, you have to be careful not to remove the heat too abruptly. Let the component cool slowly and evenly, for example in an insulating blanket or with a post-heater. If you control the cooling, you get a tough structure that you can machine and load.