Widmanstätten Ferrite Arrangement
A characteristic microstructural pattern of plate-like ferrite crystals precipitating along preferred crystallographic planes during rapid cooling from high austenite temperatures. It leads to a significant reduction in toughness.
Widmannstätten structure forms when steel cools too quickly from the austenite region after welding. Needle-shaped ferrite crystals form, making the structure brittle. This is a problem because toughness drops sharply and the material tends to crack, especially under load. In practice, you have to slow down the cooling after welding. Don't leave the part on a cold floor or in a draft; cover it or put it in a preheated sand bed. It's important that cooling is even and slow through the critical temperature zone. If you still get the structure, only normalizing will help dissolve the needles.