Widmannstätten Grain Boundary Needles

Widmanstätten needles form when cooling too quickly from the austenite region. Ferrite or bainite grows into the structure in a typical needle pattern at the grain boundaries. This makes the material harder and more brittle, and the weld becomes prone to cracking. So you have to slow down the cooling after welding, e.g., by preheating or letting it cool slowly. If you see such a structure, the heat input was too high or the cooling was too abrupt.