Widmanstätten microstructure

The Widmannstätten structure forms when the steel cools too quickly after welding. Then needle-shaped crystals form, making the structure brittle. That stuff becomes prone to cracking and can't take impacts. You can avoid this by slowing down the cooling. Let the weld cool slowly, for example with a post-heat treatment or by covering the heat-affected zone. When welding thick parts, keep an eye on the interpass temperature so it doesn't cool down too fast.