Bainite
A needle-like or feather-like microstructure formed during continuous cooling at rates intermediate between those producing pearlite and martensite.
Bainite forms during cooling in the intermediate temperature range when you deliberately guide the steel between pearlite and martensite formation. The structure is fine-needled and gives you a good compromise between strength and toughness – significantly tougher than martensite, but stronger than coarse pearlite. In welding, this means: if you let the cooling happen too fast, you get martensite and crack risk; if you cool too slowly, it becomes pearlitic and soft. The trick is to hit the cooling rate so you land in the bainite window – you achieve this through preheating and the right heat input. If you control the structure, you get crack-resistant, tough weld metal.