Bainitic needle structure

Bainite forms during cooling between pearlite and martensite formation. The structure is fine-needle-like and gives the material a good combination of strength and toughness. When welding, you have to control the cooling rate: if it cools too fast, you get martensite and the part becomes hard and brittle. If you cool too slowly, the structure reverts to pearlite and strength suffers. With the right heat input and cooling time, you hit the range where bainite forms.