Proeutectoid ferrite

Proeutectoid ferrite forms at the austenite grain boundaries during cooling before the transformation to pearlite starts. Its amount determines whether the steel becomes soft and tough or hard and brittle. In welding, this means you have to control the cooling rate. If you cool too fast, you get too hard microstructures; if you cool too slowly, the structure becomes coarse and toughness suffers. So adjust heat input and preheating to achieve a balanced microstructure.