Interlamellar spacing

Pearlite is a structure of soft ferrite and hard cementite arranged in layers. The fineness of these layers determines the hardness and strength of the steel. When welding, what matters is how fast the heat-affected zone cools: if it cools too slowly, the pearlite becomes coarse and soft. If it cools too fast, instead of pearlite, a hard, brittle structure forms. So you have to control the cooling so that the pearlite stays fine and uniform, then it's fine.