Interlamellar Pearlite Spacing

Pearlite is a lamellar structure of ferrite and cementite. The lamella spacing determines the hardness and strength of the steel. When welding, it's important that the cooling in the critical range is neither too fast nor too slow. Too fast cooling makes the structure hard and prone to cracking; too slow cooling makes it soft and tough. So you have to control the cooling to hit a reasonable middle ground, otherwise you'll have trouble with the weld.