Pearlite

Pearlite is a microstructure that forms when unalloyed steel cools slowly. It consists of fine alternating lamellae of ferrite and cementite. When welding, you often don't want to set this specifically, but it simply forms as an intermediate stage if you don't cool too quickly. If you cool down too fast, you get martensite, which is hard and brittle. If you cool slowly, the structure becomes softer and tougher. For practice, this means: the cooling rate after welding determines whether you get a hard or a tough structure, and pearlite is the middle ground.