Ferrite-Pearlite Composite Microstructure

Ferrite and pearlite form from austenite during cooling. Ferrite makes the structure tough and soft, pearlite gives it strength and hardness. During welding, this mixed structure forms normally in the heat-affected zone and in the weld if cooling is not too fast. If you cool too quickly, you get hard, brittle structure like martensite. If you cool too slowly, the grain becomes coarse and toughness suffers. So adjust the cooling rate to the material, then you'll have a usable structure without cracks.