Proeutectoid ferrite

When the weld cools, austenite transforms into ferrite if you give the weld enough time. Ferrite makes the structure softer and tougher and prevents you from getting hard, brittle martensite that cracks under stress. You have to control the cooling so that enough ferrite forms. If it's too fast, it becomes martensitic and prone to cracking; if it's too slow, the grain becomes coarse and the weld soft. So adjust preheating and heat input accordingly. If in doubt, look at the microstructure: a good ferrite content shows you that the balance is right.