Metastable fine structure

When cooling down quickly, the structure doesn't stay in a stable state but freezes an intermediate state. This can make the material harder, but also more brittle and prone to cracking. When you weld, you have to watch the cooling rate. Don't let the part cool too fast, otherwise you'll have problems with the structure later. A controlled, slower cooling keeps the material calmer and prevents things from contracting or building up stress.