Two-stage martensitic transformation

Be careful when welding such steels: The cooling after welding determines whether the structure becomes hard and crack-prone or soft and tough. If you cool too quickly, hard, brittle martensite forms, which tends to crack. If you cool too slowly, you get a soft structure that lacks the necessary strength. So you have to hit the cooling rate so that a tough, strong structure forms – in practice, this is usually achieved by preheating and controlled cooling.