Critical Cooling Rate

If the steel cools too quickly, the carbon gets trapped in the structure and hard, brittle martensite forms. The critical cooling rate tells you how fast that happens. The more carbon and alloying elements in the steel, the slower the cooling must be to avoid martensite. To manage that, you preheat the part and let the weld cool slowly, for example with an insulating blanket. With thick plates or when you don't know the steel, you should definitely preheat. And remember: the less hydrogen in the weld, the lower the risk of cracking.