Degree of supercooling
The temperature difference between the theoretical equilibrium temperature and the actual nucleation temperature, representing the driving force for solidification.
The degree of undercooling describes how far the temperature drops below the transformation temperature before the structure changes. It determines whether you get a soft or hard structure when cooling. If you cool too quickly, the material becomes hard and brittle because martensite forms. You usually don't want that when welding because it leads to cracks. If you cool too slowly, you get a soft, tough structure, but it may not have the required strength. In practice, this means you have to control the cooling rate. For higher-strength or carbon-rich steels, you must choose the heat input and preheating so that the cooling curve fits. If you're unsure, refer to the material specification or ask the foreman what the steel can handle.