Thermal supercooling window
The temperature difference below the theoretical phase transformation point required to overcome the activation energy for phase changes.
The thermal undercooling window describes the range in which the metal undergoes the transformation from austenite to another microstructure during cooling. If you fall through this window too quickly, you get hard but brittle martensite. If you cool too slowly, the structure becomes coarse and tough, which increases the risk of cracking. In practice, it's about controlling the cooling rate to get a structure that suits the application. You do this via heat input and cooling. For critical steels, you must control the cooling, for example by preheating or letting it cool slowly instead of quenching it with water.