Time-temperature-transformation diagram (TTT diagram)
A graphical tool illustrating the timing and extent of phase transformations under specific cooling rates or isothermal conditions. It helps welding engineers predict microstructural development upon cooling.
The CCT diagram shows you what happens in the material during cooling. If you can read it, you know which microstructures form at which cooling rate. This is important in welding because the heat input always creates a heat-affected zone that cools differently than the base material. If you cool too slowly, the grain can become coarse and toughness suffers; if you cool too fast, it can become hard and prone to cracking. So before welding, check what the material can handle and adjust the heat input and preheating accordingly to land in the good range.