Incubation Period of Transformation
The time delay between reaching the transformation temperature and the actual onset of phase nucleation in the microstructure.
The incubation time is the period that the microstructure needs to actually start the phase transformation after reaching the transformation temperature. During this time, the necessary diffusion processes in the lattice take place before new crystal structures can form. If you don't have the temperature under control or cool too quickly, the transformation is suppressed and you get a hard, brittle structure. If it goes too slowly, the grain can grow too much and the material becomes soft and tough. So make sure that the workpiece has actually reached the correct temperature everywhere and keep it constant until the transformation is complete. Especially with higher alloyed steels, you need to develop a feel for this, otherwise you'll burn the part or it will become brittle under the torch.