In addition, the transformation requires a certain amount of time, so when the surface of the metal workpiece reaches the required heating temperature, it is also necessary to maintain this temperature for a certain period of time to ensure that the internal and external temperatures are consistent and that the microstructure is completely transformed. This period of time is called holding time. When using high energy density heating and surface heat treatment, the heating speed is extremely fast, and generally there is no heat preservation time, while the heat preservation time for chemical heat treatment is often longer.

The heat treatment process generally includes three processes: heating, heat preservation, and cooling, sometimes only two processes: heating and cooling. These processes are interconnected and cannot be interrupted. Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment, the earliest of which was using charcoal and coal as heat sources, and the recent use of liquid and gaseous fuels. The application of electricity makes heating easy to control and free from environmental pollution. These heat sources can be used for direct heating, as well as indirect heating through molten salts or gold, as well as floating particles.