Heating up to a certain temperature range can produce a purer and harder metal. Heat treatment often creates stronger metals. However, it is possible for some processes to make the metal weaker.

An increase in the temperature of the metal will increase its surface area, volume, and length. Therefore, heat treatment expands the metal (thermal expansion). The degree of expansion will depend on the type of metal used.

Every steel material has an upper and lower limit for toughness. The temperature at which the toughness of a steel falls is called the 'Transition Temperature from Ductility to Brittleness'. This temperature is usually around 75°C for 0.01% carbon steel. This temperature differs for various steel materials.