Normalizing is used to provide uniformity in grain size and composition throughout an alloy. It is mainly used on carbon and low-alloyed steels to normalize the structure after forging, hot rolling or casting.
In normalizing, the material is heated to a temperature approximately equivalent to the hardening temperature, at which new austenitic grains are formed. The austenitic grains are much smaller than the previous ferritic grains. After heating and a short soaking time, the components are cooled freely in air. During cooling, new ferritic grains are formed with a further refined grain size. In some cases, both heating and cooling take place under protective gas to avoid oxidation and decarburization. Through normalizing, steel can obtain a more fine-grained homogeneous structure with predictable properties and machinability.
Normalizing differs from annealing in that the metal is heated to a higher temperature and then removed from the furnace for air cooling. The purpose of normalizing is to remove the internal stresses induced by heat treating, welding, casting, forging, forming or machining. Stress, if not controlled, leads to metal failure; therefore, before hardening steel, it must be normalized to ensure the maximum desired results. Low-carbon steels do not usually require normalizing.
Normalized steels are harder and stronger than annealed steels. In the normalized condition, steel is much tougher than in any other structural condition. Parts subjected to impact and those that require maximum toughness with resistance to external stress are usually normalized.