Normalizing is applied to give steel an even, fine grain structure to provide both predictable and desired strength properties as well as to improve machinability. After, for example, hot forging, hot rolling or casting, the microstructure is not homogeneous but can contain grains that are both too big and too small and contain undesirable micro-structural constituents such as bainite and carbide precipitates. This type of microstructure has a negative effect on strength, toughness and machinability.

Normalizing is begun by heating to a temperature in the austenite range, 800-920 ºC, with a short soaking time to allow new, smaller austenite grains to form from the original course-grained, normally ferrite-pearlite, structure. This is followed by cooling at a high enough, controlled rate to form a ferrite-pearlite microstructure. The resulting structure has finer grains and a more homogeneous grain size distribution than the original one.

Ref. Steel and its Heat Treatment – A Handbook (8.2.1)