Normalizing of steel: air cooling and grain refinement

What normalizing does

Normalizing heats steel into the austenite range, holds it until the section is uniform in temperature, and cools it in air. The austenite transforms to a fine mixture of ferrite and pearlite.

The heating temperature is typically about 55 °C above the upper critical line: Ac3 for hypoeutectoid steels and Acm for hypereutectoid steels. This is slightly higher than the temperature normally used for hardening carbon steel by water quenching.

Normalizing vs annealing

The main difference is the cooling rate. Air cooling transforms the austenite at a lower temperature than furnace cooling, so the pearlite is finer and the steel is somewhat stronger and harder.

Normalizing also frees the furnace sooner, because parts are cooled outside it.

Grain refinement and use on forgings

Hot forging is done at temperatures well above Ac3, and parts often finish forging at different temperatures across the section. The result can be coarse or uneven grain and a structure that machines and hardens inconsistently.

Normalizing re-austenitizes the whole forging at a controlled temperature and transforms it uniformly on air cooling. This refines the grain, evens out hardness and improves toughness. Forging specifications such as ASTM A668 and EN 10250 include normalized and normalized-and-tempered delivery conditions.

Large or alloy forgings may be tempered after normalizing to lower hardness and relieve stress from air cooling.

Limits

Air cooling rates depend on section size. Thick sections cool more slowly at the centre, so core properties approach those of an anneal.

Higher-alloy steels can partly harden in air. These grades are usually tempered after normalizing, or annealed instead, before machining.