In the normalizing process, steel is uniformly heated to a temperature that results in complete transformation to austenite. The steel is held at this temperature for a sufficient period of time to form a uniform structure throughout its mass. It is then allowed to cool in a uniform manner in still air. Air cooling results in a faster cooling rate compared to the furnace cooling rate. As a result, the cooling time for normalizing is greatly reduced compared to annealing.
The normalizing uniform heat time is usually 1 hour per 25 mm workpiece thickness, but not less than 2 hours at the uniform heat temperature. The quality of the workpiece can have a significant effect on the cooling rate and therefore on the final microstructure. Thin workpieces cool faster and are therefore harder after normalizing than thicker workpieces. This is in contrast to the case of annealing, where thin and thick workpieces are equally hard after furnace cooling.
Low carbon steels do not normally require normalizing. However, if these steels are normalized, there are no harmful effects. If the castings have uniform wall thicknesses and cross-sectional dimensions, they are usually annealed rather than normalized. Other types of castings, especially those with complex shapes or interconnected thin and thick walled sections, are prone to high levels of residual stresses and can benefit from a normalizing treatment. The microstructure obtained by normalizing depends on the composition of the casting and the cooling rate.
Normalizing of steels is usually considered from the thermal and microstructural points of view. From the thermal point of view, the normalizing process consists of austenitization and subsequent relatively slow cooling. From the microstructure point of view, the region of microstructure containing about 0.80% carbon is pearlite, while the low carbon region is ferrite.
Normalization is typically performed for either of the following purposes.
Altering and/or refining the grain structure and eliminating the coarse grain structure obtained during previous machining operations (e.g., rolling, forging, etc.).
Modify and improve the casting dendrite structure and reduce segregation by homogenizing the microstructure.
Produces a homogeneous microstructure and obtains the desired microstructure and mechanical properties.
Improve the machinability of mild steel
Improves dimensional stability
Reduces banding
Improves ductility and toughness
Provides a more consistent response when hardening or case hardening.
Removes macro-structures created by irregular molding or welding
Fine grain pearlite is tougher than coarse grain pearlite. Normalizing imparts hardness and strength to steel workpieces. In addition, normalizing helps reduce internal stresses caused by operations such as forging, casting, machining, forming or welding. Normalizing also improves microstructural uniformity and response to heat treatments such as annealing or hardening, and enhances stability by imparting a "thermal memory" for subsequent lower temperature processes. Workpieces requiring maximum toughness and workpieces subjected to impact are often normalized. When large cross sections are normalized, they are also tempered to further reduce stresses and more tightly control mechanical properties.
Normalizing removes internal stresses, strains and improves the mechanical properties of the steel, such as increasing its toughness and machinability. Better ductility is also obtained without affecting hardness and strength.
