The normalizing process is generally applied to unalloyed steels or low-alloy (hypoeutectoid) steels (i.e., steels with a carbon content lower than 0.77%). Normalizing is applied to hypereutectoid steels (i.e., steels with a carbon content greater 0.77%) only in special cases.  

Normalizing should not be confused with the process of annealing. Although the austenitizing temperatures may be similar, the cooling rates are different. Annealing is generally a slow cool, whereas the normalizing cooling rate will be faster to create the particular microstructure and mechanical properties.

 

What process temperature?

The selected process temperature must be in the austenite region and at a temperature above the final heat-treatment temperature. For example, if the final heat treatment is carburizing and the carburizing temperature is 925°C (1700°F), then the normalize temperature should be higher by approximately 15°C (40°F).

This means that any residual stresses left over from the forging procedure will be relieved prior to the final carburizing process. This will assist in reducing the potential for distortion. It will not guarantee a distortion-free process, but it will reduce the potential for distortion.

It should further be remembered that the steel will decarburize in an air atmosphere or most probably with the products of combustion in a direct-fired combustion furnace. There is a very strong likelihood that either surface oxidation (scale) or decarburization will occur in the furnace atmosphere (air or products-of-combustion).

Care should be taken with the normalizing process temperature because higher temperatures will result in the formation of coarse grain structures. The temperature selection will also influence the dissolution of carbides and carbide networks that may have been formed as a result of the pre-forging temperature selection.

 

Time at Austenitizing Temperature

The time at austenitizing temperature is based on the steels’ maximum cross-sectional thickness. It is also critical not to oversoak at the selected austenitizing temperature because of the potential for grain growth to occur. The characteristics that will further govern the soak time at temperature are:

  • The furnace and its heating method
  • Part geometry
  • Load geometry
  • Load density
  • Steel composition
  • Steel thermal properties
  • Steel surface radiation emissivity
  • Air circulation and air movement within the furnace
  • Atmosphere analysis

The general rule-of-thumb for soak time at temperature is one minute per 1 mm of maximum cross section at the selected temperature OR 30 minutes per 1 inch of maximum cross section at the selected temperature.