Normalizing Normalising of Gray iron
Gray iron is normalized by being heated to a temperature above the transformation range, held at this temperature for a period of about 1 hour per inch of maximum section thickness, and cooled in still air to room temperature. Normalizing may be used to enhance mechanical properties, such as hardness and tensile strength, or to restore as-cast properties that have been modified by another heating process, such as graphitizing or the preheating and postheating associated with repair welding.
The temperature range for normalizing gray iron is approximately 885 to 925°C (1625 to 1700°F). Austenitizing temperature has a marked effect on microstructure and on mechanical properties such as hardness and tensile strength.
The tensile strength and hardness of a normalized gray iron casting depend on the following parameters:
Combined carbon content
Pearlite spacing (distance between cementite plates)
Graphite morphology.
The graphite morphology does not change to any significant extent during normalization, and its effect on hardness and tensile strength is omitted in this discussion on normalizing.
Combined carbon content is determined by the normalizing (austenitizing) temperature and the chemical composition of the casting. Higher normalizing temperatures increase the carbon solubility in austenitic (that is, the cementite volume in the resultant pearlite). A higher cementite volume, in turn, increases both the hardness and the tensile strength. The alloy composition of a gray iron casting also influences carbon solubility in austenite. Some elements increase carbon solubility, some decrease it, and others have no effect on it. The carbon content of the matrix is determined by the combined effects of the alloying elements.
The other parameter affecting hardness and tensile strength in a normalized gray iron casting is the pearlite spacing. Pearlite spacing is determined by the cooling rate of the casting after austenitization and the alloy composition. Fast cooling results in small pearlite spacing, higher hardness, and higher tensile strength. Too high a cooling rate may cause partial or full martensitic transformation. The addition of alloying elements may change hardness and tensile strength significantly.
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