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Mechanical Properties of Gray Iron - Composition

The properties of gray iron are primarily dependent on its composition. The lower strength grades of gray iron can be consistently produced by simply selecting the proper melting stock. Iron castings in the higher strength grades require close control of their processing as well as their composition.

The majority of the carbon in gray iron is present as graphite which has little strength or hardness. Increasing amounts of graphite result from increasing the total carbon content of the iron. This decreases the strength and hardness of the iron, but increases other desirable characteristics listed above. An appreciable silicon content is also necessary in gray iron because this element causes the precipitation of the graphite in the iron. The silicon also contributes to the distinctive properties of gray iron. It maintains a moderate hardness level even in the fully annealed condition and thus assures excellent machinability. Silicon also imparts corrosion and elevated temperature oxidization resistance to gray iron. The effects of carbon, silicon and phosphorous on the tensile properties of gray iron are combined into a number called the carbon equivalent by the following equation:

% Silicon + % Phosphorous Carbon Equivalent = % Total Carbon + 3

This equation indicates the relative amount of the eutectic that forms during solidification of the iron with 4.3% as 100% eutectic composition. This equation also, in effect, indicates that on the basis of percent by weight, the silicon and phosphorous contents of gray iron influence its tensile properties in the same manner as its total carbon content but only by one third as much. Other characteristics such as thermal shock resistance and properties in the foundry do not follow this relationship.

Gray irons can be alloyed to increase their strength and hardness as cast or to increase their response to hardening by heat treatment. Gray iron can be annealed to a low hardness to increase machinability. Of course, this also decreases its strength.

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