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Mechanical Properties of Gray Iron - Base Chemical Composition

The tensile strength of gray iron is influenced by both the normal elements present in plain irons such as carbon, silicon, phosphorus, sulfur and manganese, and the presence of alloying additions and trace elements. Carbon and silicon are very important elements and are combined, usually with phosphorus, in a carbon equivalent expression.

Irons with a carbon equivalent of more than 4.3 are hypereutectic and usually contain coarse graphite. They are of lower strength, but are excellent in thermal shock applications and for vibration damping. Gray irons with less than 4.3 carbon equivalent are hypoeutectic and of higher strength because the amount and size of the graphite flakes decrease with the CE value.

The effect of higher carbon equivalent is to reduce strength because of the formation of larger amounts of coarser graphite and, commonly, more ferrite. Manganese, sulfur and phosphorus are present in plain gray irons and influence the tensile strength to some extent. Sulfur is a very significant element because it exerts marked effects on the solidification behavior of iron. For this reason, the sulfur content in iron is usually controlled within limits and with a selected ratio to the manganese content since sulfur combines chemically with manganese to form manganese sulfide. The minimum manganese content in iron is generally 1.7 times the sulfur content plus 0.12% manganese. This assures sufficient manganese so that all of the sulfur is combined with manganese rather than with iron. Manganese in excess of this amount is a mild carbide stabilizer, refining the pearlite and increasing the hardness and tensile strength. An excess of manganese or phosphorus can cause dispersed internal porosity in heavier sections such as bosses. For this reason, phosphorous is kept as low as practical except for special purpose irons. Increasing phosphorus provides a somewhat higher tensile strength, but contents over 0.20% reduces machinability particularly in drilling operations.

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