Due to the extremely low strength and hardness of graphite, it can be regarded as zero with respect to iron. In addition, the severe splitting effect of flake graphite on the matrix, so the higher the carbon content in grey cast iron, the lower the strength and hardness. That is, the tensile strength of gray cast iron decreases as the carbon equivalent increases.
In the development of high-strength gray cast iron, it has been an important measure to reduce the carbon equivalent and increase the manganese content, thereby increasing the proportion of pearlite in gray cast iron and improving the tensile strength of gray cast iron. It is generally believed that under the same carbon equivalent conditions, the silicon to carbon ratio is increased, and the tensile strength can be increased by 30 to 60 MPa. This is because, under the same carbon equivalent, the austenite of gray cast iron increases with the increase of the silicon to carbon ratio. The number of dendrites increases.
Manganese is an element that enlarges the austenite region. Therefore, increasing the manganese content in molten iron can effectively reduce the austenite eutectoid transformation temperature, favor the formation of pearlite and increase the amount of austenite dendrites, and austenite. The conversion to pearlite is carried out at a lower temperature, thus promoting the refinement of the pearlite sheet and reducing the pitch of the pearlite sheet. Manganese is a strong carbide forming and stabilizing carbide element. It can be infinitely dissolved in austenite, solid solution in matrix structure, strengthen matrix and improve the strength of gray cast iron.
Sulfur plays a dual role in gray cast iron. It is generally believed that sulfur is a strong stable cementite in the cast iron and hinders the carbonization of the stone. However, from the thermodynamic analysis, sulfur can reduce the solubility of carbon in the molten iron, with manganese, Rare earth formation of MnS and ReS will become the core of non-spontaneous nucleation of graphite, so it can promote graphitization, so sulfur is an indispensable element to promote the inoculation reaction.