Gray cast iron usually refers to the cast iron with gray section and the carbon mainly exists in the form of flake graphite. The metallographic structure of gray cast iron is composed of metal matrix and flake graphite. Metal matrix mainly includes pearlite, ferrite, pearlite and ferrite. Flake graphite can be distributed in the matrix in different quantities, sizes and shapes. In addition, there are a small amount of non-metallic inclusions, such as sulfide, phosphide, etc.
Graphite is a form of carbon in gray cast iron in free state. It is no different from graphite in nature, which contains trace impurities. Graphite is soft and brittle with very low strength（ σ B < 20MPa, elongation close to zero), the density is about 2.25g/cm3, about 1 / 3 of iron, that is, about 3% (mass ratio) of free carbon can form graphite accounting for about 10% of the volume in cast iron. The existence of graphite splits the continuity of metal matrix and greatly weakens the strength of metal matrix. Therefore, gray cast iron is often regarded as carbon steel with a large number of micro cracks or holes.
The amount and dispersion of pearlite are related to the undercooling during eutectoid transformation of cast iron. The greater the undercooling (such as reducing carbon equivalent and increasing cooling rate), the higher the proportion of pearlite and the greater the dispersion. The metal matrix of ordinary gray cast iron is composed of pearlite and ferrite in different proportions. Its distribution feature is that ferrite mostly appears around graphite. High strength gray cast iron is mainly pearlite matrix or sorbite matrix. At this time, the sheet spacing between cementite and ferrite is very small (generally less than 0.3-0.8) μ m) , it needs to be magnified more than 400 times to distinguish. Because the layered structure is dense, its tensile strength σ B and hardness values are also high.
Similarly, the distribution form of flake graphite is related to the undercooling of cast iron. With the increase of undercooling, the flake graphite of Hypoeutectic gray cast iron can appear in different distribution forms such as a, B, C, D and E, which have a great influence on the mechanical properties of cast iron. According to the traditional argument and the provisions of many drawings at home and abroad, it is considered that A-shaped graphite is the best. Type B graphite shall be avoided, and type D and e graphite are not allowed. The directionality of e-type graphite arrangement is very strong. Under the action of small external force, cast iron may be banded brittle fracture along the graphite arrangement direction. Therefore, when E-type graphite appears, the strength performance will decline. In the past, it was considered that the low strength of D-type graphite may be due to the presence of a large amount of ferrite.
For more than 20 years. Studies at home and abroad have pointed out that, especially since the advent of horizontal continuous casting cast iron profile, under the condition of the same matrix, the strength performance of D-type graphite cast iron is not only not lower than but higher than that of A-type graphite cast iron. In addition to matrix and graphite, there are still a certain number of non-metallic inclusions in cast iron, the most common are sulfide inclusions and phosphorus eutectic.
Sulfur can be completely dissolved in molten iron in the form of iron sulfide, but the solubility of sulfur in solid solution (austenite) or cementite is very small during solidification. When the sulfur content is 0.02%, independent sulfide may appear. For example, when the manganese content is low and the cooling rate is large, ternary sulfide eutectic (fe-fes-fe3c) is formed, or it exists on the eutectic grain boundary in the form of iron rich sulfide, which can reduce the strength properties of cast iron. When the manganese content is high, MNS or (Fe, Mn) s particles with high melting point are formed, which has no great effect on the strength properties. Manganese sulfide particles are gray dots under optical microscope.
Phosphorus eutectic is often distributed in network, island or fishbone shape along the eutectic grain boundary. Its property is hard and brittle, which reduces the toughness and increases the brittleness of cast iron. Therefore, the phosphorus content of castings with high quality requirements is usually limited.