Gray cast iron is mainly composed of matrix structure and graphite, and others include eutectic group, carbide and cementite eutectic. The quality of its microstructure determines its performance. When observing its microstructure, it is mainly analyzed and studied from the aspects of flake graphite, metal matrix and eutectic at grain boundary.
The stable state of metal is the state with the lowest free energy. When the metal solidifies, its free energy increases. The actual crystallization temperature must be lower than the theoretical crystallization temperature to generate phase change energy to make the solidification proceed. Therefore, in the solidification process of molten iron, the temperature does not solidify when it reaches the melting point, but must drop to a temperature below the melting point, that is, a certain degree of undercooling is required in the solidification process, Δ T=T fusion -T compaction.
Primary graphite or primary austenite will be formed when the carbon equivalent solidifies in the molten iron near the eutectic composition. Because the heterogeneous nucleation of graphite requires lower undercooling than the homogeneous nucleation, and there are a lot of impurities in the molten iron, the nucleation of graphite is heterogeneous. Austenite nucleation takes place preferentially at the mold wall or on the surface of other solid impurities in the solder body, with carbides, nitrides and other carbonitrides as the core.
(1). Graphite in gray cast iron
The graphite structure in gray cast iron is shown in the figure. It is characterized by low strength, plasticity and hardness, soft and brittle. Its density is about 2.25g/cm3, about 1/3 of iron, that is, about 3% of free carbon can form graphite in cast iron, accounting for 10% (volume ratio). Graphite exists in cast iron matrix, which is equivalent to crack defects, which will significantly reduce its strength, This property can not be brought into full play, but it is beneficial to improve the shock absorption and wear resistance of gray cast iron. The influence of graphite on the matrix structure is mainly related to the state, size, quantity and distribution of graphite.
(2). Matrix structure of gray cast iron
The main structure of ferrite gray cast iron is that there are many coarse graphite flakes distributed on the ferrite matrix, which has low mechanical properties and small application range; The structure of pearlitic gray cast iron is mainly that the pearlitic matrix is distributed with uniform and fine graphite flakes. Its mechanical properties are high, and it is often used to manufacture important parts such as bed and body; Pearlitic ferritic gray cast iron is a kind of coarse graphite flakes distributed on the matrix of pearlitic and ferritic fusion. Although the mechanical properties of this kind of cast iron are lower than those of pearlitic gray cast iron, it can still meet the requirements of general engine body. Its castability and shock absorption are good, and its application range is wide.