The soul of gray cast iron is graphite. To study the properties of gray cast iron materials, we mainly study the distribution and morphology of graphite, and then further study the nucleation ability of graphite. The stronger the nucleation ability of graphite, the more graphite there will be, so as to achieve the purpose of refining graphite and making the distribution of graphite more uniform, This helps to improve all aspects of the properties of the material.
As for the form of graphite, the American Society for materials and experiments implements it according to the standard astma-247, which divides the types of graphite in gray cast iron into seven types. Figure 1 shows the seven types of graphite shape. However, the international iso945-75 and gb7216-1987 specifically divide the form of graphite into six types, and we are used to adopting the international standard, The six graphite forms are: uniformly distributed flake A-type graphite, chrysanthemum shaped B-type graphite, block flake C-type graphite, dendritic point D-type graphite, dendritic flake E-type graphite and star shaped F-type graphite, as shown in Figure 2.
In gray cast iron, the structural characteristics of graphite determine the crystal growth mode of graphite. The binding force of carbon atoms on the edge of graphite lattice is relatively weak, that is to say, the thickening speed of graphite sheet is slow. However, because there is a covalent bond on the base plane (0001) of graphite lattice that is not combined, the unbound covalent bond is easy to attract the surrounding carbon atoms to firmly combine with the base plane. At this time, graphite will continue to grow to flake. However, due to the instability of graphite growth in molten iron, that is, the growth of graphite is not only restricted by undercooling, but also restricted by the composition of molten iron and some other elements in molten iron. With the addition of rare earth elements, lanthanum in rare earth is easily adsorbed on (0001) surface. Among them, the carbon atom spacing on the (0001) plane is 1.4210×10-10, and the rare earth atom radius is between 1.74×10-10-2.04×15-10. The rare earth atom radius is much larger than the carbon atom spacing. The adsorption of rare earth greatly inhibits the thickening of flake graphite and the continuous bonding of unbound covalent bonds of graphite, which hinders the growth of graphite and plays the role of refining graphite.