The figure shows the graphite morphology of cast iron alloy prepared with different addition amount of spheroidizing agent. Among them, figures (a) – (d) show the graphite morphology of cast iron with the addition amount of nodulizing agent of 1.4%, 1.1%, 0.8% and 0,5% respectively.
It can be seen from the figure that when the amount of spheroidizing agent added to the EPC cast iron alloy decreases from 1.4% to 1.1% and then to 0.8%, the graphite morphology in the cast iron changes from spherical graphite to spherical graphite + vermicular graphite, and the spherical graphite is the main graphite morphology. When the addition of spheroidizing agent is further reduced to 0.5%, the graphite morphology is mainly vermicular graphite and contains a small amount of spheroidal graphite, that is, the type of cast iron alloy is nodular cast iron. With the decrease of the amount of spheroidizing agent, the vermicular graphite in cast iron alloy increases and the globular graphite decreases. This is because the less amount of nodulizing agent is added, the less amount of residual magnesium and rare earth in the cast iron alloy, that is, the promotion of graphite balls in the cast iron alloy is reduced
The content of sulfur and oxygen in cast iron is increased. The sulfur and oxygen elements in the metal melt will be adsorbed on the graphite edge, reducing the diffusion of carbon atoms to the graphite edge, making the graphite grow unevenly in all directions, thus distorting the graphite and changing the graphite form from spherical to vermicular.
According to the principle of crystallography, the growth morphology of crystal is closely related to the growth kinetics of each crystal plane. When the growth driving force is constant, the growth mechanism of the interface determines the growth rate of the interface, and the microstructure of the interface determines the growth mechanism of the interface. If the interface is smooth, it can be either two-dimensional nucleation growth or screw dislocation growth.
In addition, the graphite crystal is anisotropic, and its growth mechanism must also be anisotropic. Previous studies have shown that the growth mechanism of graphite crystal base is screw dislocation growth mechanism, while the growth mechanism of graphite crystal edge is rotating twin step growth mechanism. Because the growth of graphite crystal is anisotropic, the growth morphology of graphite must be various.
When cast iron crystallizes, if the main growth direction of graphite is along the a direction, the final graphite is flake graphite. When the spheroidizing agent is added to the metal solution, the growth of graphite along the c direction is dominant, and the graphite finally grows into a sphere. This is because the nodulizer contains more magnesium and rare earth elements. When the nodulizing agent is added to the molten iron, the magnesium element in the nodulizing agent will be vaporized under the high temperature of molten iron, which will generate high local pressure in the metal solution and promote the nucleation of graphite. On the other hand, the affinity between magnesium and rare earth elements in the nodulizing agent and sulfur and oxygen is strong, which can generate high-temperature sulfides and oxides and reduce the sulfur The anti spheroidization of oxygen. In addition, magnesium will be segregated on the graphite base surface as the nucleation core of graphite, increasing the number of graphite crystal nuclei in cast iron and promoting the spheroidization of graphite.
Reducing the amount of spheroidizing agent will reduce the content of residual magnesium and residual rare earth in cast iron. On the one hand, it will increase the content of sulfur and oxygen in cast iron, increase the anti spheroidization of sulfur and oxygen, distort graphite balls, reduce spherical graphite and increase deformed graphite; On the other hand, with the reduction of the amount of residual magnesium and rare earth, the growth mode of graphite has gradually changed from mountain to and to. Therefore, with the decrease of the amount of spheroidizing agent, the globular graphite in cast iron decreases and the vermicular graphite increases, that is, the type of cast iron changes from nodular cast iron to vermicular cast iron.