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Graphite spheres will continue to grow after being surrounded by austenite(aluminum oxide sandblasting suppliers kenya). Because the carbon concentration at the austenite hot metal interface is greater than that at the austenite graphite ball interface, there is a carbon concentration gradient from outside to inside in the austenite cladding, resulting in the diffusion of carbon atoms at the austenite hot metal interface to the austenite graphite interface through the cladding.

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The concentration gradient of iron is opposite to that of carbon, and iron atoms will diffuse themselves in the opposite direction(aluminum oxide sandblasting suppliers kenya). The carbon atoms reaching the austenite graphite interface deposit on the graphite surface, which increases the size of graphite spheres. The diffusion rate of carbon in the solid phase is much lower than that in the liquid phase(steel grid). The size increase of graphite spheres in austenite is not very significant.

The reason of nucleation in these parts is that there are many crystal defects on the graphite interface with high energy(aluminum oxide for sale); Moreover, the atomic diffusion distance is short, and the desolved atoms adjacent to graphite are easy to reach. Therefore, the eutectic transformation product is the eutectic grain of austenite coated with some large or small graphite balls(glass bead blasting media suppliers). This can be seen on castings with uneven thickness(aluminum oxide sandblasting suppliers kenya).

When the growth rate is properly increased, the internal defects produced by the growth of the product increase, and the graphite appears frequent branching due to the limitation of interface undercooling and the interference of eutectic austenite(garnet sand). Although the graphite is still type A, it is significantly refined and the growth direction deviates from the heat flow direction(aluminum oxide sandblasting suppliers kenya). The microstructure of cast iron with large and small eutectic is formed.

However, after coating spherical graphite, due to the limitation of isotropic diffusion of carbon atoms, the preferred growth direction is replaced by isotropic growth, and secondary dendrites and multiple dendrites are no longer produced(aluminum oxide sandblasting media). But there are some round protrusions on the surface, forming a cauliflower like morphology(aluminum oxide sandblasting suppliers kenya). The solid phase transformation of eutectic particles of nodular cast iron is carried out around a single graphite ball.

The solid-state transformation begins with the desolvation of carbon in austenite and y + a transformation occurs(black aluminum oxide). The diffusion rate and self diffusion of iron atoms in the crystallization process are the controlling factors for the lattice recombination of carbon and iron atoms to form a new phase(aluminum oxide sandblasting suppliers kenya). The premise of Y + a phase transition is that a ferrite nucleates and has a suitable position to receive desolved carbon atoms.

The ferrite formed on the surface of graphite is composed of many equiaxed ferrite grains(80 grit aluminum oxide). During the growth of eutectic austenite, a considerable part of the carbon discharged at the interface enters the molten iron before the interface. The carbon atoms establish the graphite crystal core on the effective nucleation matrix and grow into small spherical graphite(aluminum oxide sandblasting suppliers kenya). These small graphite balls will also be coated by the growing austenite.

When the temperature of Hypereutectic molten iron is lower than the liquidus, there will be primary high carbon phase precipitation(black silicon carbide). The figure is a schematic diagram formed by overlapping the Fe-C-Si equilibrium diagram of stable system (practice) and metastable system(aluminum oxide sandblasting suppliers kenya). The figure shows that the temperature range for the formation of metastable eutectic is lower than that for the formation of stable system co products.

Ferrite nucleation began to appear at the spheroidal graphite austenite interface and nucleated and grew in many places(100 grit aluminum oxide). Comparing the crystallization processes of the two high carbon phases, the formation of graphite requires large migration of carbon atoms and iron atoms, while the combination of iron and carbon atoms to form cementite is basically a site reaction, and the amount of atomic movement (especially iron atoms) is small(aluminum oxide sandblasting suppliers kenya).

Another theory is that if the diffusion movement of iron atoms is blocked or slow enough to provide enough vacancies for the formation of graphite crystals, it is difficult to produce primary graphite(pink corundum). Increasing the cooling rate of molten iron is helpful to inhibit the crystallization of graphite and create conditions for the formation of cementite(aluminum oxide sandblasting suppliers kenya). Increasing the cooling rate of molten iron will increase the degree of crystal undercooling.

Once the molten iron is subcooled to the temperature below the phase line of carburized body fluid, primary cementite will precipitate(aluminum oxide abrasive powder). Low activity indicates that the activity of carbon atoms is poor, the diffusion rate is low, and it is difficult to form graphite crystals(aluminum oxide sandblasting suppliers kenya). The influence degree first depends on the relative solubility of elements in cementite and austenite(120 grit aluminum oxide). Once cementite precipitates on the austenitic surface, it will grow preferentially along the [010] crystal direction. 

The crystal with high growth rate is the preferred phase component. Figure 41 shows that cementite can become a dominant growth crystal only when the growth rate is higher than the intersection of the two curves (to the left of the intersection). The intersection temperature is slightly lower than the eutectic transformation temperature of cementite(aluminium oxide blasting). Carbon activity in molten iron is also an important factor determining the type of high carbon phase(aluminum oxide sandblasting suppliers kenya).

In the as cast structure of nodular cast iron, it can be seen that each spherical graphite is always surrounded by ferrite halo(green silicon carbide). On the contrary, low silicon content or carbide forming elements in molten iron will reduce the carbon activity in molten iron and contribute to cementite crystallization(aluminum oxide sandblasting suppliers kenya). Therefore, the chemical composition of molten iron is an important factor affecting the eutectic formation of cementite. 

Therefore, when the molten iron temperature drops below the lower limit of Eutectic Transformation Temperature of the stable system, cementite eutectic composed of cementite and eutectic austenite will be produced(aluminum oxide sandblasting suppliers kenya). The carbon content of graphite phase is close to 100%, which is much higher than that of cementite (6.67%)(alumina sand). If the solubility of element in cementite is greater than that in austenite, the element will enter cementite, improve the stability of cementite and promote the formation of cementite eutectic.

Austenite is the leading precipitate of cementite eutectic(alumina abrasive). Moreover, some carbide forming elements also reduce the carbon activity of molten iron and contribute to the formation of cementite eutectic. In the orthogonal lattice of cementite, the growth rate in a-axis direction and b-axis direction is greater than that in e-axis direction(aluminum oxide sand). The crystal grows two-dimensional in a layered growth mode along the [010] product direction(aluminum oxide sandblasting suppliers kenya).

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