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A large number of fine graphite spheres with a diameter of about lum can be seen in the liquid quenched samples of Hypereutectic and hypoeutectic magnesium nodular cast iron(best silicon carbide powder wholesale malaysia). Such experiments can show that the undercooling of eutectic reaction of nodular cast iron is significantly improved due to the addition of spheroidizing elements. 


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The content of eutectic graphite and austenite is very different, and the nucleation and growth modes are also different, resulting in the crystallization of the two phases outside the symbiotic zone, which are separated in space and time(best silicon carbide powder wholesale malaysia).  In the solidification process of cast iron, the concentration of silicon in the center of austenite is higher than that at the edge of grain(fine grit aluminum oxide).


If the spherical graphite precipitates from the liquid phase first, the molten liquid around it becomes a carbon poor area, in which the carbon concentration changes, forming a concentration gradient in which the carbon concentration gradually rises outward from the spherical surface(best silicon carbide powder wholesale malaysia). Austenite grows alone along the preferred direction in the liquid phase, and secondary dendrites are produced(1200 grit aluminum oxide). 


This is because after the undercooling is further improved, the composition of eutectic structure moves to high carbon, and the carbon concentration of molten liquid at the interface front decreases, which promotes the growth of austenite dendrite(best silicon carbide powder wholesale malaysia). When the undercooling reaches 100 ℃, 90% carbon comes from about 8R range(240 grit aluminum oxide). E-type graphite grows close to A-type graphite, but the growth rate is faster than A-type graphite.


The cooling rate is low, the undercooling is reduced, the carbon has sufficient time to diffuse, and the range of carbon supply from molten iron to graphite is relatively large(best silicon carbide powder wholesale malaysia). With the increase of undercooling, the carbon concentration gradient is steep(aluminium oxide blasting media). When the undercooling degree of the melt is 10 ℃, 90% of the carbon of the spherical graphite comes from the range of about 26r (R is the radius of the graphite ball).


However, the undercooling of the molten liquid in the alkali poor area around the spherical graphite is higher than that at the graphite molten liquid interface, and austenite will nucleate in these areas first(brown aluminium oxide). In addition, it shows that the nucleation position of eutectic austenite is close to the edge of spheroidal graphite with a certain size in the melt with high undercooling(best silicon carbide powder wholesale malaysia). 


As mentioned earlier, the graphite phase has the ability to lead or induce the nucleation of austenite phase, and austenite will grow rapidly relying on graphite(best silicon carbide powder wholesale malaysia). Once the two phases contact, austenite grows rapidly along the surface of the sphere(aluminum oxide sandblasting abrasive). The growth rate of austenite on the surface of graphite and the growth rate of [0001] crystal direction of graphite are related to the undercooling of molten liquid.


With the increase of undercooling, the growth rate of both should be improved(glass bead abrasive). The properties and contents of common elements and alloying elements in Fe-C alloys also affect the interface morphology and solidification structure. Graphite spheres grow in the solid phase, of course, this will not happen(aluminum oxide anti slip grit). Before the solidification volume fraction is less than 30%, there is thermal convection in the melt(best silicon carbide powder wholesale malaysia).


It can be seen that under the condition of eutectic undercooling of nodular cast iron melt, the growth rate of austenite on graphite surface is greater than that of spherical graphite(white aluminium oxide). Therefore, once austenite grows on the graphite surface, it will coat the graphite sphere and form the coating layer of the sphere(best silicon carbide powder wholesale malaysia). There is another way to make the graphite ball surrounded by austenite dendrites. 


The smaller graphite spheres in the liquid phase may be fixed at some places on the dendrite surface when the convection stops(steel shot abrasive). Due to the high thermal conductivity of graphite, grooves appear on the interface between dendrite and graphite. Under specific reagent etching, the silicon concentration in cast iron shows different colors in different parts(best silicon carbide powder wholesale malaysia). This concentration gradient is related to the undercooling of the melt.


Therefore, dendrites will grow around the sphere and discharge more carbon, so as to increase the carbon concentration in the groove, and the groove depth will increase due to the change of local melting point of liquid phase, and finally make the graphite sphere fall into the enclosure of dendrites(best silicon carbide powder wholesale malaysia). No matter which way austenite surrounds the graphite sphere, the two phases will continue to grow in the form of divorced eutectic(alumina blasting).


However, the dendrite volume expands outward, and the graphite receives the carbon atoms discharged from austenite and continues to increase in the solid state(black corundum). Dendrite growth is controlled by alkali atom diffusion rate, while graphite growth is controlled by iron atom self diffusion. In other words, the eutectic crystallization of nodular cast iron is carried out according to the mode of divorced eutectic transformation(best silicon carbide powder wholesale malaysia). 


The figure shows that it is easy to be misunderstood that each austenite graphite eutectic cluster contains only one graphite ball, that is, the divorced eutectic transformation products appear on each spherical graphite monomer(best silicon carbide powder wholesale malaysia). However, through the study of the solidification process of nodular cast iron, it is confirmed that there are not only one graphite ball, but some graphite balls in a eutectic of austenite spherical graphite(aluminum oxide blasting abrasive). 


Under the action of thermal convection, the graphite ball in molten iron will contact with the separately growing austenite dendrite(pink aluminum oxide). In the etched samples, the position and trend of the product boundary can be identified by observing the distribution of the color region corresponding to the lowest silicon concentration(best silicon carbide powder wholesale malaysia). Austenite spheroidal graphite eutectic grains in as cast microstructure of nodular cast iron(alumina abrasive powder). 


Eutectic grains have no specific shape, and the outline shape of grains is related to the size and quantity of graphite spheres trapped in them(best silicon carbide powder wholesale malaysia). Whether the molten iron is hypoeutectic, eutectic or hypereutectic, spheroidal graphite has been precipitated directly from the molten iron before the Eutectic Transformation of nodular cast iron, that is, spheroidal graphite is precipitated directly from the liquid phase(aluminium oxide shot blasting grit). 


This situation has also been confirmed in Color Metallographic examination(garnet blasting media). Each grain contains a plurality of graphite balls with different sizes. The larger graphite spheres were first surrounded by austenite and located in the grain(best silicon carbide powder wholesale malaysia). The smaller graphite balls precipitate and sink into austenite at a lower temperature when austenite has grown to a certain extent, so most of them are located at the grain edge. 


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