EFFECT OF CARBON

Sources

Carbon is present in almost all charge materials. While there is very little carbon in steel, there is enough that it must be taken into consideration when calculating a charge. Pig iron,purchased cast scrap, and returns all have considerably higher percentages of carbon than steel does. Carbon is also purchased for addition. The addition materials are called graphite or carbon raiser. Graphite is a crystalline material that when added to a ladle may have an inoculating effect. Carbon Raisers, on the other hand, are amorphous are not believed to have any inoculating effect Certain alloy additions can contain significant amounts of carbon and must be taken into consideration when calculating a charge. In particular, silicon carbide is usually considered a source of silicon, but it contains a significant amount of carbon.

Effects and Comments

Carbon has long been recognized as one of the most important elements in effecting the microstructure and strength of gray and ductile iron. This has led to great efforts in controlling it. It is a rare iron foundry that doesn't have some control of their carbon in order to minimize effects from changes. The quality of the control will vary from foundry to foundry. Those foundries that don’t have good control of their carbon can expect significant changes in their iron’s properties.

In general, carbon is the most potent of graphitizers. The more carbon in the iron the more graphite will be in the matrix. Also, the more carbon there is in the iron the greater the probability that the matrix will have ferrite in it. If that is the case, that also means that there will be less chance to have carbides.

Higher carbon irons are less likely to shrink and have better fluidity than lower carbon irons. In addition, higher carbon leads to less likelihood of producing massive carbides. If a foundry is producing thin castings it is likely that they will run a higher carbon than a foundry On the other hand, attempting to correct shrink with higher carbons can lead to other problems, especially in big castings. High carbons and slow cooling rates (thick castings) can lead to a condition called carbon flotation. Graphite typically forms first during solidification and is lighter than iron. If the solidification of the entire casting is slow enough, the graphite floats toward the surface of the casting. Recovery of carbon when added to ladles is usually less than 50%.