The role of alloy elements in grey-cast iron

1. Manganese and sulfur
Generally speaking, sulfur is a harmful element. But for gray cast iron, a small amount of sulfur plays a very important role in the refinement of graphite core and cocrystal groups. Therefore, the sulfur content in gray cast iron should not be less than 0.06%, and it is best to keep between 0.06% and 0.08%. If the sulfur content is too high (> 0.18%), all kinds of harmful effects will appear and damage the quality of the casting.
Sulfur is an element with strong chemical activity. When the manganese content in cast iron is very low, sulfur and iron produce compound FeS (melting point 1193℃) and also form low melting cocrystals with iron and carbon (carbon 0.17%, sulfur 31.7%, the rest is iron, melting point 975℃).
FeS Can be completely dissolved in the iron solution. When iron solidification, sulfur or FeS in austenite and carbonization solid solution is very small, gradually enriched in the remaining liquid phase, and finally precipitated in the form of sulfide, when the sulfur content in cast iron is 0.02%, independent sulfide can appear.
In grey cast iron, the affinity of sulfur and manganese is much greater than that of iron, and the formation of MnS is predominant in the reaction described in the following equation.
Mn+FeS MnS + Fe
The melting point of MnS is 1620℃. In gray cast iron, MnS is a small and harmless blue-gray inclusion, which is scattered distributed in the metal matrix, and helps to improve the processing performance of cast iron and improve the life of the tool.
In gray cast iron, MnS and FeS can interdissolve, so the amount of manganese and sulfur in compound sulfide (MnFe) S is variable. Sulfur and oxygen can also be mutually soluble, so in gray cast iron, the sulfide situation is very complex, the resulting compounds can be represented by FexMnySvOw, where the values of x, y, v and W will change by the specific conditions of cast iron.
In recent years, the study by quantitative metallographic analysis has shown that there are about 43 million inclusions of MnS and (MnFe) S inclusions in 1cm3 ash cast iron, and the size of inclusions is 2~23 micro. The total amount of the inclusions is less than 0.01% of the weight of the cast iron.
When sulfur exists in FeS or ternary co-crystal, it segregation in the liquid phase at the interface of the cocrystal block in the process of solidification, hinders the growth of the co-crystal group, and increases the supercooling of the co-crystal solidification, thus increasing the white mouth tendency of cast iron. After solidification, the FeS or ternary cocrystal at the boundary of the cocrystal group will make the mechanical properties of cast iron low and the brittleness large. This is the harmful effect of sulfur.
If sufficient amounts of manganese are added to the cast iron to inhibit FeS production and form fine and dispersed sulfide inclusion, sulfur will play a positive and beneficial role in the nucleation and growth of graphite. RESULTS Cast iron small crystal small, graphite is type A, there will be no white mouth. To make the shape of the graphite sheet satisfactory, the minimum content of sulfur in the gray cast iron should be 0.06%. The sulfur content is lower than this value, and the distribution of graphite sheet in cast iron is bad, and it is easy to produce white mouth.
The main role of manganese in grey cast iron is to restrain sulfur and inhibit FeS formation. Because the ratio of the atomic weight of manganese to sulfur is 1.71. The MnS equivalent ratio of manganese and sulfur formed in cast iron is 1.71. Mangn higher than equilibrium, called