(1)Cu
Copper plays an important role in controlling the matrix structure of as cast nodular cast iron. Adding copper is the best measure to control the amount of pearlite. However, when the amount of copper is less than 0.4%, it has no significant effect on the number of pearlite. When the amount of copper is more than 0.5%, it has obvious effect. However, when the copper content exceeds 2%, the copper rich phase will appear, resulting in the deterioration of mechanical properties. Therefore, in the production of as cast nodular cast iron, the amount of copper is 0.5% ~ 1.5%. Under the same casting wall thickness, the amount of pearlite increases with the increase of copper; With the increase of casting wall thickness, the amount of copper should be increased accordingly.
(2)Si
∞ (SI) > 2.0%, silicon can promote the formation of ferrite. When the silicon content is more than 3%, it is easy to obtain ferrite matrix structure in as cast state. However, silicon can increase the brittle transition temperature of nodular cast iron, which is very disadvantageous to the parts working at low temperature. For pearlite matrix structure, the final silicon content is required to be in the range of 2.0% ~ 2.5%.
(3)Mn
Manganese can significantly increase the pearlite content in nodular cast iron, but manganese is also a carbide forming element and is easy to enrich at the eutectic boundary, resulting in the deterioration of mechanical properties. For pearlite matrix structure, the manganese content shall not exceed O.3%. Although the amount of pearlite increases sharply with the increase of manganese content, in order to prevent free cementite (manganese containing carbide) in as cast nodular cast iron, the manganese content should generally be controlled to a lower level.
(4)P
Phosphorus has a significant effect on the mechanical properties of nodular cast iron. On the one hand, the precipitation of phosphorus eutectic (Ca (P) > O.05%) significantly reduces the elongation after fracture of as cast nodular cast iron; On the other hand, phosphorus increases the brittle transition temperature. The brittle transition temperature of nodular cast iron increases by 4 ~ 4.5 ℃ with the increase of phosphorus by 0.01%. Therefore, we should strive to reduce the phosphorus content of as cast nodular cast iron and keep it at 60 (P) < 0.06%.
(5)Sn
The literature (tin and copper promote pearlization of iron matrix) holds that tin and copper are enriched on one side of the matrix at the interface between graphite and matrix. The tin and copper enrichment layer destroys the nucleation substrate of bovine eye ferrite and hinders the diffusion of carbon to graphite sphere and the formation of ferrite. So as to promote the pearlization of ductile iron matrix.
(6)Cr
Chromium is an element that strongly forms pearlite, but at the same time, it will also strongly form carbides, resulting in white mouth structure. Therefore, except for rolls, chromium is not used as an alloy element in the production of general nodular cast iron.
(7) Other alloy elements
Lead, bismuth, arsenic, antimony and other elements have a strong tendency to form pearlite, and their effect is much greater than that of manganese and copper. However, because they are elements that interfere with spheroidization, their application is limited to a certain extent.
The factors that determine the as cast microstructure of nodular cast iron are the influence of cooling rate and alloy elements. In addition, the inoculation effect directly affects the number of graphite balls. Strong inoculation measures will increase the number of graphite balls per unit area, so it is easy to obtain ferrite matrix. On the contrary, to obtain full pearlite matrix, appropriate inoculation measures must be controlled, otherwise the expected purpose may not be achieved.