Molybdenum is an element with strong affinity with carbon, which is conducive to the formation of the second phase particles of gray cast iron carbides during solidification. As shown in Figure 1, gray cast iron carbides begin to precipitate when the molybdenum content is 0.26%. With the increase of molybdenum content, the number and size of precipitates increase significantly. When the molybdenum content reaches 0.56%, the size of gray cast iron carbides is mainly distributed in 8 ~ 30 μ m。 J-Mat pro (cast iron database) simulation calculation of carbide precipitation of gray cast iron is carried out according to the chemical composition in Table 1. The calculation results are shown in Figure 2.
As can be seen from Fig. 2 (a), when the molybdenum content is 0.034%, the carbides of gray cast iron generated in cast iron are mainly M7C3 type, and a small amount of M23C6 carbides. The total amount of carbides of gray cast iron is about 1.2%; When the molybdenum content exceeds 0.26%, although M7C3 and M6C carbides are formed during cooling, the final carbides will be transformed into M23C6 and M2C carbides. Therefore, it is not difficult to infer that the added molybdenum finally precipitates in the form of M23C6 and M2C carbides with high Mo content. In addition, after adding molybdenum, the total amount of carbides in gray cast iron is increasing, which is consistent with the phenomenon observed in Figure 1. When the molybdenum content reaches 0.77%, the total amount of carbides in gray cast iron is about 2.0%.
It is precisely because of the precipitation of carbides in gray cast iron that the strength properties of gray cast iron can be significantly improved, mainly in two aspects: fine grain strengthening and hindering dislocation movement. Fig. 3 is a schematic diagram of the solidification and crystallization process of gray cast iron with and without molybdenum. The gray cast iron carbides precipitated at the grain boundary and cell will hinder the growth of austenite eutectic cells and pearlite, thus forming a large number of eutectic cells and pearlite clusters, which plays the role of fine grain strengthening. Such gray cast iron carbides usually exist in the form of long strips, This is consistent with the results of Ding et al.
After studying the effect of niobium on niobium rich phase in high carbon equivalent gray cast iron, Zhou Wenbin also obtained the same conclusion. It is pointed out that Y-type or bar type niobium carbide precipitates at the grain boundary, which will hinder the growth of eutectic cells. In addition, Y-type or bar type gray cast iron carbides have high bonding strength and high hardness with the matrix, which can hinder the dislocation movement. Molybdenum and niobium are adjacent to each other in the same period in the periodic table, and have many similarities in physical and chemical properties. Therefore, it is inferred that molybdenum gray cast iron carbides also hinder the dislocation movement and improve the tensile strength. In combination with the effect of shortening the length and increasing the number of graphite and reducing the pearlite layer spacing after molybdenum addition as shown in Fig. 3, the microstructure of gray cast iron can be significantly improved after molybdenum addition, which can play a significant role in the tensile strength of gray cast iron.