Graphite parameters such as type, content and length in gray cast iron have an important influence on its tensile strength. It can be seen from Figure 1 that molybdenum does not change the type of graphite in gray cast iron, but with the increase of molybdenum content, the graphite is obviously shorter and thicker, which is consistent with the research results. According to the statistical results in Fig. 2 and Fig. 3, it can be found that the addition of molybdenum significantly shortens the length of graphite in gray cast iron and increases the number of graphite per unit area.
In S3 (0.56% Mo) and S4 (0.77% Mo) specimens, almost no length exceeds 300 μ M, which is the result of the dual effect of molybdenum on the thermodynamics and kinetics of graphite precipitation in gray cast iron. Molybdenum can increase the undercooling degree of eutectic solidification and the growth range of primary austenite. The equilibrium phase diagram of gray cast iron (fe-xmo-3.31c-2.0si-0.7mn-0.017p-0.132s-0.33cr-0.34cu-0.008nb) is calculated by using the “phase diagram” module of factsage 8.0, in which the database used is fsstel.
As shown in Figure 4, molybdenum increases the solid-liquid two-phase region of austenite, fully develops austenite, increases the branching degree of austenite dendrite, develops secondary dendrite and refines the secondary dendrite spacing. Eutectic crystallization can only be formed between dendrites. The formation and growth of graphite in gray cast iron will be strongly affected by dendrites. At the same time, the strong affinity between molybdenum and carbon slows down the diffusion rate of carbon, and then delays the precipitation of eutectic graphite. After inoculation, the core of graphite nucleation in gray cast iron is increased, However, the growth of graphite in the length direction in gray cast iron is still affected by dendrite spacing, so a large number of type a graphite with short length can be formed on the basis of avoiding the formation of D and e graphite. Because the stress concentration of graphite tip in gray cast iron is much higher than that in the middle part, the crack is easier to form at the interface between graphite tip and matrix in gray cast iron, and extends along the length direction of graphite in gray cast iron without being affected by matrix structure.
Therefore, the finer and curved graphite in gray cast iron, the more passivated the end, and the higher the tensile strength of gray cast iron. In addition, the graphite content in gray cast iron also decreases slightly with the increase of molybdenum (Fig. 3). Xu et al. Believe that this is because the amount of carbides formed increases with the increase of molybdenum content, which consumes carbon and slightly reduces the graphite content in gray cast iron. The decrease of graphite content in gray cast iron reduces the probability of crack formation with graphite in gray cast iron as the core, so as to improve the tensile strength of gray cast iron.