Factors affecting the graphitization of cast iron
The structure of cast iron depends on the extent to which graphitization is carried out. In order to obtain the desired structure, the key is to control the extent to which graphitization proceeds. Practice has proved that the chemical composition of cast iron, the cooling rate of cast iron crystallization and the overheating and standing of molten iron affect the microstructure of graphitization and cast iron.
1. The influence of chemical composition
Among C, Si, Mn, P, and S which are common in bismuth iron, C and Si are elements which strongly promote graphitization, and S is an element which strongly inhibits graphitization. In fact, the influence of each element on the graphitization ability of cast iron is extremely complicated. The influence is related to the content of each element and whether it interacts with other elements, such as Ti, Zr, B, Ce, Mg, etc., which hinder graphitization, but if its content is extremely low (such as B, Ce<0.01%, T< At 0.08%), they also showed a role in promoting graphitization.
2. The effect of cooling rate
In general, the casting cooling rate tends to be slow, and the more favorable it is to perform crystallization and transformation according to the Fe-G stable state diagram, and to fully perform graphitization; otherwise, it is advantageous to perform crystallization and transformation according to the Fe-Fe3C metastable state diagram. Finally, white iron was obtained. In particular, in the graphitization at the eutectoid stage, since the temperature is low, the cooling rate is increased, and atomic diffusion is difficult, it is usually difficult to sufficiently carry out graphitization in the eutectoid stage.
The cooling rate of cast iron is a comprehensive factor related to the casting temperature, the thermal conductivity of the molding material, and the wall thickness of the casting. And usually these factors have the same effect on the two phases.
Increasing the casting temperature retards the cooling rate of the casting, which promotes both the first stage of graphitization and the second stage of graphitization. Therefore, increasing the pouring temperature can pulverize graphite to a certain extent, and can also increase the eutectoid transformation.
3. The effect of overheating and high temperature standing on cast iron
In a certain temperature range, increasing the superheat temperature of molten iron and prolonging the time of high temperature standing will lead to the refinement of the graphite base in the cast iron and increase the strength of the cast iron. Further increasing the degree of superheat, the nucleation ability of the cast iron is reduced, so that the shape of the graphite is deteriorated, and even the free cementite is present, so that the strength is rather decreased, and thus there is a 'critical temperature'. The critical temperature depends mainly on the chemical composition of the molten iron and the cooling rate of the casting. It is generally believed that the critical temperature of ordinary gray cast iron is about 1500-1550 °C, so it is always desirable to have a higher iron temperature.
