Graphitization process of cast iron

The formation process of graphite in cast iron is called the graphitization process. The basic process of microstructure stress of cast iron is the formation process of graphite in cast iron. Therefore, it is very important to understand the conditions and influencing factors of the graphitization process for mastering the microstructure and properties of cast iron materials.

According to the dual state diagram of Fe-C alloy, the graphitization process of cast iron can be divided into three stages:

The first stage is the liquid phase hypoeutectic crystallization stage. It includes primary graphite directly crystallized from the liquid phase of Hypereutectic composition and austenite plus graphite crystallized from the liquid phase of eutectic composition, and graphite formed by the decomposition of primary cementite and eutectic cementite during high-temperature annealing.

The intermediate stage is the stage between Eutectic Transformation and hypereutectoid transformation. It includes secondary graphite directly precipitated from austenite and graphite formed by the decomposition of secondary cementite in this temperature range.

The third stage is the transformation stage. It includes eutectoid graphite formed during the co-bending transformation and graphite formed during the decomposition of eutectoid cementite during annealing.

Factors affecting graphitization of cast iron

The structure of cast iron depends on the degree of graphitization. To obtain the required structure, the key is to control the degree of graphitization. The practice has proved that many factors such as the chemical composition of cast iron, the cooling rate of cast iron crystallization, and the overheating and standing of molten iron affect the graphitization and microstructure of cast iron.

1. Influence of chemical composition

Among the common C, Si, Mn, P, s in iron, C, and Si are the elements that strongly promote graphitization, and s are the elements that strongly hinder graphitization. The influence of various elements on the graphitization ability of cast iron is extremely complex. Its influence is related to the content of each element itself and whether it acts with other elements, such as Ti, Zr, B, CE, Mg, etc. all hinder graphitization, but if its content is very low (such as B, ce<0.01%, t<0.08%), they also show the effect of promoting graphitization.

2. Influence of cooling rate

Generally speaking, the slower the cooling rate of castings, the more conducive it is to crystallize and transform according to the state diagram of the Fe-G stable system, and fully graphitize; On the contrary, it is conducive to crystallization and transformation according to the state diagram of Fe-Fe3C metastable system, and finally obtain white iron. Especially in the eutectoid stage of graphitization, because the temperature is low, the cooling rate increases, and the atomic diffusion is difficult, so generally, the eutectoid stage of graphitization is difficult to be fully carried out.

The cooling rate of cast iron is a comprehensive factor, which is related to pouring temperature, the thermal conductivity of molding materials, and the wall thickness of castings. And usually, these factors have the same impact on the two stages.

Increasing the pouring temperature can delay the cooling rate of the casting, which not only promotes the graphitization in the first stage but also promotes the graphitization in the second stage. Therefore, increasing the pouring temperature can make the graphite powdered to a certain extent, and also increase the eutectoid transformation.

3. Influence of overheating and high-temperature standing of cast iron

In a certain temperature range, increasing the overheat temperature of molten iron and prolonging the high-temperature standing time will lead to the refinement of graphite-based structure in cast iron and improve the strength of cast iron. If the superheat is further increased, the nucleation ability of cast iron will be reduced, so the graphite morphology will become worse, and even free cementite will appear, which will reduce the strength instead, so there is a 'critical temperature'. The critical temperature mainly depends on the chemical composition of molten iron and the cooling rate of castings. It is generally believed that the critical temperature of ordinary gray cast iron is about 1500-1550 ℃, so it is always hoped that the tapping temperature will be higher.

Searching for a cast iron tagine, cast iron collector, bsr cast iron supplier, wholesaler, manufacturer from China, you can get high-quality products at a nice price.