factors affecting graphitization of cast iron

The formation process of graphite in cast iron is called graphitization process.

It can be divided into three stages:

1.Liquid phase hypoeutectic crystallization stage.It include the direct crystallization of primary graphite from the liquid phase of hypereutectic components, the crystallization of austenite from the liquid phase of eutectic components and the graphite formed by primary cementite decomposed during high temperature annealing.

2.Eutectic transition phases between hypoeutectoid transitions.It include the direct precipitation of secondary graphite from austenite and the graphite formed by the decomposition of secondary cementite in this temperature range.

3.Co-folding transition stage.Including the eutectoid graphite formed during the co-folding transformation and the graphite formed by the decomposition of the eutectoid cementite during annealing.

The structure of cast iron depends on the degree of graphitization. In order to obtain the required structure, the key is to control the degree of graphitization. Practice has proved that many factors such as the chemical composition of cast iron, the cooling rate of cast iron crystals, the overheating and standing of molten iron, etc., all affect graphitization and the microstructure of cast iron.

Factors affecting graphitization of cast iron:

1.Influence of chemical composition.

Among the common C, Si, Mn, P, and S in cast iron, C and Si are elements that strongly promote graphitization, and S is an element that strongly hinders graphitization. In fact, the influence of various elements on the graphitization ability of cast iron is extremely complicated. For example, Ti, Zr, B, Ce, Mg, etc. all hinder graphitization, but if its content is extremely low (such as B, Ce<0.01%, T<0.08%) ), they show the effect of promoting graphitization.

2.Influence of cooling rate.

The cooling rate of cast iron is a comprehensive factor, which is related to the pouring temperature, the thermal conductivity of the molding material, and the wall thickness of the casting. Increasing the pouring temperature can delay 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 of cast iron.

Within a certain temperature range, increasing the overheating temperature of the molten iron and extending the high-temperature standing time will result in the refinement of the graphite-based structure in the cast iron and increase the strength of the cast iron. If the superheat is further increased, the nucleation ability of the cast iron will decrease, and the graphite morphology will be deteriorated, and even free cementite will appear, which will decrease the strength. Therefore, there is a ‘critical temperature’. The critical temperature mainly depends 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 ℃, so it is always hoped that the iron tapping temperature is higher.

The graphitization of cast iron will involve much more raw materials such as recarburizer, spheroidizing agent, inoculant, etc, Anyang Huatuo Metallurgy has in such materials for more than 20 years, if any confused can contact us, we would like to provide any help.