White tendency of gray cast iron

The solidification process of iron carbon alloy plays a decisive role in its final structure. Depending on the cooling rate, graphite (gray cast iron) or Fe3C (white cast iron) will be crystallized from the molten iron with hypoeutectic composition during the eutectic process. The two different structures determine their unique properties and the service conditions of the two materials. The carbon in gray cast iron exists in the form of graphite, and its section is dark gray. Due to the special properties of graphite, it has good castability, shock absorption and wear resistance. C in white cast iron exists in the form of Fe3C, and its cross section is gray white. It has good wear resistance, high hardness, but very brittle. It is suitable for manufacturing parts with small impact load. Due to the special requirements of the working conditions of the engine cylinder block, we can only choose gray cast iron, but the difference between the two is only the crystallization process. Therefore, in the casting production, we must strictly control the white tendency of gray cast iron.

The figure below shows the cooling curve of Hypoeutectic gray cast iron and white cast iron with carbon equivalent of 3.07%. When the molten iron is cooled to a temperature slightly lower than the liquidus equilibrium temperature (T, point), the slope of the cooling curve begins to change, reflecting that the pre eutectic austenite begins to precipitate at the temperature of ty, and the released latent heat of crystallization slows down the slope of the curve. The temperature of TEU point is slightly lower than the eutectic temperature, and the eutectic structure begins to form at this temperature. For gray cast iron, a large amount of latent heat of crystallization is released during the growth of graphite austenite eutectic. This heat is greater than the heat that the sample dissipates around at the same time, causing the melt temperature to rise until the TEM point temperature (the highest temperature reached by eutectic reaction).

The precipitation temperature of austenite in white cast iron is basically the same as that of gray cast iron, but the eutectic transformation temperature of white cast iron is lower than that of gray cast iron (TEU is lower than TEU point). Due to the long time required for eutectic transformation, the latent heat of crystallization released per unit time is relatively small. Therefore, the recovery of eutectic temperature is very small (or not), which is one of the most obvious differences between the cooling curve of white cast iron and the cooling process of gray cast iron.