White Cast Iron

The topic of this article is white cast iron. What is the chemical composition of white cast iron. Why is white cast iron brittle. We’ll cover topics such as the microstructure and applications of white cast iron. Our topics are as follows:

Properties of White Cast Iron

Iron containing less than 1.5% silicon when cast is hard and brittle. This is because the 3-4% carbon it contains is held in its structure in the form of iron carbide (cementite). This type of iron, whose fracture surfaces have a bright, white appearance, is called white cast iron. Its inability to be easily machined, and its use in places requiring only wear-resistant materials, are among its most notable characteristics.

Why is white cast iron brittle?

White cast iron forms when the iron poured into the mold cools rapidly, causing the carbon to remain locked within the structure. The cooling rate doesn’t allow the carbon to precipitate as graphite flakes, as it does in gray cast iron. Instead, the carbon combines with iron, chromium, or molybdenum to form a carbide. The resulting iron carbide (Fe3C — cementite) is hard and brittle.

The microstructure of white cast iron

The composition of white cast iron is generally 1.8-3.6% C and 0.5-1.9% Si. White cast iron consists of pearlite and cementite. While this makes the metal hard and brittle, it’s possible to soften the material by breaking down the cementite into ferrite and graphite through prolonged heating at high temperature. Metal treated this way is called “malleable cast iron.” This process is carried out between 900-1000°C for a period of 100 hours.

Cast iron mostly fractures through these cementite particles. When the fracture surface is examined, it appears light or white in color, which is why it’s called white cast iron. A high proportion of iron carbide forms during casting, and even though the structure has the same chemical composition as gray cast iron, it ends up harder and more brittle than gray cast iron. White cast iron normally cannot be welded because it lacks the ductility needed to withstand the thermal stresses that form in the base metal.

Photograph of the microstructure of white cast iron; the white regions are cementite (Fe3C), the dark regions are pearlite. (figure above)

The texture of white cast iron is a fairly coarse crystal with a silvery, silky appearance.

Using a method developed in the U.S.A., if white cast iron is heated to a high temperature inside iron oxide and left to cool slowly, its carbon content decreases by about 20%, and the properties of the cast iron come to resemble those of steel.

Where is white cast iron used?

Camshafts and rolling mill rolls, where wear resistance is required, are among the machine parts made of white cast iron. Through changes made to the cooling rate and internal structure, this type of machine part can have a white exterior surface while its interior remains gray (mottled) cast iron, which increases the usability of the machine part. White cast iron, which is normally impossible to machine, can only be shaped by grinding.

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