Cast Irons

Cast iron is produced by smelting iron-carbon alloys that have a carbon content greater than 2%. Although both steel and cast iron contain traces of carbon and appear similar, there are significant differences between the two metals. Steel contains less than 2% carbon, which enables the final product to solidify in a single microcrystalline structure. The higher carbon content of cast iron means that it solidifies as a heterogeneous alloy, and therefore has more than one microcrystalline structure present in the material.

It is the combination of high carbon content, and the presence of silicon, that gives cast iron its excellent castability. Various types of cast irons are produced using different heat treatment and processing techniques, including gray iron, white iron, malleable iron, ductile iron, and compacted graphite iron.

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• Gray iron

Gray iron is characterized by the flake shape of the graphite molecules in the metal. When the metal is fractured, the break occurs along the graphite flakes, which gives it the gray color on the fractured metal’s surface. The name gray iron comes from this characteristic.

It is possible to control the size and matrix structure of the graphite flakes during production by adjusting the cooling rate and composition. Gray iron is not as ductile as other forms of cast iron and its tensile strength is also lower. However, it is a better thermal conductor and has a higher level of vibration damping. It has a damping capacity that is 20–25 times higher than steel and superior to all other cast irons. Gray iron is also easier to machine than other cast irons, and its wear resistance properties make it one of the highest volume cast iron products.

Vibration damping and wear resistance are properties that make this the right material for many applications. Raw grey iron also produces a patina that keeps it safe from destructive corrosion even outdoors.  Gray iron is used to make engine blocks and cylinder heads, manifolds, gas burners, gear blanks, enclosures, and housings.

 

 

• White iron

 With the right carbon content and a high cooling rate, carbon atoms combine with iron to form iron carbide. This means that there are little to no free graphite molecules in the solidified material. When white iron is sheared, the fractured face appears white due to the absence of unbounded graphite. The cementite microcrystalline structure is hard and brittle with a high compressive strength and good wear resistance. In certain specialized applications, it is desirable to have white iron on the surface of the product. This can be achieved by using a good conductor of heat to make part of the mold. This will draw heat out of the molten metal quickly from that specific area, while the rest of the casting cools at a slower rate.

One of the most popular grades of white iron is Ni-Hard Iron. The addition of chromium and nickel alloys gives this product excellent properties for low impact, sliding abrasion applications.

The chilling process used to make white iron results in a brittle material that is very resistant to wear and abrasions. For this reason, it is used to make mill linings, shot-blasting nozzles, railroad brake shoes, slurry pump housings, rolling mill rolls, and crushers.

Ni-Hard Iron is specifically used for mixer paddles, augers and dies, liner plates for ball mills, coal chutes, and wire guides for drawing wires.

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• Malleable iron

White iron can be further processed into malleable iron through a suitable heat treatment. An extended program of heating and cooling, results in the breakdown of the iron carbide molecules, releasing free graphite molecules into the iron. Different cooling rates, and the addition of alloying elements, produces a malleable iron with a specific microcrystalline structure.

Different grades of malleable iron correspond to different microcrystalline structures. Specific attributes that make malleable iron attractive are its ability to retain and store lubricants, the non-abrasive wear particles, and the porous surface which traps other abrasive debris. Malleable iron is used for heavy duty bearing surfaces, chains, sprockets, connecting rods, drive train and axle components, railroad rolling stock, and farm and construction machinery.

 

 

• Ductile iron (Nodular iron)

 Ductile iron or nodular iron, obtains its special properties through the addition of magnesium into the alloy. The presence of magnesium causes the graphite to form in a spheroid shape as opposed to the flakes of gray iron. Composition control is very important in the manufacturing process. Small amounts of impurities such as sulfur and oxygen react with the magnesium, affecting the shape of the graphite molecules. Different grades of ductile iron are formed by manipulating the microcrystalline structure around the graphite spheroid. This is achieved through the casting process, or through heat treatment, as a downstream processing step.

Ductile iron itself can be broken down into different grades, each with their own property specifications and most suitable applications. It is easy to machine, has good fatigue and yield strength, while being wear resistant. Its most well-known feature, however, is ductility. Ductile iron can be used to make steering knuckles, plow shares, crankshafts, heavy duty gears, automotive and truck suspension components, hydraulic components, and automobile door hinges.

 

 

• Compacted graphite iron

 

Compacted graphite iron has a graphite structure and associated properties that are a blend of gray and white iron. The microcrystalline structure is formed around blunt flakes of graphite which are interconnected. An alloy, such as titanium, is used to suppress the formation of spheroidal graphite. Compacted graphite iron has a higher tensile strength and improved ductility compared to gray iron. The microcrystalline structure and properties can be adjusted through heat treatment or the addition of other alloys.

Compacted graphite iron is beginning to make its presence known in commercial applications. The combination of the properties of gray iron and white iron create a high strength and high thermal conductivity product—suitable for diesel engine blocks and frames, cylinder liners, brake discs for trains, exhaust manifolds, and gear plates in high pressure pumps.​