Knowledge
Cast Iron Definition, Properties, And Uses
Iron is found in the composition of many different metal alloys. Without it, carbon steel, stainless steel, superalloys, and several other metal alloy groups could not be manufactured.
However, cast iron, ironically the metal alloy group with “iron” in its name, is often misunderstood as to what it is, how it is made, and what it can be used for.
This article takes an in-depth look at cast iron.
What is Cast Iron?
Cast iron is a class of iron-carbon alloys with a carbon content of more than 2% and a silicon content of around 1–3%. Its usefulness derives from its relatively low melting temperature.
Cast iron is one of the oldest ferrous metals used in construction and outdoor ornament. It has a relatively high carbon content of 2% to 5%. It is primarily composed of iron (Fe), carbon (C) and silicon (Si), but may also contain traces of sulphur (S), manganese (Mn), and phosphorus (P).
It is hard, brittle, nonmalleable (i.e. it cannot be bent, stretched or hammered into shape) and more fusible than steel.
Its structure is crystalline and it fractures under excessive tensile loading with little prior distortion. Cast iron is, however, very good in compression.
The composition of cast iron and the method of manufacture are critical in determining its characteristics.
Cast iron is used in pipes, machinery, and automotive parts such as cylinder heads, cylinder blocks, and gearboxes. It is resistant to oxidation damage but difficult to weld.
How Is Cast Iron Made?
The manufacturing process of cast iron is how the metal gets its name. To make cast iron, iron ore is heated in a furnace until it becomes molten. Then the molten metal is cast (poured out and allowed to harden, in the shape of an ingot).
The cast iron ingots are melted again into a final mold. During this subsequent remelting, the cast iron may have several metallurgical modifications made to it through the introduction of alloying elements or heat-treating processes.
Oftentimes, this is where a cast iron falls into the specifications of one of the four groups mentioned above.
Cast iron, when molten, has better fluidity than steel. Cast iron also has a low melting temperature. These two attributes make cast iron an ideal candidate for the casting process. This is partially why it is so popular.
However, with improved technology for manufacturing and forming steel, use of cast iron has diminished over the past several centuries.
What is Cast Iron Made of?
Cast iron is made from pig iron, which is the product of melting iron ore in a blast furnace. Cast iron can be made directly from the molten pig iron or by re-melting pig iron, often along with substantial quantities of iron, steel, limestone, and carbon (coke), and taking various steps to remove undesirable contaminants.
Phosphorus and sulfur may be burnt out of the molten iron, but this also burns out the carbon, which must be replaced. Depending on the application, carbon, and silicon content are adjusted to the desired levels, which may be anywhere from 2–3.5% and 1–3%, respectively.
If desired, other elements are then added to the melt before the final form is produced by casting.
Cast iron is sometimes melted in a special type of blast furnace known as a cupola, but in modern applications, it is more often melted in electric induction furnaces or electric arc furnaces. After melting is complete, the molten cast iron is poured into a holding furnace or ladle.
What is the Difference Between Cast Iron & Steel?
The main difference between the two elements is that steel is produced from iron ore and scrap metals, and is called an alloy of iron, with controlled carbon. Whereas around 4% of carbon in iron makes it cast iron, and less than 2% of carbon makes it steel.
Cast iron is cheaper than most steel. Also, the cast iron melting temperature is lower than that of steel, but it has high compressive strength, high hardness, and high wear resistance.
Therefore, the important difference between steel and cast iron is that steel is ductile and malleable, whereas cast iron is hardened and has high compressive strength.
As another important difference between steel and cast iron, we can say that carbon in steel is in the form of iron carbide while cast iron has carbon as graphite or iron carbide or both. In addition, cast iron has excellent fluidity, with no steel.
Properties of Cast iron
A few common mechanical properties of cast iron include:
- Hardness Cast iron is hard and it can be hardened by heating and sudden cooling. This makes it quite durable. Mild steel can be hardened and tempered by using relevant processes.
- Toughness. Material’s ability to absorb energy
- Ductility. Material’s ability to deform without fracture
- Elasticity. Material’s ability to return to its original dimensions after it has been deformed
- Malleability. Material’s ability to deform under compression without rupturing
- Tensile strength. The greatest longitudinal stress a material can bear without tearing apart
- Fatigue strength. The highest stress that a material can withstand for a given number of cycles without breaking
- Melting Point. Cast iron has a lower melting point of 12000°C compared to the melting point of mild steel, which ranges between 13000°C and 14000°C.
- Castability. Cast iron is easier to work with when it comes to casting shapes out of the material. Due to the extra carbon present in cast iron, it’s molten form is more fluid and this makes it easier to cast the material into complex shapes.
- Machinability. Cast iron is almost elastic up to ultimate tensile strength and produces discontinuous chips which break away from the sample easily. This helps to improve the cutting ability. Due to this, cast iron is the preferred material
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