"Cast iron" is an iron that contains a large amount of carbon (2.1%~6.7%). Iron containing large amounts of carbon melts at approximately 1,400°C and can be poured into moulds and formed with comparative ease.
In case of steel, that has a relatively low carbon content, all of the carbon is dissolved into the iron. However, due to the high carbon content, the excess carbon precipitates as graphite. Cast iron has high wear resistance as the graphite formed in the cast iron composition works as a lubricant. However, the graphite formed divides the structure of the iron and as such makes the material "brittle".
When steel is heated up to temperatures of 1,500°C, it can be melted down and cast into shape. This is "cast steel" and not "cast iron". Cast iron is generally referred to as castings, but the material itself is classified based on the carbon content. Therefore not all castings are "cast iron", they could be "cast steel".
Normal cast iron is widely used for
machine chassis and parts.
Ductile cast iron is used in many cases
where a hard-to-break material comes
in handy.
Grey cast iron
Grey cast iron is another term for normal cast iron. It is named gray cast iron due to the large carbon content that appears as graphite. When looking at the surface of the material it has a gray colouring to it. As previously mentioned cast irons can be cast with relative ease, cast irons also has another property of being able to absorb vibrations and as such it is often used in applications such as machine beds and machine parts.
Grey cast iron can be cast into thin walled components offering comparatively lightweight parts. Therefore gray cast iron is often used for manufacturing of pipes.
Ductile cast iron
Cast iron sometimes does not have enough tensile strength and toughness. There are certain types of cast irons that do have high tensile strength. These are general termed as ductile cast irons. They are also known as "spherodial graphite cast irons" and "nodular cast irons".
I
n the composition of gray cast iron the graphite is in a linear formation. However for the ductile cast irons the graphite tends to be in a spherodial formation. The spherodial formation of the graphite improves the toughness and reduces problems relating to brittleness.
With it's high tensile strength, ductile cast iron is more suited for machine parts that going to be exposed to high forces, such as hydraulic cylinder parts and press moulds. The demand for the use of ductile cast irons is expected to increase in the future.
x