Cast iron is produced by melting iron and pouring the molten iron into a mould, then allowing the iron to harden into the desired shape. This process is called casting. Mechanical properties describe how materials respond to applied forces. Alloys are metals with varying amounts of other elements mixed in to alter their properties.
Iron atoms can be arranged in a number of different crystal structures. Cast iron can also contain varying amounts of carbon atoms. The different crystal structures are called phases. Each phase of iron has slightly different properties. The main phases of iron are ferrite, perlite, austenite, cementite, bainite and martensite. Each phase has a slightly different crystal structure and contains slightly different proportions of carbon.
Alloying elements are any other elements in the cast iron that are not metal. The main alloying element in cast iron is carbon. Other elements are also present in types of cast iron, and are capable of causing a major change in mechanical properties, even if the alloying elements are only present in small concentrations. Apart from carbon, the main alloying elements in cast iron are silicon, sulphur, manganese, nickel, chromium, tin and molybdenum.
Some important mechanical properties are strength, toughness, hardness, plasticity, elasticity, brittleness and ductility. Strength is a measure of the ability of a material to withstand a stress, which is a force applied over an area of the surface of the material. Toughness is a measure of the ability of a material to change shape without fracturing. Hardness is the ability of a material to withstand scratches. Plasticity is the extent to which a material undergoes nonreversible shape change when a force is applied to it. Elasticity is the extent to which a material returns to its original shape when it has been deformed by the action of a force. Brittleness is a measure of the tendency of a material to fracture on impact by a force. Ductility measures how easily a material deforms when it is stretched.
Gray Cast Iron
There are two main types of cast iron. These are grey cast iron and white cast iron. Gray cast iron is the most common, and is used in applications where strength is not critical. This is because grey cast iron is cheap and relatively easy to manufacture. It contains fragments of graphite and high levels of silicon. The cheapest and lowest strength grey cast irons have a ferritic crystal structure. The more expensive and stronger grey cast irons have a higher concentration of nodules of graphite. Gray cast iron is highly ductile and plastic, so is easy to mould into different shapes.
White Cast Iron
White cast iron has lower silicon content than grey cast iron. It consists of nodules of cementite crystal structure embedded within austenite, and it contains no graphite nodules. White cast iron is harder than grey cast iron, but less tough. This means it is more scratch-resistant but less resistant to fracture on impact. White cast iron is also brittle, making it unsuitable for many uses. Because of its resistance to scratches and hardness, white cast iron is used in ball bearings and pump propellers.