Cast iron is an iron alloy containing 2 wt.% – 4 wt.% carbon, 1 wt.% – 3 wt.% silicon and smaller amounts of minor elements . In comparison, steel has a lower carbon content of up to 2 wt.% and a lower silicon content.
Cast iron can also be further optimized by alloying with small quantities of manganese, molybdenum, cerium, nickel, copper, vanadium and titanium before being cast.
Depending on the silicon content of the cast iron, it is classified as either white cast iron or grey cast iron and may be treated further under particular temperatures to produce malleable or ductile cast iron.
General properties of cast iron
Cast iron is highly favoured for its ability to be easily cast into complex shapes when molten and for its low cost. In addition, its properties can be easily altered by adjusting the composition and cooling rate without significant changes to production methods.
Its other main advantages over cast steel include ease of machining, vibration dampening, compressive strength, wear resistance and corrosion resistance . Corrosion resistance of cast iron is improved via the addition of minor elements such as silicon, nickel, chromium, molybdenum and copper .
Types of cast iron and their applications
Cast iron can be classified into grey cast iron, white cast iron, malleable cast iron and ductile cast iron, depending on its composition.
Grey cast iron
Grey cast iron, or gray iron, has a dark grey fracture colour due to a graphitic microstructure. The presence of graphite flakes is due to the addition of silicon, which acts to stabilise carbon in the form of graphite as opposed to iron carbide. Grey cast iron typically has a composition of 2.5 wt.% – 4.0 wt.% carbon and 1.0 wt.% – 3.0 wt.% silicon .
Grey cast iron applications
Grey cast iron is the most common form of cast iron. It is used in applications where its high stiffness, machinability, vibration dampening, high heat capacity and high thermal conductivity are of advantage, such as internal combustion engine cylinder blocks, flywheels, gearbox cases, manifolds, disk brake rotors and cookware.
A commonly used classification for grey cast iron is ASTM International standard A48. Under this system, grey cast irons are graded according to their tensile strength, with class 20 grey cast iron, for example, having a minimum tensile strength of 20,000 psi (140 MPa).
White cast iron
White cast iron has a white fracture colour due to the presence of iron carbide, or cementite Fe3C. The presence of carbon in this form, as opposed to graphite, is the result of a lower silicon content compared to grey cast iron. White cast iron typically contains 1.8 wt.% – 3.6 wt.% carbon, 0.5 wt.% – 1.9 wt.% silicon and 1.0 wt.% – 2.0 wt.% manganese.
White cast irons are extremely wear-resistant yet brittle. They exhibit high hardness as a result of their microstructure containing large iron carbide particles and are not easily machined.
White cast iron applications
White cast irons are used in abrasion-resistant parts where it’s brittleness is of minimum concern such as shell liners, slurry pumps, ball mills, lifter bars, extrusion nozzles, cement mixers, pipe fittings, flanges, crushers and pump impellers.
A popular grade of white iron is high chrome white irons, ASTM A532. This contains nickel and chromium for good low-impact abrasion applications .
Malleable cast iron
Malleable cast irons are formed via a slow annealing heat treatment of white cast iron. This results in the conversion of carbon, in the form of iron carbide in white iron, to graphite, with the remaining matrix being composed of ferrite or pearlite . The graphite is present in the form of spherical or nodular shapes.
Malleable cast iron exhibits good malleability and good ductility. Due to the lower silicon content compared to other cast irons, it exhibits good fracture toughness at low temperature.
Malleable cast iron applications
As a result of its good tensile strength and ductility, malleable cast iron is used for electrical fittings and equipment, hand tools, pipe fittings, washers, brackets, farm equipment, mining hardware, and machine parts.
A common classification for malleable cast iron is ASTM A47.
Ductile cast iron
Ductile cast iron, also known as nodular cast iron and spheroidal graphite cast iron, is defined by the presence of graphite in the form of spherical nodules, as with malleable cast iron. Differently to malleable cast iron, ductile cast iron is formed not by heat treatment of white iron, but through a specific chemical composition.
Ductile cast iron contains 3.2 wt.% – 3.6 wt.% carbon, 2.2 wt.% – 2.8 wt.% silicon and 0.1 wt.% – 0.2 wt.% manganese, as well as smaller amounts of magnesium, phosphorus, sulfur and copper. The presence of manganese is responsible for the spherical form of the graphite inclusions .
Ductile cast iron applications
Due to its microstructure, this material is more ductile than grey or white cast irons. For this reason, it is used as ductile iron pipe for water and sewage infrastructure. It can also withstand thermal cycling and is therefore used in vehicle gears and suspension components, brakes and valves, pumps and hydraulic parts, and housings for wind turbines.
Ductile cast iron is commonly classified as ASTM A536.
Production and processing
To produce cast iron, the iron must be extracted from iron ore. The ore is smelted in a blast furnace where it separates into pig iron and slag. The furnace is heated to around 1800 degrees Celsius in an oxygen atmosphere and the slag formed rises to the top and can be removed.
The molten pig iron below contains around 3 wt.% – 5 wt.% carbon. This is then combined with iron, steel, coke and limestone.
Once impurities are removed selectively from this iron, the carbon content is reduced. At this point, silicon may be added to convert the carbon content to graphite or cementite. The iron is then cast into various forms.