Types of Cast Irons
Casting is one of the most basic mechanical processes and the most direct method of producing a product from the chosen material. The basic steps in casting involve preparation of the material in the liquid state, transferring it into a shaping mould, allowing the material to cool into a solid form. The solid object can be then used directly or subjected to secondary processes for further treatment. While ceramics and polymers can also be casted, by far the most common material used for castings are metals, and especially iron and its alloys.
Types of Cast Irons
The history of iron castings is quite old – at least 2500 years old. Iron castings were made in China in about 500 B.C. and introduced to Europe a thousand year later. One of the first iron castings that were made were cast directly from the blast furnace.
Cast iron is essentially an iron-carbon alloy that may contain some other elements like silicon, manganese, sulphur or phosphorous. The addition of these elements even in miniscule proportion changes the properties of cast iron alloys radically. Steels and cast irons are both primarily iron with carbon. The difference between them is that steels contain less than 2% carbon, while cast irons contain more than 2% carbon. However, this demarcation is not defined properly. So, the best way is to just remember that cast irons contain less carbon than steels. Cast irons can be classified into two groups – those that involve ferrous materials (cast irons and steels) and cast nonferrous materials like cast magnesium, aluminium, copper, zinc and other alloys. Unalloyed cast irons cannot be designated by chemical composition and are designated by their mechanical properties.
The carbon in cast iron is present in two forms – as free carbon / graphite, or as cementite. Nucleating agents called as inoculants are also necessary for the successful production of each type of cast iron. Various types of cast iron can be classified according to their microstructure. It is based on the form and shape in which the major portion of carbon occurs in the cast irons. While there are many types of cast irons, for the purpose of this article we will focus on those that are most popular today to manufacture machined components.
Gray Cast Iron
Gray cast iron is a ferrous alloy which has been heated until it liquefies and is then poured into a mould to solidify. Gray cast iron contains about 1.5 to 4.0 % carbon, about 1 to 3 % silicon and maybe a few other elements like manganese, sulphur or phosphorous. As the name implies, gray cast iron exhibits a gray fracture surface. It is characterized by its graphitic microstructure and the gray colour is caused by graphite fractures in the material. It is this graphite (crystalline form of carbon) flake structure which is created during the cooling process that makes gray iron 'gray'. When people say ‘cast iron’, they usually mean ‘gray cast iron’. While the graphite has little strength, it imparts other qualities to the cast iron that make up for it – economic castings in complex shapes, good machinability, dimensional stability, high vibration damping, etc. Gray cast iron can be alloyed with other elements to increase its strength and hardness, and is commonly classified by its minimum tensile strength or by hardness. Gray cast iron is used to manufacture machine tool bodies, pipe fittings, blocks, and agricultural implements.
Spheroidal Graphite (S.G.) Cast Iron / Ductile Iron
Ductile iron, also called as spheroidal graphite cast iron (S.G. iron) or nodular iron, is made by treating molten iron with appropriate amount of magnesium before casting. Ductile iron / S.G. iron is characterized by its graphite content that occurs in microscopic spherolites. Even though this graphite constitutes about 10% by volume in the cast iron, its compact sheroidal shape minimizes the effect on mechanical properties. What separates S.G. iron from gray cast iron is that the graphite in S.G. iron is in the form of spherical nodules rather than flakes. As the name suggests, ductile cast iron has a certain degree of ductility and is used extensively in the manufacturing of pipes, machines, household utensils and a few automotive parts.
Malleable Cast Iron
Malleable cast iron is one in which the carbon and silicon content is arranged so that it is graphite-free after solidification. The structure of malleable cast iron consists of metastable carbide in a pearlitic matrix. It is cast through slow annealing, and this results in a cast iron that is structurally tough but not brittle. While white malleable cast iron is the most popular form, black malleable cast iron is available too. The white malleable cast iron is decarbonized during the heat treatment, while the black is not. Malleable cast iron is predominantly used to manufacture all sorts of fittings.
Ausferrite spheroidal graphite cast iron
Also called as austempered cast iron, ausferrite S.G. iron is obtained by a process called austempering. Austempering is a special heat treatment process that hardens steel by quenching it. In essence, the process consists of heating the melt and then cooling it quickly. After the austempering process, the cast matrix consists of acicular ferrite and residual austenite without carbides. Austempered alloys results in developing a series of materials with a unique combination of mechanical and physical properties like high strength-to-weight ratio, toughness, fatigue strength, and wear resistance. Austempered ductile cast iron is used where high strength is needed coupled with excellent wear resistance and fatigue strength. Gears are usually made from austempered iron, as are other machined parts that need to resist wear and tear.
Other cast irons
These are but a few types of cast iron alloys. Adding different elements, in various proportions and using different processes results in many different types of alloys. For example, white cast iron is so called because the carbon in this cast iron takes on a silvery-white look due to the presence of crystalline cementite. It is a metastable compound consisting of iron and carbons, wherein the carbon atoms are located in interstitial sites. Compacted graphite cast iron, another type of cast iron, (also known as CGI) is a form of cast iron in which the shape of the graphite particles is between that of conventional grey iron flakes and ductile iron where the graphite is in spheres. CGI displays very useful properties for a range of engineering applications with the tensile strength and elastic modulus being considerably higher than grey cast iron.
We will cover the major types of cast alloys in detail in subsequent articles.
Types of Cast Irons
The history of iron castings is quite old – at least 2500 years old. Iron castings were made in China in about 500 B.C. and introduced to Europe a thousand year later. One of the first iron castings that were made were cast directly from the blast furnace.
Cast iron is essentially an iron-carbon alloy that may contain some other elements like silicon, manganese, sulphur or phosphorous. The addition of these elements even in miniscule proportion changes the properties of cast iron alloys radically. Steels and cast irons are both primarily iron with carbon. The difference between them is that steels contain less than 2% carbon, while cast irons contain more than 2% carbon. However, this demarcation is not defined properly. So, the best way is to just remember that cast irons contain less carbon than steels. Cast irons can be classified into two groups – those that involve ferrous materials (cast irons and steels) and cast nonferrous materials like cast magnesium, aluminium, copper, zinc and other alloys. Unalloyed cast irons cannot be designated by chemical composition and are designated by their mechanical properties.
The carbon in cast iron is present in two forms – as free carbon / graphite, or as cementite. Nucleating agents called as inoculants are also necessary for the successful production of each type of cast iron. Various types of cast iron can be classified according to their microstructure. It is based on the form and shape in which the major portion of carbon occurs in the cast irons. While there are many types of cast irons, for the purpose of this article we will focus on those that are most popular today to manufacture machined components.
Gray Cast Iron
Gray cast iron is a ferrous alloy which has been heated until it liquefies and is then poured into a mould to solidify. Gray cast iron contains about 1.5 to 4.0 % carbon, about 1 to 3 % silicon and maybe a few other elements like manganese, sulphur or phosphorous. As the name implies, gray cast iron exhibits a gray fracture surface. It is characterized by its graphitic microstructure and the gray colour is caused by graphite fractures in the material. It is this graphite (crystalline form of carbon) flake structure which is created during the cooling process that makes gray iron 'gray'. When people say ‘cast iron’, they usually mean ‘gray cast iron’. While the graphite has little strength, it imparts other qualities to the cast iron that make up for it – economic castings in complex shapes, good machinability, dimensional stability, high vibration damping, etc. Gray cast iron can be alloyed with other elements to increase its strength and hardness, and is commonly classified by its minimum tensile strength or by hardness. Gray cast iron is used to manufacture machine tool bodies, pipe fittings, blocks, and agricultural implements.
Spheroidal Graphite (S.G.) Cast Iron / Ductile Iron
Ductile iron, also called as spheroidal graphite cast iron (S.G. iron) or nodular iron, is made by treating molten iron with appropriate amount of magnesium before casting. Ductile iron / S.G. iron is characterized by its graphite content that occurs in microscopic spherolites. Even though this graphite constitutes about 10% by volume in the cast iron, its compact sheroidal shape minimizes the effect on mechanical properties. What separates S.G. iron from gray cast iron is that the graphite in S.G. iron is in the form of spherical nodules rather than flakes. As the name suggests, ductile cast iron has a certain degree of ductility and is used extensively in the manufacturing of pipes, machines, household utensils and a few automotive parts.
Malleable Cast Iron
Malleable cast iron is one in which the carbon and silicon content is arranged so that it is graphite-free after solidification. The structure of malleable cast iron consists of metastable carbide in a pearlitic matrix. It is cast through slow annealing, and this results in a cast iron that is structurally tough but not brittle. While white malleable cast iron is the most popular form, black malleable cast iron is available too. The white malleable cast iron is decarbonized during the heat treatment, while the black is not. Malleable cast iron is predominantly used to manufacture all sorts of fittings.
Ausferrite spheroidal graphite cast iron
Also called as austempered cast iron, ausferrite S.G. iron is obtained by a process called austempering. Austempering is a special heat treatment process that hardens steel by quenching it. In essence, the process consists of heating the melt and then cooling it quickly. After the austempering process, the cast matrix consists of acicular ferrite and residual austenite without carbides. Austempered alloys results in developing a series of materials with a unique combination of mechanical and physical properties like high strength-to-weight ratio, toughness, fatigue strength, and wear resistance. Austempered ductile cast iron is used where high strength is needed coupled with excellent wear resistance and fatigue strength. Gears are usually made from austempered iron, as are other machined parts that need to resist wear and tear.
Other cast irons
These are but a few types of cast iron alloys. Adding different elements, in various proportions and using different processes results in many different types of alloys. For example, white cast iron is so called because the carbon in this cast iron takes on a silvery-white look due to the presence of crystalline cementite. It is a metastable compound consisting of iron and carbons, wherein the carbon atoms are located in interstitial sites. Compacted graphite cast iron, another type of cast iron, (also known as CGI) is a form of cast iron in which the shape of the graphite particles is between that of conventional grey iron flakes and ductile iron where the graphite is in spheres. CGI displays very useful properties for a range of engineering applications with the tensile strength and elastic modulus being considerably higher than grey cast iron.
We will cover the major types of cast alloys in detail in subsequent articles.