Cementite
Encyclopedia
Cementite, also known as iron carbide, is a chemical compound
of iron
and carbon
, with the formula Fe3C (or Fe2C:Fe). By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic
in its pure form, though it is more important in metallurgy
.
In the iron–carbon system (i.e. plain-carbon steel
s and cast iron
s) it is a common constituent because ferrite
contains at most 0.02wt% of carbon. Therefore, in carbon steels and cast irons that are slowly cooled a portion of the elements are in the form of cementite. It forms directly from the melt in the case of white cast iron. In carbon steel
, it either forms from austenite
during cooling or from martensite
during tempering
. An intimate mixture with ferrite
, the other product of austenite, forms a lamellar structure
called pearlite
.
A natural iron carbide (containing minor amounts of nickel and cobalt) occurs in iron meteorite
s and is called cohenite
after the German mineralogist Emil Cohen
, who first described it.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, with the formula Fe3C (or Fe2C:Fe). By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
in its pure form, though it is more important in metallurgy
Metallurgy
Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...
.
Metallurgy
Plain-carbon steel
Carbon steel, also called plain-carbon steel, is steel where the main interstitial alloying constituent is carbon. The American Iron and Steel Institute defines carbon steel as: "Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt,...
s and cast iron
Cast iron
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
s) it is a common constituent because ferrite
Ferrite (iron)
Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure. It is the component which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material...
contains at most 0.02wt% of carbon. Therefore, in carbon steels and cast irons that are slowly cooled a portion of the elements are in the form of cementite. It forms directly from the melt in the case of white cast iron. In carbon steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
, it either forms from austenite
Austenite
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...
during cooling or from martensite
Martensite
Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...
during tempering
Tempering
Tempering is a heat treatment technique for metals, alloys and glass. In steels, tempering is done to "toughen" the metal by transforming brittle martensite or bainite into a combination of ferrite and cementite or sometimes Tempered martensite...
. An intimate mixture with ferrite
Ferrite (iron)
Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure. It is the component which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material...
, the other product of austenite, forms a lamellar structure
Lamellar structure
Lamellar structures or microstructures are composed of fine, alternating layers of different materials in the form of lamellae. They are often observed in cases where a phase transformation front moves quickly, leaving behind two solid products, as in rapid cooling of eutectic or eutectoid ...
called pearlite
Pearlite
Pearlite is often said to be a two-phased, lamellar structure composed of alternating layers of alpha-ferrite and cementite that occurs in some steels and cast irons...
.
Pure form
Cementite changes from ferromagnetic to paramagnetic at its Curie temperature of approximately 480 K.A natural iron carbide (containing minor amounts of nickel and cobalt) occurs in iron meteorite
Iron meteorite
Iron meteorites are meteorites that consist overwhelmingly of nickel-iron alloys. The metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to humans.-Occurrence:...
s and is called cohenite
Cohenite
Cohenite is a naturally occurring iron mineral with the chemical structure 3C. This forms a hard, shiny, silver mineral which was named by E. Weinschenk in 1889 after the German mineralogist Emil Cohen, who first described and analysed it. Cohenite is found in rod-like crystals in iron meteorites...
after the German mineralogist Emil Cohen
Emil Cohen
Emil Wilhelm Cohen was a German mineralogist, born in Jutland. He studied at the universities of Berlin and Heidelberg and from 1867 to 1872 was assistant in mineralogy in the latter university. He then spent a year and a half in South Africa, where he studied diamond and gold deposits...
, who first described it.