Cementite

Last updated
Iron carbide
Iron carbide.jpg
Iron carbide plates
Kristallstruktur Zementit.png
Names
IUPAC name
Iron carbide
Other names
Cementite
Identifiers
3D model (JSmol)
ECHA InfoCard 100.031.411 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 234-566-7
  • InChI=1S/C.3Fe
    Key: TXAHJXBWFZQNQY-UHFFFAOYSA-N
  • [Fe]=[C]=[Fe].[Fe]
Properties
Fe3C
Molar mass 179.546 g/mol
Appearancedark gray or black crystals, odorless
Density 7.694 g/cm3, solid [1]
Melting point 1,227 °C (2,241 °F; 1,500 K) [1]
insoluble
Structure [2]
Orthorhombic, oP16
Pnma, No. 62
a = 0.509 nm, b = 0.6478 nm, c = 0.4523 nm
4
Thermochemistry [3]
105.9 J·mol−1·K−1
Std molar
entropy (S⦵298)
104.6 J·mol−1·K−1
25.1 kJ·mol−1
20.1 kJ·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. [4] It is a hard, brittle material, [4] normally classified as a ceramic in its pure form, and is a frequently found and important constituent in ferrous metallurgy. While cementite is present in most steels [5] and cast irons, it is produced as a raw material in the iron carbide process, which belongs to the family of alternative ironmaking technologies. The name cementite originated from the theory of Floris Osmond and J. Werth, in which the structure of solidified steel consists of a kind of cellular tissue, with ferrite as the nucleus and Fe3C the envelope of the cells. The carbide therefore cemented the iron.

Contents

Metallurgy

In the iron–carbon system (i.e. plain-carbon steels and cast irons) it is a common constituent because ferrite can contain at most 0.02wt% of uncombined carbon. [6] Therefore, in carbon steels and cast irons that are slowly cooled, a portion of the carbon is in the form of cementite. [7] Cementite forms directly from the melt in the case of white cast iron. In carbon steel, cementite precipitates from austenite as austenite transforms to ferrite on slow cooling, or from martensite during tempering. An intimate mixture with ferrite, the other product of austenite, forms a lamellar structure called pearlite.

The iron-carbon phase diagram Iron carbon phase diagram.svg
The iron-carbon phase diagram

While cementite is thermodynamically unstable, eventually being converted to austenite (low carbon level) and graphite (high carbon level) at higher temperatures, it does not decompose on heating at temperatures below the eutectoid temperature (723 °C) on the metastable iron-carbon phase diagram.

Mechanical properties are as follows: room temperature microhardness 760–1350 HV; bending strength 4.6–8 GPa, Young's modulus 160–180 GPa, indentation fracture toughness 1.5–2.7 MPa√m. [8]

The morphology of cementite plays a critical role in the kinetics of phase transformations in steel. The coiling temperature and cooling rate significantly affect cementite formation. At lower coiling temperatures, cementite forms fine pearlitic colonies, whereas at higher temperatures, it precipitates as coarse particles at grain boundaries. This morphological difference influences the rate of austenite formation and decomposition, with fine cementite promoting faster transformations due to its increased surface area and the proximity of the carbide-ferrite interface. Furthermore, the dissolution kinetics of cementite during annealing are slower for coarse carbides, impacting the microstructural evolution during heat treatments​ [9] .

Pure form

Cementite changes from ferromagnetic to paramagnetic upon heating to its Curie temperature of approximately 480 K (207 °C). [10]

A natural iron carbide (containing minor amounts of nickel and cobalt) occurs in iron meteorites and is called cohenite after the German mineralogist Emil Cohen, who first described it. [11]

Other iron carbides

There are other forms of metastable iron carbides that have been identified in tempered steel and in the industrial Fischer–Tropsch process. These include epsilon (ε) carbide, hexagonal close-packed Fe2–3C, precipitates in plain-carbon steels of carbon content > 0.2%, tempered at 100–200 °C. Non-stoichiometric ε-carbide dissolves above ~200 °C, where Hägg carbides and cementite begin to form. Hägg carbide, monoclinic Fe5C2, precipitates in hardened tool steels tempered at 200–300 °C. [12] [13] It has also been found naturally as the mineral Edscottite in the Wedderburn meteorite. [14]

Related Research Articles

<span class="mw-page-title-main">Steel</span> Alloy of iron and carbon

Steel is an alloy of iron and carbon with improved strength and fracture resistance compared to other forms of iron. Because of its high tensile strength and low cost, steel is one of the most commonly manufactured materials in the world. Steel is used in buildings, as concrete reinforcing rods, in bridges, infrastructure, tools, ships, trains, cars, bicycles, machines, electrical appliances, furniture, and weapons.

<span class="mw-page-title-main">Cast iron</span> Iron-carbon alloys with a carbon content more than 2% and silicon content between 1 and 3%

Cast iron is a class of iron–carbon alloys with a carbon content of more than 2% and silicon content around 1–3%. Its usefulness derives from its relatively low melting temperature. The alloying elements determine the form in which its carbon appears: white cast iron has its carbon combined into an iron carbide named cementite, which is very hard, but brittle, as it allows cracks to pass straight through; grey cast iron has graphite flakes which deflect a passing crack and initiate countless new cracks as the material breaks, and ductile cast iron has spherical graphite "nodules" which stop the crack from further progressing.

<span class="mw-page-title-main">Heat treating</span> Process of heating something to alter it

Heat treating is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

<span class="mw-page-title-main">Martensite</span> Type of steel crystalline structure

Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation.

<span class="mw-page-title-main">Austenite</span> Metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element

Austenite, also known as gamma-phase iron (γ-Fe), 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 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902). It exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.

<span class="mw-page-title-main">Bainite</span> Plate-like microstructure in steels

Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C. First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite is cooled past a temperature where it is no longer thermodynamically stable with respect to ferrite, cementite, or ferrite and cementite. Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite.

<span class="mw-page-title-main">Bulat steel</span> Steel alloy known in Russia from medieval times

Bulat is a type of steel alloy known in Russia from medieval times; it was regularly mentioned in Russian legends as the material of choice for cold steel. The name булат is a Russian transliteration of the Persian word fulad, meaning steel. This type of steel was used by the armies of nomadic peoples. Bulat steel was the main type of steel used for swords in the armies of Genghis Khan. Bulat steel is generally agreed to be a Russian name for wootz steel, the production method of which has been lost for centuries, and the bulat steel used today makes use of a more recently developed technique.

<span class="mw-page-title-main">High-strength low-alloy steel</span> Type of alloy steel

High-strength low-alloy steel (HSLA) is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. HSLA steels vary from other steels in that they are not made to meet a specific chemical composition but rather specific mechanical properties. They have a carbon content between 0.05 and 0.25% to retain formability and weldability. Other alloying elements include up to 2.0% manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare-earth elements, or zirconium. Copper, titanium, vanadium, and niobium are added for strengthening purposes. These elements are intended to alter the microstructure of carbon steels, which is usually a ferrite-pearlite aggregate, to produce a very fine dispersion of alloy carbides in an almost pure ferrite matrix. This eliminates the toughness-reducing effect of a pearlitic volume fraction yet maintains and increases the material's strength by refining the grain size, which in the case of ferrite increases yield strength by 50% for every halving of the mean grain diameter. Precipitation strengthening plays a minor role, too. Their yield strengths can be anywhere between 250–590 megapascals (36,000–86,000 psi). Because of their higher strength and toughness HSLA steels usually require 25 to 30% more power to form, as compared to carbon steels.

<span class="mw-page-title-main">Pearlite</span> Lamellar structure of ferrite and cementite

Pearlite is a two-phased, lamellar structure composed of alternating layers of ferrite and cementite that occurs in some steels and cast irons. During slow cooling of an iron-carbon alloy, pearlite forms by a eutectoid reaction as austenite cools below 723 °C (1,333 °F). Pearlite is a microstructure occurring in many common grades of steels.

<span class="mw-page-title-main">Carbon steel</span> Steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

<span class="mw-page-title-main">Widmanstätten pattern</span> Crystal patterns found in some meteorites

Widmanstätten patterns, also known as Thomson structures, are figures of long phases of nickel–iron, found in the octahedrite shapes of iron meteorite crystals and some pallasites.

<span class="mw-page-title-main">Tempering (metallurgy)</span> Process of heat treating used to increase the toughness of iron-based alloys

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures.

Cryogenic hardening is a cryogenic treatment process where the material is cooled to approximately −185 °C (−301 °F), usually using liquid nitrogen. It can have a profound effect on the mechanical properties of certain steels, provided their composition and prior heat treatment are such that they retain some austenite at room temperature. It is designed to increase the amount of martensite in the steel's crystal structure, increasing its strength and hardness, sometimes at the cost of toughness. Presently this treatment is being used on tool steels, high-carbon, high-chromium steels and in some cases to cemented carbide to obtain excellent wear resistance. Recent research shows that there is precipitation of fine carbides in the matrix during this treatment which imparts very high wear resistance to the steels.

<span class="mw-page-title-main">Malleable iron</span> Flexible form of cast iron

Malleable iron is cast as white iron, the structure being a metastable carbide in a pearlitic matrix. Through an annealing heat treatment, the brittle structure as first cast is transformed into the malleable form. Carbon agglomerates into small roughly spherical aggregates of graphite, leaving a matrix of ferrite or pearlite according to the exact heat treatment used.

In metallurgy and materials science, annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for an appropriate amount of time and then cooling.

<span class="mw-page-title-main">Alloy steel</span> Steel alloyed with a variety of elements

Alloy steel is steel that is alloyed with a variety of elements in amounts between 1.0% and 50% by weight, typically to improve its mechanical properties.

<span class="mw-page-title-main">Ledeburite</span> Eutectic mixture of austenite and cementite

In iron and steel metallurgy, ledeburite is a mixture of 4.3% carbon in iron and is a eutectic mixture of austenite and cementite. Ledeburite is not a type of steel as the carbon level is too high although it may occur as a separate constituent in some high carbon steels. It is mostly found with cementite or pearlite in a range of cast irons.

<span class="mw-page-title-main">Allotropes of iron</span> Different forms of the element iron

At atmospheric pressure, three allotropic forms of iron exist, depending on temperature: alpha iron, gamma iron, and delta iron (δ-Fe). At very high pressure, a fourth form exists, epsilon iron. Some controversial experimental evidence suggests the existence of a fifth high-pressure form that is stable at very high pressures and temperatures.

<span class="mw-page-title-main">Austempering</span>

Austempering is heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.

κ-Carbides are a special class of carbide structures. They are most known for appearing in steels containing manganese and aluminium where they have the molecular formula (Fe,Mn)
3AlC.

References

  1. 1 2 Haynes, p. 4.67
  2. ↑ Herbstein, F. H.; Smuts, J. (1964). "Comparison of X-ray and neutron-diffraction refinements of the structure of cementite Fe3C". Acta Crystallographica. 17 (10): 1331–1332. Bibcode:1964AcCry..17.1331H. doi: 10.1107/S0365110X64003346 .
  3. ↑ Haynes, p. 5.23
  4. 1 2 Smith & Hashemi 2006 , p. 363
  5. ↑ Verhoeven, John D. (2007). Steel Metallurgy for the Non-Metallurgist. ASM International. p. 35. ISBN   978-1-61503-056-9.
  6. ↑ Ashrafzadeh, Milad; Soleymani, Amir Peyman; Panjepour, Masoud; Shamanian, Morteza (2015). "Cementite Formation from Hematite–Graphite Mixture by Simultaneous Thermal–Mechanical Activation". Metallurgical and Materials Transactions B. 46 (2): 813–823. Bibcode:2015MMTB...46..813A. doi:10.1007/s11663-014-0228-3. S2CID   98253213.
  7. ↑ Smith & Hashemi 2006 , pp. 366–372
  8. ↑ Bhadeshia, H. K. D. H. (2020). "Cementite". International Materials Reviews. 65 (1): 1–27. Bibcode:2020IMRv...65....1B. doi: 10.1080/09506608.2018.1560984 .
  9. ↑ Alvarenga HD, Van Steenberge N, Sietsma J, Terryn H (Feb 2017). "The Kinetics of Formation and Decomposition of Austenite in Relation to Carbide Morphology". Metall Mater Trans A. 48: 828–840. doi:10.1007/s11661-016-3874-z.
  10. ↑ Smith, S.W.J.; White, W.; Barker, S.G. (1911). "The Magnetic Transition Temperature of Cementite". Proc. Phys. Soc. Lond. 24 (1): 62–69. Bibcode:1911PPSL...24...62S. doi:10.1088/1478-7814/24/1/310.
  11. ↑ Buchwald, Vagn F. (1975) Handbook of Iron Meteorites, University of California Press
  12. ↑ Hägg, Gunnar (1934). "Pulverphotogramme eines neuen Eisencarbides". Zeitschrift für Kristallographie - Crystalline Materials. 89 (1–6): 92–94. doi:10.1524/zkri.1934.89.1.92. S2CID   100657250.
  13. ↑ Smith, William F. (1981). Structure and properties of engineering alloys. New York: McGraw-Hill. pp. 61–62. ISBN   978-0-07-0585607.
  14. ↑ Mannix, Liam (2019-08-31). "This meteorite came from the core of another planet. Inside it, a new mineral". The Age. Retrieved 2019-09-14.

Bibliography

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