Cementite

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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. [1] It is a hard, brittle material, [1] 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 and cast irons, [2] 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 research of Floris Osmond and J. Werth, where the structure of solidified steel consists of a kind of cellular tissue in theory, with ferrite as the nucleus and Fe3C the envelope of the cells. The carbide therefore cemented the iron. [3]

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Metallurgy

Orthorhombic Fe3C. Iron atoms are blue. Kristallstruktur Zementit.png
Orthorhombic Fe3C. Iron atoms are blue.
The iron-carbon phase diagram Iron carbon phase diagram.svg
The iron-carbon phase diagram

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. [4] Therefore, in carbon steels and cast irons that are slowly cooled, a portion of the carbon is in the form of cementite. [5] 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.

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.

Pure form

Cementite changes from ferromagnetic to paramagnetic at its Curie temperature of approximately 480 K. [6]

Molar volume vs. pressure for cementite at room temperature. Cementite-pV.svg
Molar volume vs. pressure for cementite at room temperature.

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. [7] As carbon is one of the possible minor light alloy components of metallic planetary cores, the high-pressure/high-temperature properties of cementite (Fe3C) as a simple proxy for cohenite are studied experimentally. The figure shows the compressional behaviour at room temperature.

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. [8] [9] It has also been found naturally as the mineral Edscottite in the Wedderburn meteorite [10] Characterization of different iron carbides is not at all a trivial task, and often X-ray diffraction is complemented by Mössbauer spectroscopy.

Related Research Articles

Iron Chemical element, symbol Fe and atomic number 26

Iron is a chemical element with symbol Fe and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in front of oxygen, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust.

Steel Metal alloy made by combining iron with other elements

Steel is an alloy made up of iron with typically a few tenths of a percent of carbon to improve its strength and fracture resistance compared to iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant need typically an additional 11% chromium. Because of its high tensile strength and low cost, steel is used in buildings, infrastructure, tools, ships, trains, cars, machines, electrical appliances, and weapons. Iron is the base metal of steel. Depending on the temperature, it can take two crystalline forms : body-centred cubic and face-centred cubic. The interaction of the allotropes of iron with the alloying elements, primarily carbon, gives steel and cast iron their range of unique properties.

Cast iron Iron-carbon alloys with a carbon content more than 2%.

Cast iron is a group of iron-carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its colour when fractured: white cast iron has carbide impurities which allow 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.

Heat treating 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.

Martensite

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.

Austenite 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.

Bainite

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 no longer is 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.

High-strength low-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.

Pearlite

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.

Carbon steel Steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content from about 0.05 up to 3.8 per cent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

Maraging steel

Maraging steels are steels that are known for possessing superior strength and toughness without losing ductility. Aging refers to the extended heat-treatment process. These steels are a special class of low-carbon ultra-high-strength steels that derive their strength not from carbon, but from precipitation of intermetallic compounds. The principal alloying element is 15 to 25 wt% nickel. Secondary alloying elements, which include cobalt, molybdenum and titanium, are added to produce intermetallic precipitates. Original development was carried out on 20 and 25 wt% Ni steels to which small additions of aluminium, titanium, and niobium were made; a rise in the price of cobalt in the late 1970s led to the development of cobalt-free maraging steels.

Tempering (metallurgy) Process of heat treating used to increase 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.

Gray iron

Gray iron, or grey cast iron, is a type of cast iron that has a graphitic microstructure. It is named after the gray color of the fracture it forms, which is due to the presence of graphite. It is the most common cast iron and the most widely used cast material based on weight.

Malleable 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. Three basic types of malleable iron are recognized within the casting industry: blackheart malleable iron, whiteheart malleable iron and pearlitic malleable iron.

Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low alloy steels and high alloy steels. The difference between the two is disputed. Smith and Hashemi define the difference at 4.0%, while Degarmo, et al., define it at 8.0%. Most commonly, the phrase "alloy steel" refers to low-alloy steels.

Allotropes of iron Form different types of steel

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

Austempering

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.

Boron steel refers to steel alloyed with a small amount of boron, usually less than 1 percent. The addition of boron to steel greatly increases the hardenability of the resulting alloy.

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)
3
AlC
.

References

  1. 1 2 Smith & Hashemi 2006 , p. 363
  2. Durand-Charre 2003
  3. H. K. D. H. Bhadeshia (2020). "Cementite". International Materials Reviews. 65 (1): 1–27. doi: 10.1080/09506608.2018.1560984 .
  4. 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. doi:10.1007/s11663-014-0228-3. S2CID   98253213.
  5. Smith & Hashemi 2006 , pp. 366–372
  6. S.W.J. Smith; W. White; S.G. Barker (1911). "The Magnetic Transition Temperature of Cementite". Proc. Phys. Soc. Lond. 24 (1): 62–69. doi:10.1088/1478-7814/24/1/310.
  7. Vagn F. Buchwald, Handbook of Iron Meteorites, University of California Press 1975
  8. Gunnar Hägg, Z. Krist., Vol. 89, p 92-94, 1934.
  9. Smith, William F. (1981). Structure and properties of engineering alloys. New York: McGraw-Hill. pp. 61–62. ISBN   978-0-07-0585607.
  10. 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