Cementite

Crystal structure
Fe: __ C: __ . The double capped prismatic coordination polyhedra of Fe around C are shown.
Lattice parameters	a = 0.4514 nm, b = 0.5080 nm, c = 0.6734 nm
General
Surname	Cementite
other names	Iron carbide Triiron carbide
Ratio formula	Fe 3 C
Brief description	gray orthorhombic crystals
External identifiers / databases
CAS number 12011-67-5 EC number 234-566-7 ECHA InfoCard 100.031.411 PubChem 165960 Wikidata Q189335
properties
Molar mass	179.55 g mol −1
Physical state	firmly
density	7.69 g cm −3
Melting point	1837 ° C
safety instructions
GHS hazard labeling no classification available
Thermodynamic properties
ΔH f 0	25.1 kJ / mol
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Cementite is a compound of iron and carbon with the composition Fe ₃ C (an iron carbide with intercalation compounds ) and occurs as a metastable phase in steel and white cast iron . It takes its name from "cement" (cement steel, formerly "cämented steel" = carburized steel) and in its pure form is sometimes also regarded as a non-oxide ceramic . Cementite is an intermediate phase, a so-called Häggs phase ( coordination phase ). These phases are geometrically determined, the radius ratio determines the structure.

Occurrence

In mineralogy cementite under the name cohenite as meteoritic nickel - iron - mineral in conjunction with cobalt ([Fe, Ni, Co] ₃ known C).

Extraction and presentation

As Primärzementit (Fe ₃ C ^I ) cementite is referred to, by a crystallization is resulted from the melt (line CD in the iron-carbon phase diagram ). Secondary cementite (Fe ₃ C ^II ) is produced by precipitation from the austenite (line ES), tertiary cementite (Fe ₃ C ^III ) by precipitation from the ferrite (line PQ). The eutectic or eutectoid phase mixtures ledeburite in cast iron and pearlite in steel are particularly important . Primary cementite initially forms coarse needles from the melt. Due to the decreasing solubility of iron for carbon, secondary cementite is deposited as grain boundary or shell cementite around the austenite or later pearlite crystals. Metallographically, the cementite appears in the pearlite as strip cementite. Tertiary cementite is deposited on the closest existing cementite, from pure ferrite it precipitates at the grain boundaries. In addition, soft annealing can cause the cementite to agglomerate globularly.

Cementite is also produced by the thermal decomposition of Berlin blue (iron hexacyanidoferrate).

properties

With long annealing times or extremely slow cooling, the metastable cementite breaks down into iron and graphite . The crystal structure of cementite is relatively complex. In an orthorhombic unit cell there are twelve iron and four carbon atoms , with the carbon atoms being surrounded by eight iron atoms in a relatively irregular manner ( trigonal- prismatic double capped ). Cementite is very hard ( HV = 800) and wear-resistant, but brittle and therefore difficult to deform plastically. It has a lower density than iron and is below its Curie temperature of 215 ° C ferromagnetic .

Molar volume as a function of pressure for cementite at room temperature.

Since carbon is one of the possible light components in the iron alloys that make up the cores of planets, the properties of iron-carbon compounds are experimentally investigated even under extremely high pressures and / or temperatures. The graphic opposite shows the molar volume as a function of the pressure at room temperature for cementite as a simplified model substance for cohenite.

Primary cementite needles

The machinability is very bad. In practice, cementite cannot be machined (milling, drilling, etc.). It can occur in free form or occur as a component of pearlite or bainite and influence the machinability of this structure. Because of its great hardness, it causes high abrasive tool wear . See also: Machinability of steel .

use

Iron carbide can be used as a catalyst in chemical reactions (for example the splitting of water into hydrogen and oxygen).

See also

• Iron-carbon diagram

Individual evidence

1. ↑ ^{a b} entry on iron carbide. In: Römpp Online . Georg Thieme Verlag, accessed on August 21, 2013.
2. ↑ Triiron carbide (WebElements)
3. ↑ This substance has either not yet been classified with regard to its hazardousness or a reliable and citable source has not yet been found.
4. ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Standard Thermodynamic Properties of Chemical Substances, pp. 5-19.
5. ↑ ^{a b} Jürgen Gobrecht, Erhard Rumpler: Material Technology - Metals. Oldenbourg, Munich 2006, ISBN 3-486-57903-7 , pp. 139ff.
6. ^ Hans-Jürgen Bargel, Günter Schulze: Material science . 12th edition. Springer-Verlag GmbH Germany, Berlin 2018, ISBN 978-3-662-48629-0 , 1.5.4 Intermediate crystals, p. 43 .
7. ↑ Physical basics of materials science - Springer . doi : 10.1007 / 978-3-540-71105-6 .
8. ↑ Forms of the cementite in steel (metallograph)
9. ↑ Herbert Schönherr: Machining Technology , Oldenbourg, 2002, p 60th
10. ↑ Fritz Klocke, Wilfried König: Production Process Volume 1: Turning, Milling, Drilling , Springer, 8th Edition, 2008, p. 274.
11. ↑ Magnetic sheets - the skeleton of a sheet enables filigree iron carbide structures for efficient catalysts (Wissenschaft-aktuell)

Web links

• Crystal Structure of Cementite, Cambridge University (English)