Cementite

• Iron carbide
• Trieisencarbid

Gray orthorhombic crystals

Fixed

7.69 g · cm -3

1837 ° C

25.1 kJ / mol

Template: Infobox chemical / molecular formula search available

Cementite is a compound of iron and carbon in the composition Fe3C ( iron carbide, a ) and occurs as a metastable phase in steels and white cast iron. It takes its name from " cement " (cement steel, formerly " cämentierter steel " = carburized steel).

Occurrence

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

Production and representation

As Primärzementit ( Fe3CI ) cementite is called, which emerged by crystallization from the melt ( line CD in the iron -carbon diagram). Secondary cementite ( Fe3CII ) is formed by elimination from the austenite ( line ES), Tertiärzementit ( Fe3CIII ) by elimination from the ferrite ( line PQ ). Particularly important are the eutectic or eutectoid phase mixtures ledeburite on cast iron and pearlite steel. Primärzementit forms from the melt initially coarse needles. Secondary cementite precipitates, due to decreasing solubility of iron for carbon, as grain boundaries or Schalenzementit to the austenite or later Perlitkristalle from. Metallographically of cementite in pearlite appears as Streifenzementit. Tertiärzementit attaches to the nearest existing cementite of pure ferrite he separates out at the grain boundaries. In addition, can be achieved by annealing that the cementite is highly concentrated globular.

Properties

For long annealing times or extremely slow cooling decays of metastable cementite in iron and graphite. The crystal structure of cementite is relatively complicated. In an orthorhombic unit cell contains twelve iron and four carbon atoms, wherein the carbon atoms relatively irregular ( doubly capped trigonal- prismatic) are surrounded by eight iron atoms. Cementite is very hard ( HV = 800 ) and wear resistant, but brittle and therefore difficult to plastically deformable. It has a lower density than iron, and is ferromagnetic below its Curie temperature of 215 ° C.

Use

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