Chromium carbide (
CrChromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable...
3CCarbon 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...
2) is an extremely hard
refractoryIn metallurgy, refraction is a property of metals that indicates their ability to withstand heat. Metals with a high degree of refraction are referred to as refractory. These metals derive their high melting points from their strong intermolecular forces...
ceramicA ceramic is an inorganic, non-metallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
material. It is usually processed by
sinteringSintering is a method for making objects from powder, by heating the material in a sintering furnace below its melting point until its particles adhere to each other...
. It has the appearance of a gray powder with orthorhombic
crystal structureIn mineralogy and crystallography, a crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
. The orthorhombic Cr
3C
2 occurs extremely rarely as mineral tongbaite.
Other compositions are available, eg. Cr
23C
6 (occurring as extremely rare mineral isovite, cubic) and Cr
7C
3.
Chromium carbide is used as a thermal spray material for protecting the underlying metal surface, and as an additive to corrosion-resistant and wear-resistant materials. It is used in coatings of
bearingsA bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can...
, seals, orifices, and valve seals, and as a fine-crystal phase in other sintered carbides, where, like the
vanadium carbideVanadium carbide, VC, is an extremely hard refractory ceramic material. It is commercially used in tool bits cutting tools. It has the appearance of gray metallic powder with cubic crystal structure....
, it inhibits growth of submicrometre grains during pressing and sintering. It is highly
corrosionCorrosion is the disintegration of a material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen. Weakening of iron due to oxidation of the iron atoms is a...
-resistant, and does not oxidize even at high temperature (1000-1100 °C). The powder for coating costs $40-$65 per pound.
The
thermal expansionThermal expansion is the tendency of a matter to change in volume in response to a change in temperature. When a substance is heated, its particles begin moving and become active thus maintaining a greater average separation. Materials which contract with increasing temperature are rare; this...
coefficient of chromium carbide is almost equal to that of
steelSteel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten...
, reducing the mechanical stress buildup at the layer boundary.
Precipitation of chromium carbide at the grain boundaries, depleting the grain edges of chromium, is the cause of
intergranular corrosionthumb|right|Microscope view of a polished cross section of a material attacked by intergranular corrosionIntergranular corrosion , also termed intergranular attack , is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides...
of
stainless steelIn metallurgy, stainless steel, also known as inox steel or inox, is defined as a steel alloy with a minimum of 11% chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as ordinary steel...
near the
weldWeld most commonly refers to a joint formed by welding.Weld may also refer to:-People:* Weld family, an extended family of New England** Theodore Dwight Weld** Tuesday Weld* Weld-Blundell family* Cecil Weld-Forester, 1st Baron Forester...
s.
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