Chromium carbide is a ceramic compound that exists in several different chemical compositions: Cr3 C2, Cr7 C3 and Cr23 C6. Under standard conditions, it exists as a gray matter. Chrome is a very hard and corrosion resistant metal. It also has a refractory composition, which means maintaining strength and at high temperatures.
These properties of chromium make it useful as an additive to metal alloys. When carbide crystals are integrated into the surface of the substance, this improves wear resistance and corrosion resistance, and also retains these properties at elevated temperatures. The most complex and most commonly used formulation for this purpose is Cr3 C2.
Related minerals include tongbaite and isovite (Cr, Fe) 23 C6, both extremely rare. Another rich carbide mineral is jarlongite Cr4 Fe4 NiC4.
Chrome properties
There are three different crystalline structures for carbide, corresponding to three different chemical compositions:
- Cr23 C6 has a cubic structure and a Vickers hardness of 976 kg / mm 2 .
- Cr7 C3 has a hexagonal crystal structure and a microhardness of 1336 kg / mm 2 .
- Cr3 C2 is the most durable of the three compositions and has a rhombic structure with a microhardness of 2280 kg / mm 2 .
For this reason, Cr3 C2 is the basic formula for chromium carbide used in surface treatment.
Synthesis
A carbide compound can be achieved by mechanical alloying. In this type of process, metallic chromium and carbon in the form of graphite are loaded into a ball mill and crushed into fine powder. After crushing the components, they are combined into granules and are subjected to hot isostatic pressing. This action uses an inert gas, primarily argon in a sealed oven.
This substance under pressure exerts pressure on the sample from all sides while the furnace heats up. Heat and onslaught cause graphite and metal to react with each other and form chromium carbide. A decrease in the percentage of carbon in the initial mixture leads to an increase in the yield of the forms Cr7 C3 and Cr23 C6.
Another method for the synthesis of chromium carbide uses oxide, pure aluminum and graphite in a self-propagating exothermic reaction, which proceeds as follows:
3Cr 2 O3 + 6Al + 4C → 2Cr 3 C 2 + 3Al 2 O 3
In this method, the reagents are ground and mixed in a ball mill. The homogeneous powder is then pressed into a tablet and placed in an inert atmosphere of argon. The sample is then heated. A hot wire, spark, laser, or stove can provide heat. An exothermic reaction is initiated, and the resulting vapor spreads the effect throughout the rest of the sample.
Chromium Carbide Production
Many companies create a substance by combining aluminothermic reduction and vacuum treatment at a temperature of 1500 ° C and above. A mixture of metallic chromium, oxide and carbon is prepared and then loaded into a vacuum oven. The pressure in the oven decreases, and the temperature rises to 1500 ° C. After this, carbon reacts with the oxide to form metal and gaseous monoxide, which is discharged into vacuum pumps. Then, chromium combines with the remaining carbon to form carbide.
The exact balance between these components determines the content of the resulting substance. This is carefully monitored to ensure that product quality is suitable for demanding markets such as aerospace.
Chrome metal production
- Researchers are discovering a new class of carbides that derive stability from a disordered structure.
- The results of the study lay the foundation for future investigations of new carbides useful in practical applications.
- The creation of two-dimensional nitrides has become easier.
The metal, which is used in many companies, is produced by aluminothermic reduction, where a mixture of chromium oxide and aluminum powder is formed. Then they are loaded into the firing tank, where the mixture breaks out. Aluminum reduces chromium oxide to metal and alumina slag at a temperature of 2000–2500 ° C. This substance forms a molten pool at the bottom of the firing chamber, where it can be collected when the temperature has dropped enough. Otherwise, contact will be difficult and very dangerous. Then the initial substance is converted into powder and used as a raw material for the production of chromium carbide.
Further grinding
Crushing of chromium carbide and its initial substance is carried out in mills. There is always a risk of explosion when grinding finely divided metal powders. That is why the mills are specially designed to deal with such potential dangers. Cryogenic cooling (most often liquid nitrogen) is also applied to the structure to facilitate grinding.
Wear resistant coatings
Carbides are hard, and therefore the general use of chromium is to apply strong, wear-resistant coatings to parts that need to be protected. In combination with a protective metal matrix, both anti-corrosion and wear-resistant substances can be developed that are easy to apply and cost-effective. These coatings are carried out by welding or thermal spraying. In combination with other persistent substances, chromium carbide can be used to form cutting tools.
Welding electrodes
These chromium carbide rods are increasingly being used in place of previous ferrochromes or carbon-containing components. They give superior and more stable results. In these welding electrodes, chromium carbide II is created during the bonding process to provide a wear resistant layer. However, carbide formation is determined by the exact conditions in the finished joint. And therefore, between them there may be changes that are not visible to electrodes containing chromium carbide. This affects the wear resistance of the deposited weld.
When testing a wheel made of dry sand rubber, it was found that the wear rate of the compound deposited on ferrochrome or carbon electrodes is 250% higher. When compared to chromium carbide.
The trend in the welding industry, which is moving from the use of rod electrodes to cored wire, is beneficial to the substance. Chromium carbide is used almost exclusively in the crushed element instead of high-carbon ferrochrome, since it does not suffer from the dilution effect caused by an excess of iron in it.
This means that a coating can be obtained containing a greater amount of solid particles, which has high wear resistance. Therefore, since there is a transition from rod electrodes to cored wire due to the advantages of automation and higher productivity associated with the welding technology of the latter, the carbide market is growing.
Typical areas of use for it are: surfacing of conveyor screws, blades of a fuel mixer, impellers of a pump, and general use of chromium, which requires resistance to erosion.
Thermal spray
In thermal spraying, chromium carbide combines with a metal matrix such as nickel-chromium. Typically, the ratio of these substances is 3: 1, respectively. A metal matrix is present to bind the carbide to the coated substrate and to provide a high degree of corrosion resistance.
The combination of this property and wear resistance means that thermally sprayed CrC-NiCr coatings are suitable as a barrier to high temperature wear. It is for this reason that they are increasingly used in the aerospace market. Typical applications here are coatings for core mandrels, dies for hot manufacturing, hydraulic valves, machine parts, wear protection of aluminum components and general applications with good resistance to corrosion and abrasion at temperatures up to 700–800 ° C.
Chrome plating alternative
New application for thermally sprayed coatings as a substitute for solid saturation of products. Hard chrome plating allows you to get a wear-resistant shell with good surface quality at low cost. A chromium-plated coating is obtained by immersion of an object to be saturated in a container with a chemical solution containing chromium. An electric current is then passed through the reservoir, causing the substance to deposit on the part and form a coherent coating. However, growing environmental problems are associated with the removal of wastewater from the used plating solution, and these issues have caused an increase in the cost of the process.
Chromium carbide coatings have a wear resistance that is two and a half to five times better than hard chromium plating and have no problems with wastewater disposal. Therefore, they are finding increasing application due to hard chrome plating, especially if wear resistance is important or a thick coating is required for a large part. This is an interesting and rapidly developing area, which will become more important as the costs of compliance with environmental laws increase.
Cutting tools
The predominant material here is tungsten carbide powder, which is sintered with cobalt to produce extremely hard objects. To improve the toughness of these cutting tools, titanium, niobium, and chromium carbide are added to the material. The role of the latter is to prevent grain growth during sintering. Otherwise, the process will form excessively large crystals, which can degrade the toughness of the cutting tool.