Cemented carbide is mainly composed of high-hardness, refractory metal carbides (WC, TiC) micron powders, and cobalt (Co), nickel (Ni), and molybdenum (Mo) as binders. It can be used in a vacuum furnace or hydrogen Powder metallurgy products sintered in a reduction furnace.

Carbides, nitrides, borides, etc. of group IVB, ⅤB, and VIB metals are collectively referred to as cemented carbide due to their extremely high hardness and melting point. The following focuses on carbides to illustrate the structure, characteristics and applications of hard gold.

In the metal carbide formed by metals of group IVB, ⅤB, VIB and carbon, due to the small carbon atom radius, it can be filled in the voids of the metal lattice and retain the original lattice form of the metal to form interstitial solid solutions. Under proper conditions, this type of solid solution can continue to dissolve its constituent elements until it reaches saturation. Therefore, their composition can vary within a certain range (for example, the composition of titanium carbide varies between TiC0.5 and TiC), and the chemical formula does not comply with the valence rules. When the dissolved carbon content exceeds a certain limit (for example, Ti:C=1:1 in titanium carbide), the lattice pattern will change, causing the original metal lattice to transform into another form of metal lattice. The interfilling solid solution is called an interfilling compound.

Metal carbides, especially Group IVB, VB, VIB metal carbides have melting points above 3273K, among which hafnium carbide and tantalum carbide are 4160K and 4150K, respectively, which are the highest melting points among the currently known materials. The hardness of most carbides is very large, and their microhardness is greater than 1800kg·mm2 (microhardness is one of the hardness representation methods, mostly used in cemented carbide and hard compounds. The microhardness of 1800kg·mm2 is equivalent to one on the Mohs scale. Diamond-hardness 9). Many carbides are not easy to decompose at high temperatures, and their oxidation resistance is stronger than their constituent metals. Titanium carbide has the best thermal stability among all carbides and is a very important metal carbide. However, in an oxidizing atmosphere, all carbides are easily oxidized at high temperatures, which can be said to be a major weakness of carbides.

In addition to carbon atoms, nitrogen atoms and boron atoms can also enter the voids of the metal lattice to form interstitial solid solutions. They are similar in nature to interstitial carbides, and can conduct electricity, heat conduction, high melting point, high hardness, and high brittleness.

The matrix of cemented carbide consists of two parts: one part is the hardening phase; the other part is the bonding metal.

The hardened phase is the carbides of transition metals in the periodic table, such as tungsten carbide, titanium carbide, and tantalum carbide. Their hardness is very high, and their melting points are above 2000°C, and some even exceed 4000°C. In addition, transition metal nitrides, borides, and silicides have similar characteristics and can also act as hardening phases in cemented carbide. The existence of the hardening phase determines the alloy has extremely high hardness and wear resistance.

The requirements of cemented carbide for tungsten carbide WC grain size are WC (tungsten carbide) with different grain sizes according to different uses of cemented carbide. Cemented carbide cutting tools: For example, fine machining alloys such as foot cutter blades and V-CUT knives use ultra-fine, sub-fine, and fine-grained WC; rough-machining alloys use medium-grain WC; gravity cutting and heavy-duty cutting alloys use medium and coarse Granular WC is used as raw material; mining tools: rock hardness is high, impact load is large, coarse particle WC is used, rock impact is small, and medium particle WC is used as raw material; wear-resistant parts: when the wear resistance, compression resistance and surface finish are emphasized When using ultra-fine, sub-fine, fine, and medium-grain WC as raw materials, impact-resistant tools use medium- and coarse-grain WC as raw materials.

The theoretical carbon content of WC is 6.128% (atomic 50%). When the carbon content of WC is greater than the theoretical carbon content, free carbon (WC+C) appears in the WC. The presence of free carbon causes the surrounding WC grains to grow during sintering, resulting in uneven grains of cemented carbide. Tungsten carbide generally requires high combined carbon (≥6.07%), free carbon (≤0.05%), and total carbon is determined by the production process and scope of use of cemented carbide.

Under normal circumstances, the total carbon of WC used for vacuum sintering in paraffin wax process is mainly determined by the oxygen content in the briquette before sintering. Containing one part of oxygen, add 0.75 part of carbon, that is, WC total carbon=6.13%+oxygen content%×0.75 (assuming a neutral atmosphere in the sintering furnace, in fact, most vacuum furnaces are carburizing atmosphere, and the WC total carbon used is less than the calculation value).

Currently, the total carbon content of WC in my country can be roughly divided into three types: the total carbon of WC for vacuum sintering in paraffin process is about 6.18±0.03% (free carbon will increase). The total carbon content of WC for hydrogen sintering in paraffin process is 6.13±0.03%. Total carbon of WC for hydrogen sintering in rubber process=5.90±0.03%. The above processes are sometimes cross-processed, so the determination of WC total carbon should be based on specific conditions.

The total carbon of WC used in alloys with different application ranges, different Co (cobalt) content and different grain size can be adjusted slightly. Low-cobalt alloys can choose tungsten carbide with higher total carbon, and high-cobalt alloys can choose tungsten carbide with lower total carbon. In short, the specific requirements of cemented carbide have different requirements on the particle size of tungsten carbide.

Binder metals are generally iron group metals, and cobalt and nickel are commonly used.

In the manufacture of cemented carbide, the raw material powder size is between 1 and 2 microns, and the purity is very high. The raw materials are mixed according to the specified composition ratio, and alcohol or other medium is added to wet grinding in a wet ball mill to make them fully mixed and crushed. After drying and sieving, a molding agent such as wax or glue is added. The mixture is obtained by sieving. Then, when the mixture is granulated and pressed, and heated to close to the melting point of the binder metal (1300-1500°C), the hardened phase and the binder metal will form a eutectic alloy. After cooling, the hardened phase is distributed in the grid composed of the bonding metal, and is closely connected with each other to form a solid whole. The hardness of cemented carbide depends on the hardened phase content and grain size, that is, the higher the hardened phase content and the finer the grains, the greater the hardness. The toughness of cemented carbide is determined by the bond metal. The higher the content of the bond metal, the greater the bending strength.