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Cemented Carbide Material Properties

Cemented carbide is made of high-hardness, refractory metal carbide (WC, TiC) micron powder as the main component, with cobalt (Co), nickel (Ni), and molybdenum (Mo) as the binder. It can be used in a vacuum furnace or hydrogen Powder metallurgy products sintered in a reduction furnace.
The carbides, nitrides, borides, etc. of Group IVB, VB, and VIB metals are collectively referred to as hard alloys due to their extremely high hardness and melting point. The following focuses on carbides to illustrate the structure, characteristics and applications of hard gold.
The metal carbides formed by metals of groups IVB, VB, VIB, and carbon, due to the small carbon atom radius, can be filled in the voids of the metal lattice and retain the original lattice form of the metal, forming 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 conform to the valence rule. 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 the metal lattice. The interfilling solid solution is called an interfiling compound.
Metal carbides, especially metal carbides of Group IVB, VB, and VIB have melting points above 3273K, among which hafnium carbide and tantalum carbide are 4160K and 4150K, respectively, which are the highest melting points among 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 Mohs one 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. They can conduct electricity, conduct heat, have a high melting point, have high hardness, and are brittle.
The matrix of cemented carbide is composed of two parts: one part is the hardening phase; the other part is the bonding metal.
The hardened phase is the carbide of transition metals in the periodic table, such as tungsten carbide, titanium carbide, and tantalum carbide. Their hardness is very high, and the melting point is 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. Tungsten carbide WC grain size requirements for cemented carbide use different grain size WC (tungsten carbide) 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 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 WC. The existence 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 for vacuum sintering in paraffin wax process is mainly determined by the oxygen content in the compact 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 total WC carbon used is less than the calculation value).
The total carbon content of WC in my country is 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.
Different use range, different Co (cobalt) content, different grain size alloy used WC total carbon 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.
The bonding metal is generally iron group metals, and cobalt and nickel are commonly used.
When manufacturing cemented carbide, the selected raw material powder has a particle size between 1 and 2 microns and has a high purity. 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, and then dried and processed. 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 firm 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.