Cemented carbide is mainly carbide (WC, TiC) micron powder with high hardness and refractory metal.
The components are sintered with cobalt (Co) or nickel (Ni) and molybdenum (Mo) as binder and sintered in vacuum furnace or hydrogen reduction furnace.
The carbides, nitrides and borides of group B, V, B, and B are all known as cemented carbide due to their high hardness and melting point. Next, the structure, characteristics and application of hard gold are described with emphasis on carbide.
The metal type carbides formed by metal and carbon of IV B, V B and VI B can be filled in the gap of metal lattice and retain the original lattice form of metal, and form the gap solid solution. Under suitable conditions, this solid solution can continue to dissolve its constituent elements until it reaches saturation. Therefore, their composition can be changed within a certain range (for example, the composition of titanium carbide is TiC0. Between 5 and TiC, chemical formula does not conform to the valence rule. When the dissolved carbon content exceeds a certain limit (such as Ti: C=1: 1 in titanium carbide), the lattice type will change to change the lattice of the original metal into another form of metal lattice, which is called the mesenchyme.
The melting point of metal carbides, especially the metal carbides of IV B, V B and VI B, is above 3273K, of which hafnium carbide and tantalum carbide are 4160K and 4150K respectively, which are the highest melting points of the known substances. The hardness of most carbides is large and their microhardness is greater than 1800kg. Mm2. (microhardness is one of the means of hardness representation, which is used for hard alloys and hard compounds, and the microhardness of 1800kg. Mm2 is equivalent to 9 of the hardness of Mohs one diamond). Many carbides are difficult to decompose at high temperatures, and their oxidation resistance is stronger than that of their constituent metals. Titanium carbide has the best thermal stability in all carbides. It is a very important metal type carbide. However, in oxidizing atmospheres, 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 interstitial spaces in the metal lattice and form interstitial solid solutions. They are similar to the properties of interstitial carbides. They are conductive, heat conducting, high melting point, high hardness, and brittleness.
The matrix of cemented carbide consists of two parts: one is the hardening phase and the other is the bonding metal.
The hardened phase is the carbide of transition metal in the periodic table of elements, such as tungsten carbide, titanium carbide and tantalum carbide. Their hardness is very high, the melting point is above 2000 degrees C, and some even exceed 4000 degrees C. In addition, transition metal nitrides, borides and silicides also have similar properties and can also serve as hardening phases in cemented carbides. The existence of hardening phase determines that the alloy has extremely high hardness and wear resistance.
The requirement of tungsten carbide for granularity of tungsten carbide WC is based on WC of different sizes (tungsten carbide) for different uses of cemented carbide. Hard alloy cutting tools: such as cutting foot machine blade, V-CUT knife and other fine machining alloys, ultra-fine, subfine and fine particles WC, coarse grain WC, gravity cutting and heavy cutting alloys using medium and coarse particles WC as raw materials; mining tools: rock hardness, heavy impact load, coarse particles WC, rock Small impact impact load is small, medium particle WC is used as raw material; wear resistance parts: when stressing its wear resistance, compression and surface finish, superfine, subfine, fine, medium particles WC are used as raw materials, and the impact tool is mainly medium and coarse particles of WC.
The carbon content of WC theory is 6. 128% (atomic 50%), when the carbon content of WC is greater than the theoretical carbon content, free carbon (WC+C) occurs in WC. In the presence of free carbon, WC grains grow around sintering, resulting in uneven grain size of cemented carbide. Tungsten carbide generally requires high carbon (> 6). 07%), free carbon (less than 0.) 05%), the total carbon depends on the production technology and application scope of cemented carbide.
Under normal circumstances, WC total carbon used in vacuum sintering of paraffin process is mainly determined by the content of combined oxygen in the briquetting before sintering. An increase of 0 of oxygen is needed. 75 copies of carbon, that is, WC total carbon =6. The oxygen content of 13%+ is% * 0. 75 (assuming that there is a neutral atmosphere in the sintering furnace, in fact most of the vacuum furnaces are carburizing atmospheres, and the total WC used in the furnace is less than the calculated value).
At present, the total carbon content of WC in China is roughly divided into three types: the total carbon content of WC in paraffin process vacuum sintering is about 6. 18 + 0. 03% (free carbon will increase). The total carbon content of WC used in paraffin process for hydrogen sintering is 6. 13 + 0. 03%. WC total carbon =5 was used for hydrogen sintering in rubber process. 90 + 0. 03%. The above process is sometimes crossed, so the total carbon of WC should be determined according to the specific circumstances.
WC total carbon used in alloys with different ranges of use, Co content and grain size can be adjusted slightly. Tungsten carbide with high total carbon content can be selected for low cobalt alloy, and tungsten carbide with low total carbon can be used in high cobalt alloy. In short, the requirement of tungsten carbide is different from that of tungsten carbide.
Bonding metals are usually iron group metals, commonly used as cobalt and nickel.
When making cemented carbide, the particle size of raw material selected is between 1~2 microns, and the purity is very high. The raw materials are proportioning according to the prescribed proportions, adding alcohol or other media to wet grinding in the wet ball mill, making them fully mixed and crushed, adding wax or glue after drying and sieving, and then drying and sifting the mixture. Then, the mixture is granulated, pressed and heated to the melting point of the bonding metal (1300~1500 degrees). The hardening phase is bonded to the bonded metal.