Hard alloy is a powder metallurgy product sintered in vacuum furnace or hydrogen reduction furnace with high hardness refractory metal carbide (WC, TiC) micron powder as the main component and cobalt (Co), nickel (Ni), molybdenum (Mo) as the binder.
The carbides, nitrides and borides of group ⅣB, ⅤB and ⅥB metals are collectively referred to as hard alloys because of their particularly high hardness and melting point. The structure, characteristics and applications of hard gold bearing are described below with the emphasis on carbide.
Carbide tool
In the metal-type carbides formed by group ⅣB, ⅤB and Ⅵ metals and carbon, due to the small radius of the carbon atom, they can fill in the gaps of the metal lattice and retain the original lattice form of the metal to form interstitial solid solutions. Under the right conditions, such 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 (e.g. Ti: C=1:1 in titanium carbide), the lattice pattern will change, transforming the original metal lattice into another form of metal lattice. In this case, the mesenchymal solid solution is called an mesenchymal compound.
Metal type carbides, especially ⅣB, ⅤB, ⅥB metal carbides have melting points above 3273K, wherein hafnium carbide and tantalum carbide are 4160K and 4150K, respectively, which are the highest melting points among known substances. Most carbides are very hard, their microhardness is greater than 1800kgmm2(microhardness is one of the hardness methods, most used in hard alloys and hard compounds, 1800kgmm2 microhardness is equivalent to Mohs diamond hardness 9). Many carbides are not easily decomposed at high temperature and have stronger oxidation resistance than their component metals. Titanium carbide is a very important metal type carbide with excellent thermal stability among all carbides. 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 interstitial space of metal lattice to form interstitial solid solutions. They are similar to the properties of interstitial carbides in that they can conduct electricity and heat, have high melting point, high hardness and high brittleness.
The matrix of hard alloy is composed of two parts: one is hardening phase; The other part is bonded metal.
The hardening 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 their melting point is above 2000℃, and some even exceed 4000℃. In addition, the nitrides, borides and silicates of transition metals have similar properties and can also act as hardening phases in hard alloys. The existence of hardened phase determines that the alloy has very high hardness and wear resistance.
Carbide tool
Bonding metals are generally ferrous metals, cobalt and nickel are commonly used.
In the manufacture of hard alloy, the selection of raw material powder particle size between 1 ~ 2 microns, and high purity. Raw materials according to the specified proportion of ingredients, adding alcohol or other media in the wet ball mill wet grinding, so that they are fully mixed, crushed, after drying, sifting into wax or gum and other a kind of forming agent, and then after drying, sifting to make the mixture. Then, the mixture is granulated, pressed, heated to close to the bonding metal melting point (1300 ~ 1500℃), hardening phase and bonding metal to form eutectic alloy. After cooling, the hardened phase is distributed in a grid of bonded metals, closely linked to each other, forming a solid whole. The hardness of hard alloy depends on the hardening phase content and grain size, that is, the higher the hardening phase content, the finer the grain size, the greater the hardness. The toughness of cemented carbide is determined by bonding metal, the higher the content of bonded metal, the greater the bending strength.
