What is cemented carbide?

What is cemented carbide?

Cemented carbide is a kind of alloy made by powder metallurgy with one or several kinds of refractory carbides (tungsten carbide, titanium carbide, etc.) as the main component and metal powder (cobalt, nickel, etc.) as adhesive. It is mainly used to manufacture high-speed cutting tools and cutting tools for hard and tough materials, as well as cold working dies, measuring tools and high wear-resistant parts free from impact and vibration.

Characteristics of cemented carbide

(1) High hardness, wear resistance and red hardness
The hardness of cemented carbide can reach 86-93 HRA at room temperature, which is equivalent to 69-81 HRC. It can maintain high hardness and excellent wear resistance at 900 ～ 1000 ℃. Compared with high-speed tool steel, the cutting speed can be 4-7 times higher, the service life can be 5-80 times longer, and the hard materials with hardness up to 50 HRC can be machined.
(2) High strength and elastic modulus
The compressive strength of cemented carbide is as high as 6000 MPa, and the elastic modulus is (4-7) × 105 MPa, which are higher than that of high-speed steel. But its bending strength is low, generally 1000-3000mpa.
(3) Good corrosion resistance and oxidation resistance
Generally, it can resist the corrosion of atmosphere, acid and alkali, and is not easy to oxidize.
(4) The coefficient of linear expansion is small
When working, the shape and size are stable.
(5) The formed products are no longer processed and reground

Because of the high hardness and brittleness of cemented carbide, it is no longer necessary to carry out cutting or regrinding after powder metallurgy forming and sintering. When special re machining is needed, only EDM, WEDM, electrolytic grinding and other electrical machining or special grinding wheel grinding can be used. Usually, the products with certain specifications made of cemented carbide are brazed, bonded or mechanically clamped on the cutter body or die.

Properties of cemented carbide

Cemented carbide is mainly composed of carbide (WC, TIC) with high hardness and refractory metal
It is a powder metallurgy product sintered in vacuum furnace or hydrogen reduction furnace with cobalt (CO) or nickel (Ni) and molybdenum (MO) as binder.
Carbides, nitrides and borides of group Ⅳ B, Ⅴ B and Ⅵ B are called cemented carbides because of their high hardness and melting point. The structure, characteristics and application of hard gold bearing materials are described with emphasis on carbides.
Due to the small radius of carbon atom, the metallic carbides formed by Ⅳ B, Ⅴ B, Ⅵ B metals and carbon can fill in the gap of metal lattice and retain the original lattice form of metal to form interstitial solid solution. Under suitable conditions, this kind of solid solution can continue to dissolve its constituent elements until it reaches saturation. Therefore, their compositions can vary in a certain range (for example, the composition of titanium carbide varies from tic0.5 to TIC), and the chemical formula does not conform to the valence rule. When the content of dissolved carbon exceeds a certain limit (for example, Ti ∶ C = 1 ∶ 1 in titanium carbide), the lattice pattern will change, and the original metal lattice will be transformed into another form of metal lattice. At this time, the interstitial solid solution is called mesenchymal compound.
The melting points of metal type carbides, especially those of group Ⅳ B, Ⅴ B and Ⅵ B, are above 3273 K, among which hafnium carbide and tantalum carbide are 4160k and 4150k respectively, which are the highest melting points among the known materials. The hardness of most carbides is very high, and their microhardness is more than 1800kg · mm2 (microhardness is one of the methods to express hardness, which is mostly used in cemented carbide and hard compound. The microhardness 1800kg · mm2 is equivalent to Mohr diamond hardness 9). The oxidation resistance of many metal components is stronger than that of high temperature. Titanium carbide has the best thermal stability among all carbides and is a very important metal type carbide. However, in oxidizing atmosphere, all carbides are easy to be oxidized at high temperature, which is a major weakness of carbides.
In addition to carbon atoms, nitrogen atoms and boron atoms can also enter the space of metal lattice to form interstitial solid solution. They are similar to interstitial carbides in the properties of electric conduction, heat conduction, high melting point, high hardness and brittleness.
The matrix of cemented carbide consists of two parts: one is hardening phase, the other is bonding metal.
The hardening phase is the carbides of transition metals in the periodic table of elements, such as tungsten carbide, titanium carbide and tantalum carbide. Their hardness is very high, and their melting points are above 2000 ℃ and some even exceed 4000 ℃. In addition, nitrides, borides and silicides of transition metals also have similar characteristics, which can be used as hardening phases in cemented carbides. The existence of hardening phase determines that the alloy has very high hardness and wear resistance.
According to the requirements of WC particle size of WC for cemented carbide, WC (tungsten carbide) with different particle size is used according to different cemented carbide applications. Cemented carbide cutting tools: such as foot cutting machine blade, V-CUT knife and other finishing alloy using ultra-fine, sub-fine and fine-grained WC, rough machining alloy using medium particle WC, gravity cutting and heavy cutting alloy using medium and coarse-grained WC as raw materials; mining tools: high hardness rock, high impact load, using coarse-grained WC, rock impact small impact load, using medium particle WC as raw material Wear resistant parts: when the wear resistance, compression resistance and surface finish are emphasized, ultra-fine, sub fine, fine and medium particles WC are used as raw materials, and medium and coarse WC raw materials are mainly used for impact resistant tools.
The theoretical carbon content of WC is 6.128% (atom 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 makes the WC grains around it grow up during sintering, resulting in the inhomogeneous grains of cemented carbide. Tungsten carbide generally requires high combined carbon (≥ 6.07%) and free carbon (≤ 0.05%). The total carbon depends on the production process and application range of cemented carbide.
Under normal conditions, the total carbon content of WC for vacuum sintering in paraffin process is mainly determined by the oxygen content in the briquette before sintering. If one part of oxygen is contained, 0.75 parts of carbon should be added, that is, WC total carbon = 6.13% + oxygen content% × 0.75 (assuming that the sintering furnace is neutral atmosphere, in fact, most vacuum furnaces are carburized atmosphere, and the WC total carbon used is less than the calculated value).
The total carbon content of WC in China can be roughly divided into three types: the total carbon content 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%. 0.03 ± 0.03% of total carbon. Therefore, the determination of WC total carbon depends on the specific situation.
WC total carbon used in alloys with different application range, CO content and grain size can be adjusted slightly. Tungsten carbide with high total carbon can be selected for low cobalt alloy, while tungsten carbide with low total carbon can be selected for high cobalt alloy. In a word, the requirements of WC particle size are different for different cemented carbides.
Bonding metals are generally iron group metals, and cobalt and nickel are commonly used.
When making cemented carbide, the particle size of raw material is between 1 and 2 μ m, and the purity is very high. The raw materials are proportioned according to the specified composition ratio, added alcohol or other media to wet grind in the wet ball mill, so that they are fully mixed and crushed. After drying and screening, wax or glue and other molding agents are added, and then the mixture is dried and screened. Then, when the mixture is granulated and molded, and heated to the melting point of the bonding metal (1300-1500 ℃), the eutectic alloy is formed between the hardened phase and the bonding metal. They are tightly bonded to each other and form a network of closely bonded metals. The hardness of cemented carbide depends on the content of hardened phase and grain size, that is, the higher the content of hardened phase and the finer the grain, the greater the hardness. The toughness of cemented carbide is determined by the bonding metal. The higher the content of bonding metal, the greater the bending strength.

Common cemented carbide

The common cemented carbides can be divided into three types according to their composition and properties: tungsten cobalt, tungsten titanium cobalt and tungsten titanium tantalum (niobium). Tungsten cobalt and tungsten titanium cobalt cemented carbide are widely used in production.
(1) Tungsten cobalt cemented carbide
The main components are tungsten carbide (WC) and cobalt. The brand is represented by YG (Chinese phonetic alphabet for hard and cobalt), followed by the percentage value of cobalt content. For example, YG6 is the tungsten cobalt cemented carbide with 6% cobalt content, and the tungsten carbide content is WWC = 1-wco = 94%.
(2) Tungsten titanium cobalt cemented carbide
The main components are tungsten carbide (WC), titanium carbide (TIC) and cobalt. The brand name is YT (Chinese phonetic alphabet for hard and titanium), followed by the percentage value of titanium carbide content. For example, YT15 is a tungsten titanium cobalt cemented carbide with a titanium carbide content of 15%.
(3) Tungsten titanium tantalum (niobium) cemented carbide
The main composition of the cemented carbide is tantalum (NBC), or tantalum carbide. The brand name is indicated by the code YW (hard and WAN) followed by ordinal number.
Table 1 grades and chemical compositions of commonly used cemented carbides

Brand	Chemical composition w/%
	ＷＣ	ＴｉＣ	ＴａＣ	Ｃｏ
ＹＧ３Ｘ	９６．５	—	＜０．５	３
ＹＧ６	９４．０	—	—	６
ＹＧ６Ｘ	９３．５	—	＜０．５	６
ＹＧ８	９２．０	—	—	８
ＹＧ８Ｎ	９１．０	—	１	８
ＹＧ１１Ｃ	８９．０	—	—	１１
ＹＧ１５	８５．０	—	—	１５
ＹＧ４Ｃ	９６．０	—	—	４
ＹＧ６Ａ	９２．０	—	２	６
ＹＧ８Ｃ	９２．０	—	—	８
ＹＴ５	８５．０	５	—	１０
ＹＴ１４	７８．０	１４	—	８
ＹＴ３０	６６．０	３０	—	４
ＹＷ１	８４～８５	６	３～４	６
ＹＷ２	８２～８３	６	３～４	８

Note: the “X” after the brand name refers to the fine-grained alloy, “C” refers to the coarse-grained alloy, and the word “no” refers to the general granular alloy.
Table.2 physical and mechanical properties of common cemented carbide

Brand	Physical and mechanical properties (not less than)
	Density （ｇ·ｃｍ－３）	Hardness （ＨＲＡ）	Strength σｂ／ＭＰａ
ＹＧ３Ｘ	１５．０～１５．３	９１．５	１０７９
ＹＧ６	１４．６～１５．０	８９．５	１４２２
ＹＧ６Ｘ	１４．６～１５．０	９１．０	１３７３
ＹＧ８	１４．５～１４．９	８９．０	１４７１
ＹＧ８Ｎ	１４．５～１４．９	８９．５	１４７１
ＹＧ１１Ｃ	１４．０～１４．４	８６．５	２０６０
ＹＧ１５	１３．０～１４．２	８７	２０６０
ＹＧ４Ｃ	１４．９～１５．２	８９．５	１４２２
ＹＧ６Ａ	１４．６～１５．０	９１．５	１３７３
ＹＧ８Ｃ	１４．５～１４．９	８８．０	１７１６
ＹＴ５	１２．５～１３．２	８９．５	１３７３
ＹＴ１４	１１．２～１２．０	９０．５	１１７７
ＹＴ３０	９．３～９．７	９２．５	８８３
ＹＷ１	１２．６～１３．５	９１．５	１１７７
ＹＷ２	１２．４～１３．５	９０．５	１３２４

Application of cemented carbide

(1) Tool material
Cemented carbide is the largest tool material, which can be used to make turning tool, milling cutter, planer, drill bit, etc. Among them, tungsten cobalt cemented carbide is suitable for short chip processing of ferrous metals and non-ferrous metals and processing of non-metallic materials, such as cast iron, cast brass, bakelite, etc.; tungsten titanium cobalt cemented carbide is suitable for long chip processing of ferrous metals such as steel. In the same kind of alloy, the alloy with more cobalt content is suitable for rough machining, and the one with less cobalt content is suitable for finishing. The machining life of general-purpose cemented carbides for stainless steel and other difficult to machine materials is much longer than other cemented carbides.
(2) Die material
Cemented carbide is mainly used as cold drawing die, cold stamping die, cold extrusion die, cold pier die and so on.
(3) Measuring tools and wear-resistant parts
Cemented carbides are used for inlaying easily worn surfaces and parts of measuring tools, precision bearings of grinding machines, guide plates and guide rods of centerless grinder, lathe centers and other wear-resistant parts.

Hardness testing of cemented carbide

Test method 

The hardness of cemented carbide is mainly measured by Rockwell hardness tester to test the hardness value of HRA. Phr Series Portable Rockwell hardness tester is very suitable for testing the hardness of cemented carbide. The weight precision of the instrument is the same as that of the table type Rockwell hardness tester, so it is very convenient to use and carry.
Cemented carbide is a kind of metal. The hardness test can reflect the difference of mechanical properties of cemented carbide materials under different chemical composition, microstructure and heat treatment process conditions. Therefore, hardness test is widely used to inspect the properties of cemented carbide, supervise the correctness of heat treatment process and research of new materials.

Characteristic

It belongs to non-destructive test, and the test method is relatively simple. The hardness test of cemented carbide has strong adaptability to the shape and size of the specimen, and the test efficiency is high. In addition, there is a certain relationship between the hardness of cemented carbide and other physical properties. For example, cemented carbide hardness test and tensile test are basically used to test the ability of metal to resist plastic deformation, and these two tests are to detect the similar characteristics of metals to some extent. Therefore, the detection results can be compared with each other. For many metal materials, there is a conversion table for hardness test and tensile test. Therefore, when testing the mechanical properties of cemented carbide materials, more and more people use hardness test, and less use tensile test.

Tool

Generally, the hardness of cemented carbide is tested by using Rockwell hardness tester HRA scale or Vickers hardness tester. In practice, people mainly use Rockwell hardness tester to test HRA hardness. Phr Series Portable Rockwell hardness tester is very suitable for testing the hardness of cemented carbide. The weight of this instrument is only 0.7kg, and the accuracy is the same as that of the table type Rockwell hardness tester. When measuring the hardness of cemented carbide, phr Series Portable Rockwell hardness tester produced by Tianxing company can test cemented carbide workpiece with thickness or diameter less than 50mm, cemented carbide workpiece with diameter less than 2.0mm and tubular cemented carbide workpiece with inner diameter less than 30mm. It can also be used in warehouse, production site or production site. This instrument is simple, rapid and nondestructive for testing cemented carbide workpieces. It can be used to test the hardness of finished or semi-finished cemented carbide workpieces one by one.

Source: China Alloy Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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