Cemented carbide is an alloy material produced through powder metallurgy processes, consisting of hard compounds of refractory metals and a binding metal.

This material possesses a series of exceptional properties, including high hardness, wear resistance, good strength and toughness, heat resistance, and corrosion resistance. Particularly notable are its high hardness and wear resistance, which remain largely stable even at temperatures of 500°C and maintain considerable hardness at 1000°C.

Cemented carbide is widely used as a cutting tool material, for items such as lathe tools, milling cutters, planing tools, drill bits, and boring tools. It is suitable for cutting cast iron, non-ferrous metals, plastics, chemical fibers, graphite, glass, stone, and ordinary steel. Furthermore, it can also be used to process difficult-to-machine materials like heat-resistant steel, stainless steel, high-manganese steel, and tool steel.

Advantages

Due to its extremely high hardness, strength, wear resistance, and corrosion resistance, cemented carbide is renowned as the "Industrial Teeth." It is used to manufacture cutting tools, drills, and wear-resistant components, finding extensive applications in sectors such as military industry, aerospace, mechanical processing, metallurgy, petroleum drilling, mining tools, electronic communications, and construction. With the ongoing development of downstream industries, market demand for cemented carbide continues to increase. Looking ahead, the manufacturing of high-tech weaponry and equipment, advancements in cutting-edge science and technology, and the rapid development of nuclear energy will significantly boost the demand for high-technology content and high-quality stable cemented carbide products.

Applications

In 1923, Schröter in Germany invented a new alloy of tungsten carbide and cobalt by adding 10% to 20% cobalt as a binder to tungsten carbide powder. This alloy, with a hardness second only to diamond, was the first artificially produced cemented carbide in the world. However, when tools made from this alloy were used to cut steel, the cutting edge would wear quickly or even chip. In 1929, Schwarzkopf in the United States added a certain amount of complex carbides of tungsten carbide and titanium carbide to the original composition, thereby improving the steel-cutting performance of the tools. This marked another significant achievement in the history of cemented carbide development.

Cemented carbide is also used to manufacture rock drilling tools, excavation tools, drilling tools, measuring instruments, wear-resistant parts, metal molds, cylinder liners, precision bearings, nozzles, and hardware molds (such as wire drawing dies, bolt dies, nut dies, and various fastener dies). Its superior properties have gradually replaced earlier steel molds.

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Subsequently, coated cemented carbide emerged. In 1969, Sweden successfully developed titanium carbide-coated tools. The substrate of these tools was tungsten-titanium-cobalt cemented carbide or tungsten-cobalt cemented carbide, with a surface titanium carbide coating only a few micrometers thick. Compared to uncoated alloy tools of the same grade, their service life was extended by 3 times, and cutting speed increased by 25% to 50%. By the 1970s, the fourth generation of coated tools had appeared, capable of machining extremely difficult-to-cut materials.

Performance Characteristics

High hardness (86–93 HRA, equivalent to 69–81 HRC);

Excellent hot hardness (can maintain 60 HRC at temperatures up to 900–1000°C);

Outstanding wear resistance.

Cemented carbide tools exhibit cutting speeds 4 to 7 times higher than high-speed steel, and tool life is 5 to 80 times greater. When used for manufacturing molds and measuring tools, their service life is 20 to 150 times higher than that of alloy tool steel. They can cut hard materials with hardness up to approximately 50 HRC.

However, cemented carbide is inherently brittle, cannot be machined by cutting methods, and is difficult to fabricate into complex-shaped monolithic tools. Consequently, it is typically produced in the form of inserts of various shapes, which are then mounted onto tool shanks or mold bodies using methods such as brazing, adhesive bonding, or mechanical clamping.

Material Properties

Cemented carbide is a powder metallurgy product sintered in vacuum furnaces or hydrogen reduction furnaces, primarily composed of micron-sized powders of high-hardness refractory metal carbides (like WC, TiC), using cobalt (Co), nickel (Ni), or molybdenum (Mo) as the binder.

Carbides, nitrides, borides, etc., of Group IVB, VB, and VIB metals in the periodic table are collectively referred to as cemented carbides or hard metals due to their exceptionally high hardness and melting points. The structure, characteristics, and applications of these hard alloys will be explained below, focusing on carbides.

In the metal-type carbides formed by Group IVB, VB, VIB metals and carbon, due to the small atomic radius of carbon, carbon atoms can fill the interstitial sites of the metal crystal lattice while retaining the original lattice form of the metal, forming interstitial solid solutions. Under suitable conditions, these solid solutions can continue to dissolve their constituent elements until saturation is reached. Therefore, their composition can vary within a certain range (for instance, the composition of titanium carbide varies between TiC₀.₅ and TiC), and their chemical formulas do not adhere to the valency rules. When the dissolved carbon content exceeds a certain limit (e.g., Ti:C = 1:1 in titanium carbide), the crystal lattice type changes, transforming the original metal lattice into another form of metal lattice. At this point, the interstitial solid solution is called an interstitial compound.

Metal-type carbides, especially those of Group IVB, VB, and VIB metals, have melting points above 3273 K. Among them, hafnium carbide and tantalum carbide have melting points of 4160 K and 4150 K, respectively, ranking among the highest known melting points of any substances. Most carbides are extremely hard, with a microhardness greater than 1800 kg/mm² (Microhardness is one method of expressing hardness, often used for cemented carbides and hard compounds; a microhardness of 1800 kg/mm² corresponds approximately to Mohs hardness 9, close to diamond). Many carbides do not decompose easily at high temperatures and possess better oxidation resistance than their constituent metals. Titanium carbide has the best thermal stability among all carbides and is a very important metal-type carbide. However, in oxidizing atmospheres, all carbides are susceptible to oxidation at high temperatures, which can be considered a significant weakness.

Besides carbon atoms, nitrogen and boron atoms can also enter the interstitial sites of metal crystal lattices, forming interstitial solid solutions. Their properties are similar to those of interstitial carbides: they conduct electricity and heat, have high melting points and high hardness, but also significant brittleness.

The matrix of cemented carbide consists of two parts: one is the hard phase; the other is the binder metal.

The hard phases are carbides of transition metals from the periodic table, such as tungsten carbide (WC), titanium carbide (TiC), and tantalum carbide (TaC). These carbides possess high hardness, with melting points all above 2000°C, some even exceeding 4000°C. Additionally, nitrides, borides, and silicides of transition metals have similar characteristics and can also serve as hard phases in cemented carbides. The presence of the hard phase determines the alloy's extremely high hardness and wear resistance.

The requirement for tungsten carbide (WC) grain size in cemented carbide varies depending on its intended use. For cemented carbide cutting tools: For finishing alloys, such as PCB cutting knife blades and V-CUT knives, ultra-fine, sub-fine, or fine-grained WC is used. For roughing alloys, medium-grained WC is employed. For heavy-duty and severe machining alloys, medium or coarse-grained WC serves as the raw material. For mining tools: Where rock hardness is high and impact load is significant, coarse-grained WC is used. Where rock impact and load are lower, medium-grained WC is used as the raw material. For wear-resistant parts: When emphasizing wear resistance, compressive strength, and surface finish, ultra-fine, sub-fine, fine, or medium-grained WC is used. For impact-resistant tools, medium or coarse-grained WC is primarily used as the raw material.

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The theoretical carbon content of WC is 6.128% (atomic 50%). When the carbon content of WC exceeds the theoretical value, free carbon (WC + C) appears. The presence of free carbon causes the surrounding WC grains to grow during sintering, leading to inhomogeneous grain size in the cemented carbide. Generally, tungsten carbide requires high combined carbon (≥6.07%), low free carbon (≤0.05%), while the total carbon content depends on the production process and application range of the cemented carbide.

Under normal conditions, for vacuum sintering using the paraffin wax process, the total carbon content of WC is primarily determined by the combined oxygen content within the compact before sintering. For each part of oxygen, 0.75 parts of carbon need to be added, meaning: WC Total Carbon = 6.13% + Oxygen Content % × 0.75 (This calculation assumes a neutral atmosphere inside the sintering furnace; in reality, most vacuum furnaces have a carburizing atmosphere, so the actual total carbon used is less than the calculated value).

In China, the total carbon content of WC is broadly categorized into three types: For the paraffin wax process with vacuum sintering, the total carbon of WC is approximately 6.18% ± 0.03% (free carbon will increase). For the paraffin wax process with hydrogen sintering, the total carbon content of WC is 6.13% ± 0.03%. For the rubber process with hydrogen sintering, the total carbon of WC = 5.90% ± 0.03%. These processes are sometimes used interchangeably, so determining the total carbon for WC must be based on the specific circumstances.

The total carbon content of WC used can be adjusted slightly for different applications, different cobalt (Co) contents, and different grain sizes of the alloy. Low-cobalt alloys can utilize tungsten carbide with a higher total carbon content, while high-cobalt alloys can use tungsten carbide with a lower total carbon content. In summary, the specific usage requirements of the cemented carbide dictate the necessary tungsten carbide grain size.

The binder metal is typically from the iron group metals, most commonly cobalt and nickel.

During the manufacture of cemented carbide, raw material powders with particle sizes between 1 and 2 micrometers and high purity are selected. The raw materials are batched according to the specified composition ratio, and alcohol or other media are added for wet milling in a ball mill to ensure thorough mixing and pulverization. After drying and sieving, forming agents such as wax or glue are added. Following another drying and sieving step, the mixture is obtained. This mixture is then granulated and pressed into shape. When heated to a temperature close to the melting point of the binder metal (1300–1500°C), the hard phase and the binder metal form a eutectic alloy. Upon cooling, the hard phase is distributed within the network formed by the binder metal, closely interconnected to form a solid integral structure. The hardness of the cemented carbide depends on the hard phase content and the grain size; specifically, higher hard phase content and finer grains result in greater hardness. The toughness of cemented carbide is determined by the binder metal; a higher binder metal content leads to greater bending strength.