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Summary:Cemented carbide is one of the most widely used high-speed machining (HSM) tool materials. This kind of material is produced by powder metallurgy process. It consists of hard carbide (usually tungsten carbide WC) particles and soft metal bond. At pres...

Cemented carbide is one of the most widely used high-speed machining (HSM) tool materials. This kind of material is produced by powder metallurgy process. It consists of hard carbide (usually tungsten carbide WC) particles and soft metal bond. At present, there are hundreds of different components of WC-based cemented carbides, most of which use Cobalt (Co) as binder, nickel (Ni) and chromium (Cr) are also commonly used as binder elements, in addition to some other alloy elements can be added. Why are there so many cemented carbide grades? How do tool manufacturers choose the right tool materials for a particular cutting process? To answer these questions, let's first understand the properties that make cemented carbide an ideal tool material.

Hardness and toughness

WC-Co cemented carbide has unique advantages in both hardness and toughness. Tungsten carbide (WC) has a very high hardness (over corundum or alumina), and hardness rarely decreases when the working temperature rises. However, it lacks adequate toughness, which is essential for cutting tools. In order to utilize the high hardness of tungsten carbide and improve its toughness, tungsten carbide is bonded by metal binder, which makes the material have hardness far beyond that of high speed steel and can withstand cutting force in most cutting processes. In addition, it can withstand the cutting temperature generated by high-speed machining.

Today, almost all WC-Co tools and blades are coated, so the role of the matrix material seems to be less important. But in fact, it is the high elastic coefficient of the WC-Co material (a measure of stiffness, WC-Co's elastic coefficient at room temperature is about three times that of high-speed steel) that provides a non-deforming base for the coating. The WC-Co matrix also provides the required toughness. These properties are the basic properties of WC-Co materials, but they can also be produced in the production of cemented carbide powder, by adjusting the composition and microstructure of the material to customize the material properties. Therefore, the suitability of tool performance and specific machining depends to a large extent on the initial milling process.

Pulverizing process

Tungsten carbide powder is obtained by carburizing tungsten (W) powder. The characteristics of tungsten carbide powder (especially its particle size) mainly depend on the particle size of raw tungsten powder and the temperature and time of carburizing. Chemical control is also essential, and the carbon content must be kept constant (close to the theoretical ratio of 6.13% by weight). A small amount of vanadium and/or chromium can be added before carburizing in order to control the particle size of the powder through subsequent processes. Different downstream process conditions and different final processing uses require the use of specific tungsten carbide particle size, carbon content, vanadium content and chromium content of the combination of these changes, can produce a variety of tungsten carbide powder. For example, ATI Alldyne, a manufacturer of tungsten carbide powder, produces 23 standard grades of tungsten carbide powder, and customized varieties of tungsten carbide powder can reach more than five times the standard grade of tungsten carbide powder.

Various combinations of tungsten carbide powder and metal binder can be used to produce a certain grade of cemented carbide powder. The most commonly used cobalt content is 3% - 25% (by weight), while nickel and chromium need to be added to enhance the corrosion resistance of the tool. In addition, metal bonding agents can be further improved by adding other alloy components. For example, adding ruthenium to WC-Co cemented carbide can significantly improve its toughness without reducing its hardness. Increasing the binder content can also improve the toughness of cemented carbide, but it will reduce its hardness.

Reducing the size of tungsten carbide particles can improve the hardness of the material, but in the sintering process, the particle size of tungsten carbide must remain unchanged. During sintering, tungsten carbide particles combine and grow through dissolution and precipitation. In the actual sintering process, in order to form a completely dense material, metal binder to become liquid (called liquid phase sintering). The growth rate of tungsten carbide particles can be controlled by adding other transition metal carbides, including vanadium carbide (VC), chromium carbide (Cr3C2), titanium carbide (TiC), tantalum carbide (TaC) and niobium carbide (NbC). These carbides are usually added when the tungsten carbide powder is mixed with a metal binder, although vanadium carbide and chromium carbide can also be formed when the tungsten carbide powder is carburized.

The recycled tungsten carbide powder can also be used to produce grade tungsten carbide powder. The recycling and reuse of spent cemented carbide has a long history in the cemented carbide industry. It is an important part of the whole economic chain of the industry. It helps to reduce material costs, save natural resources and avoid harmless disposal of waste materials. Waste cemented carbide can be recycled by APT (ammonium paratungstate) process, zinc recovery process or through grinding. These "regenerated" tungsten carbide powders usually have better and predictable densification because their surface area is smaller than that of tungsten carbide powders made directly through the tungsten carburizing process.

The processing condition of tungsten carbide powder mixed with metal bond is also a critical process parameter. The two most commonly used milling technologies are ball milling and ultramicro milling. The two processes can uniformly mix the grind powder and reduce the particle size. An organic binder is usually added to the grinding process in order to make the later pressed workpiece strong enough to keep the shape of the workpiece and enable the operator or manipulator to pick up the workpiece for operation.

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