What is the milling process for tungsten carbide?
The tungsten carbide powder is obtained by carburizing the tungsten (W) powder. The properties of the tungsten carbide powder, especially its particle size, depend primarily on the particle size of the raw tungsten powder and the temperature and time of carburization. Chemical control is also critical, and the carbon content must be kept constant (close to the theoretical ratio of 6.13% by weight). In order to control the particle size by a subsequent process, a small amount of vanadium and/or chromium may be added before the carburizing treatment. Different downstream process conditions and different final processing applications require a combination of specific tungsten carbide particle size, carbon content, vanadium content, and chromium content, and variations in these combinations can produce a variety of different tungsten carbide powders.
When the tungsten carbide powder is mixed and ground with a metal bond to produce a certain grade of tungsten carbide powder, various combinations can be employed. The most commonly used cobalt content is 3% to 25% by weight, and nickel and chromium are required to increase the corrosion resistance of the tool. In addition, the metal bond can be further improved by adding other alloy components. For example, the addition of niobium to WC-Co tungsten carbide can significantly improve the toughness without lowering its hardness. Increasing the amount of binder can also increase the toughness of the tungsten carbide, but it will reduce its hardness.
Reducing the size of the tungsten carbide particles can increase the hardness of the material, but in the sintering process, the particle size of the tungsten carbide must remain unchanged. At the time of sintering, the tungsten carbide particles are combined and grown by the process of dissolution and re-precipitation. In the actual sintering process, in order to form a completely dense material, the metal bond is turned into a liquid state (referred to as liquid phase sintering). The growth rate of the 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 metal carbides are usually added during the mixing and milling of the tungsten carbide powder together with the metal binder, although vanadium carbide and chromium carbide can also be formed when carburizing the tungsten carbide powder.
Grades of tungsten carbide powder can also be produced from recycled solid carbide materials. The recycling and reuse of used tungsten carbide has a long history in the tungsten carbide industry and is an important part of the industry’s entire economic chain, helping to reduce material costs, conserve natural resources and avoid waste materials. Harmful disposal. Waste tungsten carbide can generally be reused by APT (ammonium paratungstate) process, zinc recovery process or by pulverization. These “recycled” tungsten carbide powders generally have better, predictable densification because their surface area is smaller than tungsten carbide powder made directly from the tungsten carburizing process.
The processing conditions for the mixing of tungsten carbide powder with a metal bond are also critical process parameters. The two most common milling techniques are ball milling and ultrafine milling. Both processes allow the milled powder to be evenly mixed and reduce particle size. In order to allow the workpiece to be pressed to have sufficient strength to maintain the shape of the workpiece and allow the operator or robot to pick up the workpiece for operation, it is usually necessary to add an organic binder during milling. The chemical composition of such a binder can affect the density and strength of the pressed workpiece. In order to facilitate the operation, it is preferable to add a high-strength binder, but this results in a lower pressing density and may cause a hard block, resulting in defects in the final product.
After the milling is completed, the powder is typically spray dried to produce a free flowing mass that is agglomerated by the organic binder. By adjusting the composition of the organic binder, the fluidity and charge density of these agglomerates can be tailored to suit the needs. By screening out coarser or finer particles, the particle size distribution of the agglomerates can be further tailored to ensure good fluidity when loaded into the mold cavity.