Hard Alloy Machining: Understanding the Properties of Hard Alloys Hard alloys are powder metallurgy products primarily composed of micron-sized powders of high-hardness, refractory metal carbides (such as WC and TiC), combined with cobalt (Co) or nickel (Ni), molybdenum (Mo) as binders. They are sintered in vacuum furnaces or hydrogen-reduction furnaces. ⅣB...
　　Cemented Carbide Machining: Understanding the Properties of Cemented Carbide
　　Cemented carbides are powder metallurgy products made primarily from micron-sized powders of hard, refractory metal carbides (such as WC and TiC), with cobalt (Co) or nickel (Ni), along with molybdenum (Mo), serving as the binding agents. They are sintered in vacuum furnaces or hydrogen-reduction furnaces.
　　The carbides, nitrides, borides, and other compounds of metals in Groups ⅣB, ⅤB, and ⅥB are collectively known as cemented carbides due to their exceptionally high hardness and melting points. Below, we will focus on carbides to discuss the structure, characteristics, and applications of these hard alloys.
　　Among the metallic carbides formed by Group ⅣB, ⅤB, and ⅥB metals with carbon, the small radius of carbon atoms allows them to fit into the interstitial spaces of the metal lattice while preserving the original metallic lattice structure, thus forming an interstitial solid solution. Under suitable conditions, these solid solutions can even continue to dissolve their constituent elements until saturation is reached. As a result, their compositions can vary within a certain range, and their chemical formulas do not strictly adhere to conventional valence rules. However, when the dissolved carbon content exceeds a specific threshold, the lattice structure undergoes a transformation, causing the original metal lattice to shift into a different type of metallic lattice arrangement. In such cases, the interstitial solid solution is referred to as an interstitial compound.
　　Metallic carbides, especially those from Groups IVB, VB, and VIB, have melting points exceeding 3,273 K. Among them, hafnium carbide and tantalum carbide boast the highest melting points to date—4,160 K and 4,150 K, respectively. Most carbides are also remarkably hard, with microhardness values surpassing 1,800 kg·mm². Many carbides exhibit excellent resistance to decomposition at high temperatures and demonstrate stronger oxidation resistance compared to their constituent metals. Notably, titanium carbide stands out as the most thermally stable among all carbides, making it a critically important metallic carbide material. However, under oxidizing atmospheres, all carbides tend to oxidize easily at elevated temperatures—a characteristic that highlights a significant vulnerability of these compounds.

　　In addition to carbon atoms, nitrogen and boron atoms can also fit into the interstitial spaces of a metal lattice, forming interstitial solid solutions. These elements share similar properties with interstitial carbides—they are conductive, have high thermal conductivity, exhibit high melting points and great hardness, yet they are also characterized by significant brittleness.
　　The matrix of cemented carbide consists of two parts: one is the hardening phase, and the other is the bonding metal.
　　The hardening phase refers to carbides of transition metals found in the periodic table, such as tungsten carbide, titanium carbide, and tantalum carbide. These compounds exhibit exceptional hardness and melting points exceeding 2000°C—some even surpassing 4000°C. Additionally, nitrides, borides, and silicides of transition metals share similar properties and can also serve as hardening phases in cemented carbides. The presence of these hardening phases is what endows the alloy with its extraordinary hardness and wear resistance.
　　Cemented carbides have varying requirements for tungsten carbide (WC) particle size, depending on their specific applications. For cemented carbide cutting tools—such as lead-cutting blades and V-CUT tools—ultrafine, sub-fine, and fine-grained WC are used in precision finishing alloys, while medium-grained WC is preferred for roughing applications. Alloys designed for heavy-duty and high-impact cutting processes utilize medium-to-coarse-grained WC as the primary material. In mining tools, where rock hardness is high and impact loads are significant, coarse-grained WC is employed. Conversely, when dealing with rocks that experience lower impact and lighter load conditions, medium-grained WC serves as the ideal raw material. For wear-resistant components, where durability, compressive strength, and surface finish are critical, ultrafine, sub-fine, fine, and medium-grained WC are typically chosen. Meanwhile, impact-resistant tools predominantly rely on medium-to-coarse-grained WC as their main material.
　　WC theory specifies a carbon content of 6.128%. When the actual carbon content in WC exceeds this theoretical value, free carbon begins to appear within the material. The presence of free carbon promotes the growth of surrounding WC grains during sintering, leading to uneven grain sizes in the cemented carbide. Generally, tungsten carbide is designed to have high levels of combined carbon, while free carbon and total carbon content are determined by the production process and application requirements of the cemented carbide.
　　Under normal circumstances, the total carbon content of WC used in vacuum sintering with the paraffin wax process is primarily determined by the amount of combined oxygen present in the compacted material before sintering. For each unit of oxygen, 0.75 units of carbon must be added; thus, the total carbon content of WC can be calculated as: Total Carbon in WC = 6.13% + Oxygen Content (%) × 0.75. (Note: This calculation assumes a neutral atmosphere within the sintering furnace. In reality, most vacuum furnaces operate under a carburizing atmosphere, resulting in a lower total carbon content in the WC than what would be predicted by this formula.)
　　Currently, the total carbon content of WC in China is broadly categorized into three types: The total carbon content of WC used for vacuum sintering via the paraffin process is approximately 6.18 ± 0.03%. For WC sintered with hydrogen gas using the paraffin process, the total carbon content is 6.13 ± 0.03%. And for WC sintered with hydrogen gas using the rubber process, the total carbon content is 5.90 ± 0.03%. Note that these processes are sometimes carried out in an overlapping manner, so determining the exact total carbon content of WC requires careful consideration of the specific circumstances.
　　The total carbon content of WC can be slightly adjusted depending on the alloy's intended application, cobalt content, and grain size. For low-cobalt alloys, tungsten carbide with a slightly higher total carbon content is recommended, while high-cobalt alloys benefit from tungsten carbide with lower total carbon levels. In short, the specific usage requirements of cemented carbides dictate varying demands on the particle size of tungsten carbide.
　　Bonding metals are generally iron-group metals, with cobalt and nickel being commonly used.