Why is tungsten carbide used for cutting tools？

Release time：2022/03/23

Why is tungsten carbide used for cutting tools？

Tungsten carbide properties.

Tungsten is an especially hard, tough metal with a high melting point. Pure metallic tungsten typically isn’t used in cutting tools, but tungsten carbide is. In cutting tools, tungsten carbide ceramic particles are bonded together using cobalt metal, which adds fracture toughness to the tool; ceramics are brittle (low fracture toughness) with high strength, and metals tend to have higher fracture toughness with lower overall strength, so tungsten carbide-cobalt parts have a nice balance of these properties. They are strong enough to cut hard materials, and tough enough to last a long time.

Tungsten carbide applications.

By far the most important application area for tungsten carbide is metal cutting. The main applications are milling, turning and bore machining, which account for around two thirds of worldwide sales of tungsten carbide. These include both indexable insert tools, in which coated carbide inserts are usually mounted on a steel body, and solid carbide tools.
Tungsten carbide plays an outstanding role in the machining process because it offers a considerably higher wear resistance than high-speed steel (HSS), can withstand higher working temperatures and can be very well optimized to meet the requirements of various processes. At the same time, it is significantly less expensive than tools with polycrystalline diamond (PCD) coating, for example.
At the cutting edge, carbide tools can withstand temperatures of up to 1,100°C with minimal creep, allowing higher cutting speeds and feed rates. In combination with modern CVD and PVD coatings, with which most tools are equipped, carbide tools are also excellently suited for current trends such as minimum quantity lubrication (MQL) and high-speed cutting (HSC), which place even higher demands on the performance of cutting tools.
Today, almost all WC-Co tools and inserts are coated, so the role of the matrix material seems less important. But in fact, it is the high modulus of elasticity of the WC-Co material (the measure of stiffness, the room temperature modulus of WC-Co is about three times that of high-speed steel) provides a non-deformable substrate for the coating. The WC-Co matrix also provides the required toughness. These properties are basic properties of WC-Co materials, but they can also be tailored to the material composition and microstructure when producing tungsten carbide powders. Therefore, the suitability of the tool performance to a particular process depends to a large extent on the initial milling process.

High-precision tools made of tungsten carbide are used wherever high cutting volumes and maximum precision are required or when tools have to meet special requirements. This is the case, for example, in machine and tool construction, the automotive industry and in the aerospace industry. Especially in the aerospace industry, materials that are difficult to machine and stacks of different materials pose special challenges for tools. Materials that are difficult to machine, such as titanium alloys, also play an important role in the medical industry - both in dental and medical technology - as well as in the energy industry.

Cutting used Tungsten carbide grades.

The combination of different types of tungsten carbide powder, mixture composition and metal binder content, type and amount of grain growth inhibitors, etc., constitutes a variety of carbide grades. These parameters will determine the microstructure and properties of the tungsten carbide. Certain specific performance combinations have become the first choice for specific processing applications, making it possible to classify multiple carbide grades.
    The two most commonly used carbide machining classification systems for machining purposes are the C grade system and the ISO grade system. Although neither of these systems fully reflects the material properties that affect the choice of carbide grades, they provide a starting point for discussion. For each taxonomy, many manufacturers have their own special grades, resulting in a wide variety of carbide grades.
Carbide grades can also be classified by composition. Tungsten carbide (WC) grades can be divided into three basic types: simple, microcrystalline and alloy. Simple grades consist primarily of tungsten carbide and cobalt binders, but may also contain small amounts of grain growth inhibitors. The microcrystalline grade consists of tungsten carbide and a cobalt binder with a few thousandths of vanadium carbide (VC) and/or chromium carbide (Cr3C2) added, and its grain size can be less than 1 μm. The alloy grade consists of tungsten carbide and a cobalt binder containing several percent of titanium carbide (TiC), tantalum carbide (TaC) and niobium carbide (NbC). These additives are also called cubic carbides because of their sintering. The resulting microstructure exhibits a non-uniform three-phase structure.

    (1) Simple carbide grade
    Such grades for metal cutting typically contain 3%-12% cobalt (by weight). The size of the tungsten carbide grains is usually in the range of 1-8 μm. As with other grades, reducing the particle size of tungsten carbide increases its hardness and transverse rupture strength (TRS), but reduces its toughness. The hardness of simple grades is usually between HRA 89-93.5; the transverse rupture strength is usually between 175-350 ksi. Such grades of powder may contain a large amount of recycled raw materials.
    Simple grades can be divided into C1-C4 in the C grade system and can be classified according to the K, N, S and H grade series in the ISO grade system. Simple grades with intermediate characteristics can be classified as general grades (eg C2 or K20) for turning, milling, planing and boring; grades with smaller grain sizes or lower cobalt content and higher hardness can be used Classified as a finishing grade (such as C4 or K01); grades with larger grain sizes or higher cobalt content and better toughness can be classified as rough grades (eg C1 or K30).
Tools made from simple grades can be used to cut cast iron, 200 and 300 series stainless steel, aluminum and other non-ferrous metals, superalloys and hardened steel. These grades can also be used in non-metal cutting applications (such as rock and geological drilling tools) with grain sizes ranging from 1.5 to 10 μm (or larger) and cobalt levels from 6% to 16%. Another non-metal cutting type of simple carbide grades is the manufacture of molds and punches. These grades typically have a medium size grain size with a cobalt content of 16%-30%.

    (2) Microcrystalline carbide grade
    Such grades usually contain 6%-15% cobalt. In the liquid phase sintering, the added vanadium carbide and/or chromium carbide can control the grain growth, thereby obtaining a fine grain structure having a particle size of less than 1 μm. This fine grain grade has a very high hardness and a transverse rupture strength of 500 ksi or more. The combination of high strength and sufficient toughness allows these grades of tools to have a larger positive rake angle, which reduces cutting forces and produces thinner chips by cutting rather than pushing metal.
    Through the strict quality identification of various raw materials in the production of grades of tungsten carbide powder and strict control of the sintering process conditions, it is possible to prevent the formation of abnormal large grains in the microstructure of the material. Material properties. In order to keep the grain size small and uniform, the recycled powder can only be used if the raw materials and recovery process are fully controlled and extensive quality testing is performed.
    Microcrystalline grades can be classified according to the M grade series in the ISO grade system. In addition, the other classification methods in the C grade system and the ISO grade system are the same as the simple grades. Microcrystalline grades can be used to make tools for cutting softer workpiece materials because the surface of the tool can be machined very smoothly and maintain an extremely sharp cutting edge.
Microcrystalline grades can also be used to machine nickel-based superalloys because they can withstand cutting temperatures up to 1200 °C. For the processing of high-temperature alloys and other special materials, the use of micro-grain grade tools and simple grade tools with enamel can simultaneously improve their wear resistance, deformation resistance and toughness. Microcrystalline grades are also suitable for making rotary tools (such as drill bits) that generate shear stress. One type of drill bit is made of a composite grade of tungsten carbide. The specific cobalt content of the material in the specific part of the same bit is different, so that the hardness and toughness of the drill bit are optimized according to the processing needs.

    (3) Alloy type carbide grade
    These grades are mainly used for cutting steel parts, which typically have a cobalt content of 5%-10% and a grain size range of 0.8-2 μm. By adding 4% to 25% of titanium carbide (TiC), the tendency of tungsten carbide (WC) to diffuse to the surface of the steel scrap can be reduced. Tool strength, crater wear resistance and thermal shock resistance can be improved by adding no more than 25% tantalum carbide (TaC) and niobium carbide (NbC). The addition of such cubic carbides also increases the redness of the tool, helping to avoid thermal deformation of the tool during heavy-duty cutting or other machining where the cutting edge can create high temperatures. In addition, titanium carbide can provide nucleation sites during sintering, improving the uniformity of cubic carbide distribution in the workpiece.
    In general, alloy-type carbide grades have a hardness range of HRA91-94 and a transverse rupture strength of 150-300 ksi. Compared with the simple type, the wear resistance of the alloy type has poor wear resistance and low strength, but its bond wear resistance is better. Alloy grades can be divided into C5-C8 in the C grade system, and can be classified according to the P and M grade series in the ISO grade system. Alloy grades with intermediate properties can be classified as general grades (eg C6 or P30) for turning, tapping, planing and milling. The hardest grades can be classified as fine grades (eg C8 and P01) for finishing and boring. These grades typically have a smaller grain size and a lower cobalt content to achieve the desired hardness and wear resistance. However, similar material properties can be obtained by adding more cubic carbides. The most resilient grades can be classified as rough grades (eg C5 or P50). These grades typically have a medium size particle size and a high cobalt content, and the amount of cubic carbide added is also small to achieve the desired toughness by inhibiting crack propagation. In the interrupted turning process, the cutting performance can be further improved by using the cobalt-rich grade having a higher cobalt content on the surface of the cutter.
    Alloy grades with low titanium carbide content are used for machining stainless steel and malleable cast iron, but can also be used to process non-ferrous metals (such as nickel-based superalloys). These grades typically have a grain size of less than 1 μm and a cobalt content of 8% to 12%. Grades with higher hardness (eg M10) can be used for turning malleable cast iron; grades with better toughness (eg M40) can be used for milling and planing steel or for turning stainless steel or superalloys.
    Alloy-type carbide grades can also be used for non-metal cutting applications, primarily for the manufacture of wear-resistant parts. These grades typically have a particle size of 1.2-2 μm and a cobalt content of 7%-10%. In the production of these grades, a large proportion of recycled materials are usually added, resulting in higher cost-effectiveness in the application of wear parts. Wear parts require good corrosion resistance and high hardness. These grades can be obtained by adding nickel and chromium carbide when producing such grades.

News Center

Why is tungsten carbide used for cutting tools？

Release time：2022/03/23

Contact Details