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Cemented Carbide Cutting Tool Material Basics Summary
Cemented carbide is the most widely used class of high-speed machining (HSM) tool materials, produced by powder metallurgy processes, consisting of hard carbide (usually tungsten carbide, WC) particles and a softer metal binder. Currently, there are hundreds of different compositions of WC-based cemented carbides, most of which use cobalt (Co) as the binder, with nickel (Ni) and chromium (Cr) also commonly used as binder elements, and other alloy elements can be added as well. Why are there so many grades of cemented carbide? How do tool manufacturers choose the right tool material for a specific machining process? To answer these questions, let’s first understand the various characteristics that make cemented carbides an ideal tool material.
Hardness and Toughness
WC-Co cemented carbides have unique advantages in combining hardness and toughness. Tungsten carbide (WC) itself has very high hardness (surpassing alumina or aluminum oxide) and its hardness does not decrease significantly when the working temperature rises. However, it lacks sufficient toughness, which is essential for cutting tools. To utilize the high hardness of tungsten carbide and improve its toughness, metal binders are used to bond tungsten carbide together, making this material much harder than high-speed steel while being able to withstand the cutting forces in most machining processes. Moreover, it can withstand the high temperatures produced by high-speed machining.
Today, almost all WC-Co tools and blades are coated, so the role of the base material seems less important. However, in reality, it is the high elastic modulus of WC-Co material (a measure of stiffness, about three times that of high-speed steel at room temperature) that provides a non-deforming substrate for the coating. The WC-Co base also provides the necessary toughness. These properties are fundamental to WC-Co materials but can also be customized by adjusting the material composition and microstructure during the production of the cemented carbide powder. Therefore, tool performance and suitability for specific machining largely depend on the initial powder-making process.
Powder-Making Process
ungsten carbide powder is obtained by carburizing tungsten (W) powder. The characteristics of tungsten carbide powder (especially its particle size) depend mainly on the particle size of the raw tungsten powder and the temperature and duration of the carburization. Chemical control is also crucial, with carbon content needing to be constant (close to the theoretical ratio of 6.13% by weight). To control powder granularity in subsequent processes, small amounts of vanadium and/or chromium can be added before carburization. Different downstream process conditions and different final applications require specific combinations of tungsten carbide granularity, carbon content, vanadium content, and chromium content. By varying these combinations, various tungsten carbide powders can be produced. For example, the tungsten carbide powder producer ATI Alldyne produces 23 standard grades of tungsten carbide powder, and the variety of custom-made tungsten carbide powders can be more than five times the number of standard grades.
In the production of a certain grade of cemented carbide powder by mixing tungsten carbide powder with a metal binder, various combinations can be used. The most common cobalt content ranges from 3% to 25% (by weight), and nickel and chromium are added when enhanced corrosion resistance of the tool is needed. Furthermore, the metal binder can be further modified by adding other alloy elements. For example, adding ruthenium in WC-Co cemented carbides can significantly improve their toughness without reducing their hardness. Increasing the binder content can also enhance the toughness of cemented carbides, but at the expense of lowering their hardness.
Reducing the size of tungsten carbide particles can increase the hardness of the material, but during the sintering process, the size of tungsten carbide particles must be maintained. During sintering, tungsten carbide particles are bonded and grow through a dissolution-reprecipitation process. In actual sintering, to form a completely dense material, the metal binder becomes liquid (known as liquid-phase sintering). Adding other transition metal carbides, including vanadium carbide (VC), chromium carbide (Cr3C2), titanium carbide (TiC), tantalum carbide (TaC), and niobium carbide (NbC), can control the growth rate of tungsten carbide particles. These metal carbides are typically added during the mixing and grinding of tungsten carbide powder with the metal binder, although vanadium carbide and chromium carbide can also be formed during the carburization of tungsten carbide powder.
Recycled waste cemented carbide materials can also be used to produce graded tungsten carbide powders. The recycling and reuse of waste cemented carbides have a long history in the cemented carbide industry, being an essential part of the entire economic chain of the industry. It helps reduce material costs, conserve natural resources, and avoid the need for harmless disposal of waste materials. Waste cemented carbides are generally processed using the ammonium paratungstate (APT) process, zinc recovery process, or by reusing them after crushing. These “regenerated” tungsten carbide powders typically have better and more predictable density because their surface area is smaller than that of tungsten carbide powders directly produced through the tungsten carburization process.
The processing conditions for mixing and grinding tungsten carbide powder with metal binders are also critical process parameters. The two most common grinding techniques are ball milling and ultrafine grinding. Both processes can uniformly mix the ground powder and reduce particle size. To ensure that the subsequently pressed workpieces have sufficient strength to maintain their shape and allow operators or robotic arms to pick up the workpieces, an organic binder is usually added during grinding. The chemical composition of this binder can affect the density and strength of the pressed workpieces. To facilitate handling, it is best to add a high-strength binder, but this can lead to lower pressing density and may produce hard lumps, causing defects in the final product.
After grinding, the powder is usually spray-dried, producing free-flowing clumps held together by the organic binder. By adjusting the composition of the organic binder, these clumps can be customized for flowability and packing density as needed. By screening out coarser or finer particles, the particle size distribution of the clumps can be further customized to ensure good flowability when loaded into the mold cavity.
Workpiece Manufacturing
Cemented carbide workpieces can be formed using multiple process methods. Depending on the size, complexity of shape, and production volume of the workpieces, most cutting blades are formed using top and bottom pressing rigid mold pressing. During each pressing, to maintain consistency in the weight and size of the workpieces, the amount of powder flowing into the mold cavity (in terms of mass and volume) must be exactly the same. The flowability of the powder is mainly controlled by the size distribution of the clumps and the properties of the organic binder. By applying a forming pressure of 10-80 ksi (thousands of pounds per square inch) to the powder loaded into the mold cavity, a pressed workpiece (or “blank”) can be formed.
Even under extremely high forming pressures, the hard tungsten carbide particles do not deform or break, but the organic binder is pressed into the gaps between the tungsten carbide particles, thereby fixing the position of the particles. The higher the pressure, the tighter the bonding of the tungsten carbide particles, and the greater the pressing density of the workpiece. The pressing characteristics of graded cemented carbide powders may vary depending on the content of the metal binder, the size and shape of the tungsten carbide particles, the degree of clumping, and the composition and amount of the organic binder added. To provide quantitative information on the pressing characteristics of graded cemented carbide powders, it is usually the responsibility of the powder manufacturer to design and construct the relationship between pressing density and forming pressure. This information ensures that the provided powder is compatible with the tool manufacturer’s pressing process.
Large-sized cemented carbide workpieces or those with a high aspect ratio (such as the shanks of milling cutters and
drills) are usually manufactured using graded cemented carbide powders pressed in a flexible bag. Although the production cycle of the balanced pressing method is longer than that of the mold pressing method, the manufacturing cost
of the tool is lower, making this method more suitable for small batch production.
This process involves loading the powder into a bag, sealing the bag, and then placing the filled bag in a chamber where a hydraulic device applies a pressure of 30-60 ksi. The pressed workpieces usually need to be machined into specific geometric shapes before sintering. The size of the bag is increased to accommodate the shrinkage of the workpiece during the pressing process and to provide enough margin for grinding operations. Since the workpieces need to be machined after pressing, the requirements for loading consistency are not as strict as those for mold pressing, but it is still desirable to ensure that the same amount of powder is loaded into the bag each time. If the packing density of the powder is too low, it may result in insufficient powder being loaded into the bag, causing the workpiece to be undersized and thus scrapped. If the packing density of the powder is too high, too much powder may be loaded into the bag, requiring more powder to be removed during machining after the workpiece is pressed. Although the excess removed powder and scrapped workpieces can be recycled, doing so ultimately reduces production efficiency.
Cemented carbide workpieces can also be formed using extrusion molds or injection molds. The extrusion molding process is more suitable for mass production of axially symmetric workpieces, while the injection molding process is typically used for mass production of complex-shaped workpieces. In both forming processes, graded cemented carbide powder is suspended in an organic binder, giving the cemented carbide mixture a uniform consistency similar to toothpaste. Then, the mixture is either extruded through a hole to form a shape or injected into a mold cavity to form. The characteristics of the graded cemented carbide powder determine the optimal ratio of powder to binder in the mixture and have a significant impact on the flowability of the mixture through the extrusion hole or into the mold cavity.
After the workpieces are formed by mold pressing, balanced pressing, extrusion molding, or injection molding, the organic binder must be removed from the workpieces before the final sintering stage. Sintering removes the pores in the workpiece, making it completely (or almost completely) dense. During sintering, the metal binder in the pressed workpiece becomes liquid, but the workpiece still retains its shape due to the combined action of capillary forces and particle contact.
After sintering, the geometric shape of the workpiece remains unchanged, but the size is reduced. To achieve the required workpiece size after sintering, it is necessary to consider its shrinkage rate when designing the tool. When designing the graded cemented carbide powder used to manufacture each tool, it must be ensured that it has the correct shrinkage rate under appropriate pressure.
In almost all cases, post-sintering treatment of the workpiece is required. The most basic treatment for cutting tools is grinding the cutting edges. Many tools also need to be ground to adjust their geometric shape and size after sintering. Some tools require grinding of the top and bottom surfaces; others require peripheral grinding (with or without grinding the cutting edges). All the cemented carbide swarf produced by grinding can be recycled.
Workpiece Coating
In many cases, the finished workpiece needs to be coated. Coatings can provide lubrication and increase hardness, as well as provide a diffusion barrier to prevent oxidation when the substrate is exposed to high temperatures. The cemented carbide substrate is crucial for the performance of the coating. In addition to customizing the main properties of the substrate powder, the surface properties of the substrate can also be customized through chemical selection and changes in the sintering method. By migrating cobalt, the outermost layer of the blade surface can be enriched with more cobalt than the rest of the workpiece, providing the substrate surface layer with better toughness and resistance to deformation.
Tool manufacturers, based on their own manufacturing processes (such as dewaxing methods, heating rates, sintering times, temperatures, and carburizing voltages), may have special requirements for the graded cemented carbide powders they use. Some tool manufacturers may sinter workpieces in a vacuum furnace, while others may use hot isostatic pressing (HIP) sintering furnaces (which only apply pressure to the workpiece near the end of the process cycle to eliminate any residual pores). Workpieces sintered in a vacuum furnace may also require additional hot isostatic pressing to increase the density of the workpiece. Some tool manufacturers may use higher vacuum sintering temperatures to improve the sintering density of mixtures with lower cobalt content, but this method may cause the microstructure to become coarser. To maintain a fine grain size, powder with smaller tungsten carbide particle sizes can be selected. Dewaxing conditions and carburizing voltages also have different requirements for the carbon content in cemented carbide powders to match specific production equipment.
All these factors have a crucial impact on the microstructure and material performance of sintered cemented carbide tools, so close communication between tool manufacturers and powder suppliers is necessary to ensure that graded cemented carbide powders are customized according to the tool manufacturer’s production process. Therefore, it is not surprising that there are hundreds of different grades of cemented carbide powders. For example, ATI Alldyne produces more than 600 different powder grades, each specifically designed for target users and specific applications.
Grade Classification
The combination of different types of tungsten carbide powder, mixture components, and metal binder content, as well as the type and amount of grain growth inhibitors, results in a wide variety of cemented carbide grades. These parameters determine the microstructure of the cemented carbide and its properties. Some specific combinations of properties have become the preferred choice for certain machining applications, making it meaningful to classify the various grades of cemented carbides.
The two most commonly used classification systems for machining applications are the C grade system and the ISO grade system. Although neither system fully reflects the material properties that influence the choice of cemented carbide grades, they provide a starting point for discussion. For each classification method, many manufacturers have their own special grades, resulting in a wide variety of different cemented carbide grades.
Cemented carbide grades can also be classified by composition. Tungsten carbide (WC) grades can be divided into three basic types: pure, microcrystalline, and alloy. Pure grades mainly consist of tungsten carbide and cobalt binder, but may also contain small amounts of grain growth inhibitors. Microcrystalline grades consist of tungsten carbide and a cobalt binder added with a few thousandths of vanadium carbide (VC) and/or chromium carbide (Cr3C2), with a grain size of less than 1μm. Alloy grades consist of tungsten carbide and a cobalt binder containing a few percent of titanium carbide (TiC), tantalum carbide (TaC), and niobium carbide (NbC), which are also known as cubic carbides because their sintered microstructure exhibits an uneven three-phase structure.
- Pure Cemented Carbide Grades
These grades are typically used for metal cutting and usually contain 3% to 12% cobalt (by weight). The size of tungsten carbide grains typically ranges from 1 to 8μm. Like other grades, reducing the size of tungsten carbide grains can increase their hardness and transverse rupture strength (TRS), but it reduces their toughness. The hardness of pure grades usually ranges from HRA89 to 93.5; transverse rupture strength usually ranges from 175 to 350ksi. These grades may contain a large amount of recycled material.
Pure grades in the C grade system can be classified as C1 to C4, and in the ISO grade system, they can be classified under the K, N, S, and H grade series. Grades with intermediate properties can be classified as general-purpose grades (such as C2 or K20), suitable for turning, milling, planing, and boring operations; grades with smaller grain sizes or lower cobalt content and higher hardness can be classified as finishing grades (such as C4 or K01); grades with larger grain sizes or higher cobalt content and better toughness can be classified as roughing grades (such as C1 or K30).
Tools made with pure grades can be used for cutting cast iron, 200 and 300 series stainless steel, aluminum and other non-ferrous metals, high-temperature alloys, and hardened steel. These grades are also used in non-metal cutting applications (such as rock and geological drilling tools), with grain sizes ranging from 1.5 to 10μm (or larger) and cobalt contents of 6% to 16%. Another non-metal cutting application for pure grades is the manufacture of molds and punches, which typically have medium-sized grains and cobalt contents of 16% to 30%.
- Microcrystalline Cemented Carbide Grades
These grades typically contain 6% to 15% cobalt. During liquid-phase sintering, the addition of vanadium carbide and/or chromium carbide controls grain growth, resulting in a fine-grained structure with a grain size of less than 1μm. These microcrystalline grades have very high hardness and a transverse rupture strength of over 500ksi. The combination of high strength and sufficient toughness allows tools made with these grades to use larger positive rake angles, reducing cutting forces and producing thinner chips by cutting rather than pushing metal materials.
By strictly identifying the quality of various raw materials in the production of graded cemented carbide powders and strictly controlling the sintering process conditions, it is possible to obtain appropriate material properties while preventing the formation of abnormally large grains in the material microstructure. To maintain small and uniform grain sizes, recycled regenerated powders can only be used when comprehensive control over raw materials and recycling processes is possible, along with extensive quality testing.
Microcrystalline grades can be classified under the M grade series in the ISO grade system, and other classification methods in the C grade system and ISO grade system are the same as for pure grades. Microcrystalline grades can be used to manufacture tools for cutting softer workpiece materials, because these tools can produce very smooth surfaces and maintain extremely sharp cutting edges.
Microcrystalline grade tools can also be used to process nickel-based superalloys, as these tools can withstand cutting temperatures up to 1200°C. For processing high-temperature alloys and other special materials, using microcrystalline grade tools and pure grade tools containing ruthenium can simultaneously improve their wear resistance, deformation resistance, and toughness. Microcrystalline grades are also suitable for manufacturing rotary cutting tools (such as drills) that generate shear stress. A type of drill is made with a composite grade of cemented carbide, with different cobalt contents in specific parts of the same drill, optimizing the drill’s hardness and toughness according to processing needs.
- Alloy Cemented Carbide Grades
These grades are primarily used for cutting steel parts, typically containing 5% to 10% cobalt, with a grain size range of 0.8 to 2μm. By adding 4% to 25% titanium carbide (TiC), the tendency of tungsten carbide (WC) to diffuse to the surface of steel chips can be reduced. Adding up to 25% tantalum carbide (TaC) and niobium carbide (NbC) can improve the strength, resistance to crescent wear, and thermal shock resistance of the tool. Adding such cubic carbides also enhances the red hardness of the tool, which is beneficial in heavy-load cutting or other machining where high temperatures are generated at the cutting edge, helping to prevent thermal deformation of the tool. Additionally, titanium carbide provides nucleation sites during sintering, improving the uniform distribution of cubic carbides in the workpiece.
Generally, alloy cemented carbide grades have a hardness range of HRA91 to 94 and a transverse rupture strength of 150 to 300ksi. Compared to pure grades, alloy grades have poorer performance against abrasive wear and lower strength, but better resistance to adhesive wear. Alloy grades can be classified under the C5 to C8 grades in the C grade system and under the P and M grade series in the ISO grade system. Grades with intermediate properties can be classified as general-purpose grades (such as C6 or P30), suitable for turning, tapping, planing, and milling operations. The highest hardness grades can be classified as finishing grades (such as C8 and P01), used for precision turning and boring operations. These grades typically have smaller grain sizes and lower cobalt content to achieve the required hardness and wear resistance. However, similar material properties can also be obtained by adding a larger amount of cubic carbides. The toughest grades can be classified as roughing grades (such as C5 or P50). These grades typically have medium-sized grains and high cobalt content, with a smaller amount of cubic carbides added, achieving the required toughness by inhibiting crack propagation. In interrupted turning operations, using tools with surfaces that have a higher cobalt content can further improve cutting performance.
Lower titanium carbide content alloy grades are used for cutting stainless steel and malleable cast iron but can also be used for machining non-ferrous metals (such as nickel-based superalloys). These grades typically have grain sizes of less than 1μm and cobalt contents of 8% to 12%. Harder grades (such as M10) can be used for turning malleable cast iron; tougher grades (such as M40) can be used for milling and planing steel parts, or for turning stainless steel or superalloys.
Alloy cemented carbide grades can also be used for non-metal cutting applications, primarily for manufacturing wear-resistant parts. These grades typically have grain sizes of 1.2 to 2μm and cobalt contents of 7% to 10%. When producing these grades, a large proportion of recycled materials is usually added, achieving higher cost-effectiveness in applications for wear-resistant parts. Wear-resistant parts require good corrosion resistance and high hardness, which can be achieved by adding nickel and chromium carbide during the production of these grades.
conclusion
To meet the dual requirements of technical and economic considerations for tool manufacturers, cemented carbide powders are a key element. Powder designed for tool manufacturers’ processing equipment and process parameters ensures the performance of finished workpieces, leading to the emergence of hundreds of cemented carbide grades. The recyclable nature of cemented carbide materials and the ability to cooperate directly with powder suppliers enable tool manufacturers to effectively control their product quality and material costs.
