Basic knowledge of carbide tool materials


Carbide is the most widely used class of high-speed machining (HSM) tool materials, which are produced by powder metallurgy processes and consist of hard carbide (usually tungsten carbide WC) particles and a softer metal bond composition. At present, there are hundreds of WC-based cemented carbides with different compositions, most of which use cobalt (Co) as a binder, nickel (Ni) and chromium (Cr) are also commonly used binder elements, and other can also be added. some alloying elements. Why are there so many carbide grades? How do tool manufacturers choose the right tool material for a specific cutting operation? To answer these questions, let’s first look at the various 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) is inherently very hard (more than corundum or alumina), and its hardness rarely decreases as operating temperature increases. However, it lacks sufficient toughness, an essential property for cutting tools. In order to take advantage of the high hardness of tungsten carbide and improve its toughness, people use metal bonds to bond tungsten carbide together, so that this material has a hardness far exceeding that of high-speed steel, while being able to withstand most cutting operations. cutting force. In addition, it can withstand the high cutting temperatures caused by high-speed machining.

Today, almost all WC-Co knives and inserts are coated, so the role of the base material seems less important. But in fact, it is the high elastic modulus of the WC-Co material (a measure of stiffness, which is about three times that of high-speed steel at room temperature) that provides the non-deformable substrate for the coating. The WC-Co matrix also provides the required toughness. These properties are the basic properties of WC-Co materials, but the material properties can also be tailored by adjusting the material composition and microstructure when producing cemented carbide powders. Therefore, the suitability of tool performance to a specific machining depends to a large extent on the initial milling process.

Milling 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 the raw material 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 stoichiometric value of 6.13% by weight). A small amount of vanadium and/or chromium may be added before the carburizing treatment in order to control the powder particle size through subsequent processes. Different downstream process conditions and different end processing uses require a specific combination of tungsten carbide particle size, carbon content, vanadium content and chromium content, through which a variety of different tungsten carbide powders can be produced. For example, ATI Alldyne, a tungsten carbide powder manufacturer, produces 23 standard grades of tungsten carbide powder, and the varieties of tungsten carbide powder customized according to user requirements can reach more than 5 times that of standard grades of tungsten carbide powder.

When mixing and grinding tungsten carbide powder and metal bond to produce a certain grade of cemented carbide powder, various combinations can be used. The most commonly used cobalt content is 3% – 25% (weight ratio), and in the case of needing to enhance the corrosion resistance of the tool, it is necessary to add nickel and chromium. In addition, the metal bond 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 content of binder can also improve the toughness of cemented carbide, but it will reduce its hardness.

Reducing the size of the tungsten carbide particles can increase the hardness of the material, but the particle size of the tungsten carbide must remain the same during the sintering process. During sintering, the tungsten carbide particles combine and grow through a process of dissolution and reprecipitation. In the actual sintering process, in order to form a fully dense material, the metal bond becomes 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 metal carbides are usually added when the tungsten carbide powder is mixed and milled with a metal bond, although vanadium carbide and chromium carbide can also be formed when the tungsten carbide powder is carburized.

Tungsten carbide powder can also be produced by using recycled waste cemented carbide materials. The recycling and reuse of scrap carbide has a long history in the cemented carbide industry and is an important part of the entire economic chain of the industry, helping to reduce material costs, save natural resources and avoid waste materials. Harmful disposal. Scrap cemented carbide can generally be reused by APT (ammonium paratungstate) process, zinc recovery process or by crushing. These “recycled” tungsten carbide powders generally have better, predictable densification because they have a smaller surface area than tungsten carbide powders made directly through the tungsten carburizing process.

The processing conditions of the mixed grinding of tungsten carbide powder and metal bond are also crucial process parameters. The two most commonly used milling techniques are ball milling and micromilling. Both processes enable uniform mixing of milled powders and reduced particle size. In order to make the later pressed workpiece have sufficient strength, maintain the shape of the workpiece, and enable the operator or manipulator to pick up the workpiece for operation, it is usually necessary to add an organic binder during grinding. The chemical composition of this bond can affect the density and strength of the pressed workpiece. To facilitate handling, it is advisable to add high strength binders, but this results in a lower compaction density and may produce lumps that can cause defects in the final product.

After milling, the powder is usually spray-dried to produce free-flowing agglomerates held together by organic binders. By adjusting the composition of the organic binder, the flowability and charge density of these agglomerates can be tailored as desired. By screening out coarser or finer particles, the particle size distribution of the agglomerate can be further tailored to ensure good flow when loaded into the mold cavity.

Workpiece manufacturing

Carbide workpieces can be formed by a variety of process methods. Depending on the size of the workpiece, the level of shape complexity, and the production batch, most cutting inserts are molded using top- and bottom-pressure rigid dies. In order to maintain the consistency of workpiece weight and size during each pressing, it is necessary to ensure that the amount of powder (mass and volume) flowing into the cavity is exactly the same. The fluidity of the powder is mainly controlled by the size distribution of the agglomerates and the properties of the organic binder. Molded workpieces (or “blanks”) are formed by applying a molding pressure of 10-80 ksi (kilo pounds per square foot) to the powder loaded into the mold cavity.

Even under extremely high molding pressure, the hard tungsten carbide particles will 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 compaction density of the workpiece. The molding properties of grades of cemented carbide powder may vary, depending on the content of metallic binder, the size and shape of the tungsten carbide particles, the degree of agglomeration, and the composition and addition of organic binder. In order to provide quantitative information about the compaction properties of grades of cemented carbide powders, the relationship between molding density and molding pressure is usually designed and constructed by the powder manufacturer. This information ensures that the powder supplied is compatible with the tool manufacturer’s molding process.

Large-sized carbide workpieces or carbide workpieces with high aspect ratios (such as shanks for end mills and drills) are typically manufactured from uniformly pressed grades of carbide powder in a flexible bag. Although the production cycle of the balanced pressing method is longer than that of the molding method, the manufacturing cost of the tool is lower, so this method is more suitable for small batch production.

This process method is to put the powder into the bag, and seal the bag mouth, and then put the bag full of powder in a chamber, and apply a pressure of 30-60ksi through a hydraulic device to press. Pressed workpieces are often machined to specific geometries prior to sintering. The size of the sack is enlarged to accommodate workpiece shrinkage during compaction and to provide sufficient margin for grinding operations. Since the workpiece needs to be processed after pressing, the requirements for the consistency of charging are not as strict as those of the molding method, but it is still desirable to ensure that the same amount of powder is loaded into the bag each time. If the charging density of the powder is too small, it may lead to insufficient powder in the bag, resulting in the workpiece being too small and having to be scrapped. If the loading density of the powder is too high, and the powder loaded into the bag is too much, the workpiece needs to be processed to remove more powder after it is pressed. Although the excess powder removed and scrapped workpieces can be recycled, doing so reduces productivity.

Carbide workpieces can also be formed using extrusion dies or injection dies. The extrusion molding process is more suitable for the mass production of axisymmetric shape workpieces, while the injection molding process is usually used for the mass production of complex shape workpieces. In both molding processes, grades of cemented carbide powder are suspended in an organic binder that imparts a toothpaste-like consistency to the cemented carbide mix. The compound is then either extruded through a hole or injected into a cavity to form. The characteristics of the grade of cemented carbide powder determine the optimum ratio of powder to binder in the mixture, and have an important influence on the flowability of the mixture through the extrusion hole or injection into the cavity.

After the workpiece is formed by molding, isostatic pressing, extrusion or injection molding, the organic binder needs to be removed from the workpiece before the final sintering stage. Sintering removes porosity from the workpiece, making it fully (or substantially) dense. During sintering, the metal bond in the press-formed workpiece becomes liquid, but the workpiece retains its shape under the combined action of capillary forces and particle linkage.

After sintering, the workpiece geometry remains the same, but the dimensions are reduced. In order to obtain the required workpiece size after sintering, the shrinkage rate needs to be considered when designing the tool. The grade of carbide powder used to make each tool must be designed to have the correct shrinkage when compacted under the appropriate pressure.

In almost all cases, post-sintering treatment of the sintered workpiece is required. The most basic treatment of cutting tools is to sharpen the cutting edge. Many tools require grinding of their geometry and dimensions after sintering. Some tools require top and bottom grinding; others require peripheral grinding (with or without sharpening the cutting edge). All carbide chips from grinding can be recycled.

Workpiece coating

In many cases, the finished workpiece needs to be coated. The coating provides lubricity and increased hardness, as well as a diffusion barrier to the substrate, preventing oxidation when exposed to high temperatures. The cemented carbide substrate is critical to the performance of the coating. In addition to tailoring the main properties of the matrix powder, the surface properties of the matrix can also be tailored by chemical selection and changing the sintering method. Through the migration of cobalt, more cobalt can be enriched in the outermost layer of the blade surface within the thickness of 20-30 μm relative to the rest of the workpiece, thereby giving the surface of the substrate better strength and toughness, making it more resistant to deformation.

Based on their own manufacturing process (such as dewaxing method, heating rate, sintering time, temperature and carburizing voltage), the tool manufacturer may have some special requirements for the grade of cemented carbide powder used. Some toolmakers may sinter the workpiece in a vacuum furnace, while others may use a hot isostatic pressing (HIP) sintering furnace (which pressurizes the workpiece near the end of the process cycle to remove any residues) pores). Workpieces sintered in a vacuum furnace may also need to be hot isostatically pressed through an additional process to increase the density of the workpiece. Some tool manufacturers may use higher vacuum sintering temperatures to increase the sintered density of mixtures with lower cobalt content, but this approach may coarsen their microstructure. In order to maintain a fine grain size, powders with smaller particle size of tungsten carbide can be selected. In order to match the specific production equipment, the dewaxing conditions and carburizing voltage also have different requirements for the carbon content in the cemented carbide powder.

Grade classification

Combination changes of different types of tungsten carbide powder, mixture composition and metal binder content, type and amount of grain growth inhibitor, etc., constitute a variety of cemented carbide grades. These parameters will determine the microstructure of the cemented carbide and its properties. Some specific combinations of properties have become the priority for some specific processing applications, making it meaningful to classify various cemented carbide grades.

The two most commonly used carbide classification systems for machining applications are the C designation system and the ISO designation 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, 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 alloyed. Simplex grades consist primarily of tungsten carbide and cobalt binders, but may also contain small amounts of grain growth inhibitors. The microcrystalline grade is composed of tungsten carbide and cobalt binder added with several thousandths of vanadium carbide (VC) and (or) chromium carbide (Cr3C2), and its grain size can reach 1 μm or less. Alloy grades are composed of tungsten carbide and cobalt binders containing a few percent titanium carbide (TiC), tantalum carbide (TaC), and niobium carbide (NbC). These additions are also known as cubic carbides because of their sintering properties. The resulting microstructure exhibits an inhomogeneous three-phase structure.

1) Simple carbide grades

These grades for metal cutting usually contain 3% to 12% cobalt (by weight). The size range of tungsten carbide grains is usually between 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 the pure type is usually between HRA89-93.5; the transverse rupture strength is usually between 175-350ksi. Powders of these grades may contain large quantities of recycled materials.

The simple type 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. Simplex grades with intermediate properties can be classified as general-purpose grades (such as C2 or K20) and can be used for turning, milling, planing and boring; grades with smaller grain size or lower cobalt content and higher hardness can be Classified as finishing grades (such as C4 or K01); grades with larger grain size or higher cobalt content and better toughness can be classified as roughing grades (such as C1 or K30).

Tools made in Simplex grades can be used for machining cast iron, 200 and 300 series stainless steel, aluminum and other non-ferrous metals, superalloys and hardened steels. These grades can also be used in non-metal cutting applications (eg as rock and geological drilling tools), and these grades have a grain size range of 1.5-10μm (or larger) and a cobalt content of 6%-16%. Another non-metal cutting use of simple carbide grades is in the manufacture of dies and punches. These grades typically have a medium grain size with a cobalt content of 16%-30%.

(2) Microcrystalline cemented carbide grades

Such grades usually contain 6%-15% cobalt. During liquid phase sintering, the addition of vanadium carbide and/or chromium carbide can control the grain growth to obtain a fine grain structure with a particle size of less than 1 μm. This fine-grained grade has very high hardness and transverse rupture strengths above 500ksi. The combination of high strength and sufficient toughness allows these grades to use a larger positive rake angle, which reduces cutting forces and produces thinner chips by cutting rather than pushing the metal material.

Through strict quality identification of various raw materials in the production of grades of cemented carbide powder, and strict control of sintering process conditions to prevent the formation of abnormally large grains in the material microstructure, it is possible to obtain appropriate material properties. In order to keep the grain size small and uniform, recycled recycled powder should only be used if there is full control of the raw material and recovery process, and extensive quality testing.

The microcrystalline grades can be classified according to the M grade series in the ISO grade system. In addition, other classification methods in the C grade system and the ISO grade system are the same as the pure grades. Microcrystalline grades can be used to make tools that cut softer workpiece materials, because the surface of the tool can be machined very smooth and can maintain an extremely sharp cutting edge.

Microcrystalline grades can also be used to machine nickel-based superalloys, as they can withstand cutting temperatures of up to 1200°C. For the processing of superalloys and other special materials, the use of 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 the manufacture of rotating tools such as drills that generate shear stress. There is a drill made of composite grades of cemented carbide. In specific parts of the same drill, the cobalt content in the material varies, so that the hardness and toughness of the drill are optimized according to processing needs.

(3) Alloy type cemented carbide grades

These grades are mainly used for cutting steel parts, and their cobalt content is usually 5%-10%, and the grain size ranges from 0.8-2μm. By adding 4%-25% titanium carbide (TiC), the tendency of tungsten carbide (WC) to diffuse to the surface of the steel chips can be reduced. Tool strength, crater wear resistance and thermal shock resistance can be improved by adding up to 25% tantalum carbide (TaC) and niobium carbide (NbC). The addition of such cubic carbides also increases the red hardness of the tool, helping to avoid thermal deformation of the tool in heavy cutting or other operations where the cutting edge will generate high temperatures. In addition, titanium carbide can provide nucleation sites during sintering, improving the uniformity of cubic carbide distribution in the workpiece.

Generally speaking, the hardness range of alloy-type cemented carbide grades is HRA91-94, and the transverse fracture strength is 150-300ksi. Compared with pure grades, alloy grades have poor wear resistance and lower strength, but have better resistance to adhesive wear. 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 purpose grades (such as C6 or P30) and can be used for turning, tapping, planing and milling. The hardest grades can be classified as finishing grades (such as C8 and P01) for finishing turning and boring operations. These grades typically have smaller grain sizes and lower cobalt content to obtain the required hardness and wear resistance. However, similar material properties can be obtained by adding more cubic carbides. Grades with the highest toughness can be classified as roughing grades (eg C5 or P50). These grades typically have a medium grain size and high cobalt content, with low additions of cubic carbides to achieve the desired toughness by inhibiting crack growth. In interrupted turning operations, the cutting performance can be further improved by using the above-mentioned cobalt-rich grades with higher cobalt content on the tool surface.

Alloy grades with a lower titanium carbide content are used for machining stainless steel and malleable iron, but can also be used for machining non-ferrous metals such as nickel-based superalloys. The grain size of these grades is usually less than 1 μm, and the cobalt content is 8%-12%. Harder grades, such as M10, can be used for turning malleable iron; tougher grades, such as M40, can be used for milling and planing steel, or for turning stainless steel or superalloys.

Alloy-type cemented carbide grades can also be used for non-metal cutting purposes, mainly for the manufacture of wear-resistant parts. The particle size of these grades is usually 1.2-2 μm, and the cobalt content is 7%-10%. When producing these grades, a high percentage of recycled raw material is usually added, resulting in a high cost-effectiveness in wear parts applications. Wear parts require good corrosion resistance and high hardness, which can be obtained by adding nickel and chromium carbide when producing these grades.

In order to meet the technical and economical requirements of tool manufacturers, carbide powder is the key element. Powders designed for tool manufacturers’ machining equipment and process parameters ensure the performance of the finished workpiece and have resulted in hundreds of carbide grades. The recyclable nature of carbide materials and the ability to work directly with powder suppliers allows toolmakers to effectively control their product quality and material costs.

Post time: Oct-18-2022