On 2018-08-08 14:48:54
Cemented carbide materials
Cemented carbide is the most widely used tool material for high speed machining (HSM), which is produced by powder metallurgy process and consists of hard carbide (usually tungsten carbide WC) particles and soft metal bonds. At present, there are hundreds of WC-based cemented carbides with different compositions. Most of them use Cobalt (Co) as binder. Nickel (Ni) and chromium (Cr) are also commonly used binder elements. In addition, some other alloying elements can be added. Why are there so many cemented carbide brands? How does a tool manufacturer choose the right tool material for a particular cutting process? In order to answer these questions, let us first understand the characteristics of cemented carbide as an ideal tool material.
Various kinds of tungsten carbide powders, mixtures and metal binder content, types and dosages of grain growth inhibitors constitute various cemented carbide brands. These parameters will determine the microstructure and properties of cemented carbides. Some specific performance combinations have become the preferred choice for some specific processing purposes, which makes it meaningful to classify various cemented carbide grades.
The two most commonly used classification systems of cemented carbides for processing purposes are C and SIO respectively. Although neither of the two systems can fully reflect the material characteristics affecting the choice of cemented carbide grades, they provide a starting point for discussion. For each classification, many manufacturers have their own special brands, which results in a variety of cemented carbide brands.
Cemented carbide grades can also be classified according to their composition. Tungsten Carbide (WC) brand can be divided into three basic types: simple type, microcrystalline type and alloy type.
Simple brands are mainly composed of tungsten carbide and cobalt binders, but may also contain a small amount of grain growth inhibitors.
The microcrystalline type is composed of tungsten carbide and cobalt binder with vanadium carbide (VC) and/or chromium carbide (Cr3C2) added in a few thousandths. Its grain size can reach less than 1 m.
The alloy type is composed of tungsten carbide and cobalt binders containing a few percent of titanium carbide (TiC), tantalum carbide (TaC) and nickel carbide (NbC). These additives are also called cubic carbides because their sintered microstructures exhibit an inhomogeneous three-phase structure.
(1) Pure cemented carbide grades
Such brands used in metal cutting usually contain 3%-12% cobalt (weight ratio). The grain size range of tungsten carbide is usually between 1 m and 8 M. As with other brands, reducing the particle size of tungsten carbide can improve its hardness and transverse fracture strength (TRS), but reduce its toughness. The hardness of the simple type is usually between 89-93.5 HRA and the transverse fracture strength is between 1.2-2.4 MPa (175-350 ksi). Such brands of powder may contain a large number of recycled raw materials.
Simple brand can be divided into C1 - C4 in C brand system and K, N, S and H brand series in ISO brand system. Simple grades with intermediate characteristics can be classified as general grades (e.g. C2 or K20) and can be used for turning, milling, planer and radium cutting; brands with smaller grain size or lower cobalt content and higher hardness can be classified as finishing grades (e.g. C4 or K01); brands with larger grain size or higher cobalt content and better toughness can be classified as rough processing grades (e.g. C1 or K30).
Tools manufactured with simple grades can be used for cutting cast iron, 200 and 300 series stainless steel, aluminium and other non-ferrous metals, superalloys and hardened steels. Such brands can also be used in the field of non-metallic cutting (such as rock and geological drilling tools), and their grain size ranges from 1.5 to 10 m (or larger) and cobalt content ranges from 6% to 16%. Another non-metallic cutting use of simple cemented carbide grades is to manufacture dies and punches. These grades usually have medium grain sizes with cobalt content ranging from 16% to 30%.
(2) Brand of microcrystalline cemented carbide
Such brands usually contain 6% to 15% cobalt. In liquid phase sintering, the addition of vanadium carbide and/or chromium carbide can control the grain growth, thus obtaining fine grain structure with particle size less than 1 m. This fine grain grade has very high hardness and transverse fracture strength of more than 3.45 MPa (500 ksi). The combination of high strength and adequate toughness enables such brands of cutters to adopt larger positive rake angles, thereby reducing cutting forces and producing thinner chips by cutting rather than pushing metal materials.
Proper material properties can be obtained by strict quality identification of various raw materials in the production of brand cemented carbide powder and strict control of sintering process conditions to prevent abnormal large grains from forming in the microstructure of materials. In order to keep the grain size fine and uniform, recycled powder can be used only when the raw materials and recycling process can be fully controlled and extensive quality testing can be carried out.
Microcrystalline brand can be classified according to M Series in ISO brand system. Besides, other classification methods in C brand system and ISO brand system are the same as those in simple brand system. The microcrystalline brand can be used to manufacture tools for cutting softer workpiece materials because the surface of such tools can be machined very smoothly and the cutting edge can be kept extremely sharp.
Microcrystal cutters can also be used to process nickel-based superalloys because they can withstand cutting temperatures as high as 1200 degrees Celsius. For the processing of superalloys and other special materials, the wear resistance, deformation resistance and toughness can be improved simultaneously by using microcrystalline cutters and pure ruthenium-containing cutters. A rotary tool (such as a drill bit) that produces shear stress when the microcrystalline number is trapped. One kind of drill is made of composite cemented carbide. The cobalt content in the material is different at the specific part of the same drill, so the hardness and toughness of the drill are optimized according to the processing requirements.
(3) Brand of Alloy-type Cemented Carbide
This kind of brand is mainly used for cutting steel parts. Its cobalt content is usually 5%-10%, and its grain size is 0.8-2 M. The tendency of tungsten carbide (WC) diffusion to the surface of steel chips can be reduced by adding 4%-25% TiC. By adding TaC and NbC not exceeding 25%, the strength, wear resistance and heat shock resistance of the tool can be improved. Adding such cubic carbides can also improve the redness and hardness of the tool, and help avoid thermal deformation of the tool in other processes where heavy-duty cutting or cutting edge will produce high temperature. In addition, titanium carbide can provide nucleation position during sintering process and improve the uniformity of distribution of cubic carbide in workpiece.
Generally speaking, the hardness range of alloy type cemented carbide is 91-94 HRA, and the transverse fracture strength is 1-2 KPa (150-300 ksi). Compared with the pure type, the wear resistance of the alloy type is worse and the strength is lower, but the bond wear resistance of the alloy type is better. Alloy type grades can be classified into C5-C8 in C system and P and M Series in ISO system. Alloy type grades with intermediate characteristics can be classified as general grades (such as C6 or P30), which can be used in turning, tapping, planer and milling. The hardest grades can be classified as finishing grades (such as C8 and P01) for finishing and key cutting. These brands usually have smaller grain size and lower cobalt content to obtain the required hardness and wear resistance. However, similar material properties can be obtained by adding more cubic carbides. The toughest grades can be classified as rough processing grades (e.g. C5 or P50). These grades usually have medium grain size and high cobalt content, and the addition of cubic carbides is less to obtain the required toughness by inhibiting crack growth. In intermittent turning, the cutting performance can be further improved by using the cobalt-rich brand with high cobalt content on the surface of the tool.
Alloy grades with low TiC content are used for cutting stainless steel and malleable iron, but also for processing non-ferrous metals (such as nickel-based superalloys). The grain size of these brands is usually less than 1 m and the cobalt content is 8%-12%. The harder grades (such as M10) can be used for turning malleable cast iron, while the tougher grades (such as M40) can be used for milling and planed steel, or for turning stainless steel or superalloys.
Alloy cemented carbide brand can also be used for non-metal cutting purposes, mainly for the manufacture of wear-resistant parts. The particle size of these brands is usually 1.2~2 m, and the cobalt content is 7%~10%. In the production of these brands, a large proportion of recycled raw materials are usually added, thus achieving higher cost-effectiveness in the application of wear-resistant parts. Wear-resistant parts need high corrosion resistance and hardness, which can be obtained by adding nickel and chromium carbide in the production of such brands.
Cemented carbide powder is a key factor to meet the technical and economic requirements of tool manufacturers. Powder designed for the processing equipment and process parameters of tool manufacturers can ensure the performance of finished workpieces, resulting in hundreds of cemented carbide brands. The recyclability 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.
Cemented carbide workpieces can be formed by various processes. According to the size, shape complexity and production batch of the workpiece, most cutting blades are moulded by top and bottom pressure rigid dies. In order to maintain the consistency of the weight and size of the workpiece during each pressing, it is necessary to ensure that the amount of powder (mass and volume) flowing into the die cavity is exactly the same. The fluidity of powder is mainly controlled by the size distribution of aggregates and the characteristics of organic binders. By applying the forming pressure of 69-551.6 kPa (10-80 ksi, or 10-80 kilopounds/square foot) on the powder loaded into the die cavity, the die-pressing workpiece (or "blank") can be formed.
Hard tungsten carbide particles will not deform or break even under very high forming pressure, while organic binder is pressed into the gap between tungsten carbide particles, thus playing a role in fixing the position of particles. The higher the pressure, the tighter the bonding of tungsten carbide particles and the higher the pressure density of the workpiece. The moulding characteristics of grade cemented carbide powders may vary depending on the content of metal binder, the size and shape of tungsten carbide particles, the degree of formation of agglomerations, and the composition and addition of organic binders. In order to provide quantitative information about the compaction characteristics of brand cemented carbide powder, it is usually designed and constructed by powder producers to correspond the density of moulding to the forming pressure. This information ensures that the powder provided is consistent with the die-pressing process of the tool manufacturer.
Large size cemented carbide workpieces or high aspect ratio cemented carbide workpieces (such as end milling cutters and drill rods) are usually manufactured by balanced pressing of brand cemented carbide powder in a flexible material bag. Although the production cycle of the balanced pressing method is longer than that of the moulding method, the manufacturing cost of the cutting tool is lower, so this method is more suitable for the production of Xiaozao female.
This process is to put the powder into the bag and seal the mouth of the bag. Then the bag filled with powder is placed in a chamber. The pressure of 207-414 kPa (30-60 ksi) is applied by hydraulic device to suppress the powder. Pressed workpieces are usually processed into specific geometric shapes before sintering. The size of the bag is increased to accommodate the shrinkage of the workpiece during the compaction process and to provide sufficient margin for grinding. Because the workpiece is processed after pressing, the requirement of consistency of loading is not as strict as that of wood sample used for moulding, but it is still hoped that the same amount of powder will be filled in each bag. If the powder density is too small, it may lead to insufficient powder in the bag, resulting in small workpiece size and have to scrap. If the powder density is too high and the powder in the bag is too much, the workpiece needs to be processed to remove more powder after pressing. Although the removal of excess powder and scrap work can be recycled, but after all, this will reduce production efficiency.
Cemented carbide workpiece can also be processed by extrusion die or injection knitting. Extrusion moulding process is more suitable for mass production of axisymmetric workpieces, while injection moulding process is usually used for mass production of complex workpieces. In these two forming processes, the brand cemented carbide powder is suspended in the organic binder, which gives the cemented carbide mixture the same uniformity as toothpaste. The mixture is then extruded through a hole or injected into a cavity. The properties of grade cemented carbide powder determine the optimum ratio of powder to binder in the mixture, and have an important influence on the fluidity of the mixture through the extrusion hole or injection cavity.
When the workpiece is formed by die pressing, balanced pressing, extrusion die or injection molding, the organic binder must be removed from the workpiece before the final sintering stage. Vacuum sintering can remove the voids in the workpiece and make it completely (or basically) dense. During sintering, the metal binder in the compacted workpiece becomes liquid, but under the combined action of capillary force and particle linkage, the workpiece can still maintain its shape.
After sintering, the geometrical shape of the workpiece remains unchanged, but the size will be reduced. In order to obtain the required workpiece size after sintering, it is necessary to consider the shrinkage rate when designing the cutting tool. When designing the grade cemented carbide powder for each cutting tool, it is necessary to ensure that it has the correct shrinkage when pressed under appropriate pressure.
In almost all cases, sintered workpieces need to be treated after sintering. The most basic way to deal with cutting tools is to grind the cutting edge. Many tools need to be grinded after sintering. Some cutters need to grind the top and bottom; others need to grind peripherally (cutting edges need or need not be sharpened). All cemented carbide debris produced by grinding can be recycled.