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.
(1) Simple carbide grades
Such grades for metal cutting usually contain 3% to 12% cobalt (by weight). The size range of tungsten carbide grains is usually between 1 and 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 89 and 93.5HRA; the transverse rupture strength is usually between 1.2 and 2.4MPa (175 to 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 wok machining; 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 (such as rock and geological drilling tools), and these grades have a grain size range of 1.5 to 10 µm (or larger) and a cobalt content of 6% to 16%. Another non-metal-cutting use for simple carbide grades is in the manufacture of dies and punches. These grades typically have a medium grain size and a cobalt content of 16% to 30%.
(2) Microcrystalline cemented carbide grades
Such grades usually contain 6% to 15% cobalt. During liquid phase sintering, the addition of vanadium carbide and/or chromium carbide can control the grain growth, resulting in a fine grain structure with a grain size of less than 1µm. This fine grained grade has very high hardness and transverse rupture strength above 3.45MPa (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, except that 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 trapped in 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% to 10%, and the grain size ranges from 0.8 to 2µm. By adding 4% to 25% of titanium carbide (TiC), the tendency of tungsten carbide (WC) to diffuse to the surface of steel chips can be reduced. The strength, crater wear resistance and thermal shock resistance of the tool 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 91-94HRA, and the transverse rupture strength is 1-2KPa (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 and key cutting. 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. The toughest grades 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 1µm and the cobalt content is 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, 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. These grades typically have a particle size of 1.2 to 2 µm and a cobalt content of 7 to 10 percent. 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 high corrosion resistance and high hardness, which can be obtained by adding nickel and chromium carbide when producing these grades.
