The material properties of hard alloys

Material Properties

Cemented carbide is a powder metallurgy product mainly composed of micrometer-sized powder of high-hardness and refractory metal carbides (such as WC, TiC) as the primary components, with cobalt (Co) or nickel (Ni), molybdenum (Mo) serving as binders. It is sintered in a vacuum furnace or hydrogen reduction furnace.

The carbides, nitrides, and borides of Group IVB, VB, and VIB metals are collectively referred to as cemented carbides due to their exceptionally high hardness and melting points. The following focuses on carbides to illustrate the structure, characteristics, and applications of cemented carbides.

In the metal-type carbides formed by Group IVB, VB, and VIB metals with carbon, due to the small atomic radius of carbon, carbon atoms can fill the voids in the metal lattice while retaining the original metal lattice structure, forming an interstitial solid solution. Under appropriate conditions, such solid solutions can continue to dissolve their constituent elements until saturation. Therefore, their composition can vary within a certain range (for example, the composition of titanium carbide varies between TiC0.5 and TiC), and the chemical formula does not conform to the valence rules. When the dissolved carbon content exceeds a certain limit (for example, in titanium carbide where the ratio of Ti to C is 1:1), the lattice type changes, transforming the original metal lattice into another form of metal lattice. This interstitial solid solution is called an interstitial compound.

Metal-type carbides, especially those of Group IVB, VB, and VIB metals, have melting points above 3273K, with hafnium carbide and tantalum carbide reaching 4160K and 4150K, respectively, which are among the highest known melting points of any substance. Most carbides are extremely hard, with microhardness values exceeding 1800 kg·mm² (microhardness is one of the methods for expressing hardness, commonly used for cemented carbides and hard compounds; a microhardness of 1800 kg·mm² is equivalent to a Mohs hardness of 9, comparable to diamond). Many carbides are resistant to decomposition at high temperatures and have stronger oxidation resistance than their constituent metals. Among all carbides, titanium carbide exhibits the best thermal stability and is a very important metal-type carbide. However, in an oxidizing atmosphere, all carbides are susceptible to oxidation at high temperatures, which can be considered a major weakness of carbides.

In addition to carbon atoms, nitrogen and boron atoms can also enter the voids in the metal lattice, forming interstitial solid solutions. They share similar properties with interstitial carbides, such as electrical and thermal conductivity, high melting points, high hardness, and brittleness.

The matrix of cemented carbide consists of two parts: one is the hardening phase, and the other is the binder metal.

The hardening phase is composed of carbides of transition metals from the periodic table, such as tungsten carbide, titanium carbide, and tantalum carbide. They have high hardness and melting points above 2000°C, with some even exceeding 4000°C. Additionally, nitrides, borides, and silicides of transition metals exhibit similar properties and can also serve as the hardening phase in cemented carbides. The presence of the hardening phase determines the alloy's extremely high hardness and wear resistance.

Cemented carbide has different requirements for the particle size of tungsten carbide (WC) depending on the intended application. For cutting tools made of cemented carbide, such as lead cutting machine blades and V-CUT blades, fine, sub-fine, or ultra-fine WC particles are used for precision machining, medium-sized particles for rough machining, and medium to coarse particles for heavy cutting and gravity cutting. For mining tools, coarse WC particles are used for rocks with high hardness and high impact loads, while medium-sized particles are used for rocks with low impact loads. For wear-resistant parts, when emphasis is placed on wear resistance, compression resistance, and surface finish, ultra-fine, sub-fine, fine, or medium WC particles are used, while medium to coarse particles are primarily used for impact-resistant tools.

The theoretical carbon content of WC is 6.128% (50% by atom). When the carbon content of WC exceeds the theoretical value, free carbon (WC+C) appears in WC. The presence of free carbon causes the surrounding WC grains to grow during sintering, resulting in uneven grain size in the cemented carbide. Tungsten carbide generally requires a high combined carbon content (≥6.07%) and a low free carbon content (≤0.05%). The total carbon content depends on the production process and application range of the cemented carbide.

Under normal circumstances, the total carbon content of WC for vacuum sintering using the paraffin wax process primarily depends on the oxygen content in the compressed block before sintering. Each part of oxygen requires an additional 0.75 parts of carbon, i.e., the total carbon content of WC = 6.13% + oxygen content% × 0.75 (assuming a neutral atmosphere in the sintering furnace; in reality, most vacuum furnaces have a carburizing atmosphere, so the actual total carbon content of WC used is less than the calculated value).

Currently, the total carbon content of WC in China is roughly divided into three categories: approximately 6.18±0.03% for WC used in vacuum sintering with the paraffin wax process (with increased free carbon), 6.13±0.03% for WC used in hydrogen sintering with the paraffin wax process, and 5.90±0.03% for WC used in hydrogen sintering with the rubber process. Sometimes, these processes are used interchangeably, so the total carbon content of WC should be determined based on specific circumstances.

Small adjustments can be made to the total carbon content of WC used in alloys with different application ranges, cobalt (Co) contents, and grain sizes. For low-cobalt alloys, WC with a slightly higher total carbon content can be selected, while for high-cobalt alloys, WC with a slightly lower total carbon content can be chosen. In summary, the specific requirements for the particle size of tungsten carbide in cemented carbides vary depending on the intended use.

The binder metal is typically an iron-group metal, with cobalt and nickel being commonly used.

When manufacturing cemented carbide, the raw material powders with a particle size of 1 to 2 micrometers and high purity are selected. The raw materials are mixed in proportion, added to alcohol or another medium, and wet-milled in a wet ball mill to ensure thorough mixing and pulverization. After drying and screening, a molding agent such as wax or resin is added, followed by additional drying and screening to obtain the mixed material. Then, the mixed material is granulated, pressed into shape, and heated to near the melting point of the binder metal (1300 to 1500°C), at which point the hardening phase and binder metal form a eutectic alloy. After cooling, the hardening phase is distributed within a network formed by the binder metal, tightly interconnected to form a sturdy whole. The hardness of cemented carbide depends on the content and grain size of the hardening phase; that is, the higher the hardening phase content and the finer the grain size, the greater the hardness. The toughness of cemented carbide is determined by the binder metal; the higher the binder metal content, the greater the bending strength. SGS Cemented Carbide End Mill.