"Super carbide" was first led by senior innovative entrepreneurs in the field of refractory metals and hard materials in China, leaders in key new materials in the 15th Five-Year Plan, Ms. Qin Hanmei, founder of Hunan Rheniumet  ltd, and innovatively proposed in 2021, which is a new high-performance cemented carbide material development concept with excellent properties such as high strength, high temperature and high hardness. To understand what is a super cemented carbide material, first analyze and understand the differences, complementarities and logical correlations between the two materials of "super alloy" and "cemented carbide" in specific application fields.

Superalloy

Superalloy refers to a class of alloy materials based on iron, nickel and cobalt, with other refractory metal components, which can work for a long time at high temperatures above 700 ° C (even 1000 ° C) and under certain stresses. Superalloy has excellent high temperature strength, good oxidation and thermal corrosion resistance, good fatigue performance, fracture toughness and other comprehensive properties, so it is called "super alloy", mainly used in the aerospace drive field and energy field.

Oxidation and corrosion are the weakness of metal materials. Under high temperature conditions, the oxidation corrosion reaction of the metal will be greatly accelerated, the metal surface will become rough, affecting its accuracy and strength, and even scrap parts. If it works under the high temperature conditions of corrosive media (phosphorus, sulfur and vanadium in the gas after high temperature and high pressure gasoline combustion), the corrosion effect will be more obvious, so the superalloy must have high oxidation resistance and corrosion resistance.

Alloys operating at high temperatures must also have sufficient creep resistance (creep resistance refers to the ability of solid materials to resist slow and continuous deformation under a certain stress) to ensure that they work for a long time under certain temperature and stress conditions, and the total deformation can still be maintained within a certain tolerance range.

Superalloys working at high temperature or under alternating temperature conditions are more prone to fatigue failure than normal temperature, or considerable thermal stress due to repeated rapid hot and cold changes during work. Therefore, superalloys must have good fatigue resistance (fatigue resistance refers to the ability of materials or parts to resist sudden breakage or failure under long-term changing loads).

Based on the characteristics that the melting point of refractory metals is generally above 2000℃ (W tungsten melting point 3400℃, Re rium 3180℃, Ta tantalum 2996℃, Mo molybdenum 2615℃, Nb Nb 2415℃), in order to meet the needs of the latest generation of high-tech aerospace equipment with large thrust-to-weight ratio and large carrying capacity. It is suitable for the manufacture of aerospace engine core components that work in high temperature and high stress environments, which means that superalloys need to be served in extremely high fever corrosion environments above 1500 ° C. This requires that a large number of refractory metal elements must be added to the superalloy for material strengthening. Among them, among the above five (tungsten, rhenium, molybdenum, tantalum, niobium) most representative refractory elements, the addition of rhenium element is the most significant, and the "rhenium effect" is the most significant "strong bone" effect in all high-temperature alloy materials including iron series, nickel series, cobalt series.

 Cemented carbide 

Cemented carbide is a refractory metal carbide (tungsten carbide, titanium carbide, etc.) powder as the main component, adding as a binder metal powder (cobalt, nickel, etc.), by powder metallurgy and obtained alloy. The matrix of cemented carbide consists of two parts: one is the hard phase; The other part is the adhesive phase.

The hard phase is generally a transition metal carbide in the periodic table, such as tungsten carbide, titanium carbide, tantalum carbide, etc. Their hardness is very high, the melting point is above 2000 ° C, and some even exceed 4000 ° C. In addition, transition metal nitrides, borides and silicates have similar properties and can also be used as hardening phases in cemented carbide. The existence of hard phase determines the extremely high hardness and wear resistance of cemented carbide. The bonding phase is the binding metal, generally iron group metals, commonly used cobalt and nickel.

In 1923, Schreiter of Germany added 10% cobalt in tungsten carbide as a binder for tungsten carbide powder, thus inventing a new alloy of tungsten carbide and cobalt, which was second only to diamond in hardness and began to be called "cemented carbide". In 1929, the United States Schwarzkov (Schwarzkov) added a certain amount of tungsten carbide and titanium carbide compound carbide in the original hard phase composition, thereby further improving the performance of cutting tool steel, which is another achievement in the history of cemented carbide.

Cemented carbide has a series of excellent properties, such as high hardness, wear resistance, high strength, heat resistance and corrosion resistance, especially its high hardness and wear resistance, even at 500 ° C temperature is basically unchanged, at 1000 ° C also has a high hardness. Carbide is widely used as cutting steel parts, stainless steel, heat-resistant alloys, cast iron, non-ferrous metals, plastics, chemical fiber, graphite, glass, stone and other difficult materials, such as turning tools, milling cutters, planers, drills, taps, boring tools and so on. The choice of tungsten carbide for cutting difficult materials, but also has a high cost-effective advantage, such as heat-resistant stainless steel, high manganese steel, tool steel, superalloys, titanium alloys, and even many refractory metal alloys (such as tungsten, molybdenum, tantalum, niobium, etc.). Now, the cutting efficiency of new cemented carbide tools has reached hundreds of times that of carbon steel, dozens of times that of high-speed steel, and it is the most commonly used cutting tool mainstream material on the market, with a market share of more than 70%.

Carbide can also be used to manufacture drilling tools, mining tools, drilling tools, measuring tools, wearing parts, metal grinding tools, cylinder liner, precision bearings, nozzles and so on. Coated carbide has been available for nearly two decades, in 1969, Sweden successfully developed titanium carbide coated tools, the basis of the tool is tungsten titanium cobalt carbide or tungsten cobalt carbide. The surface titanium carbide coating is only a few microns thick, but compared with the same grade of alloy tools, the service life is extended by three times, and the cutting speed is increased by 25% to 50%. The fourth generation of coated tools appeared in the 1970s for cutting difficult materials.

Super alloy + cemented carbide = Super cemented carbide

The following is the core idea of material innovation in this paper:

In superalloys, the deep mechanism of rhenium effect is more complex, so far material scientists have not fully understood, is still in the exploration stage. However, as far as the results of the addition of rhenium in superalloys are concerned, one thing is very certain, that is, the addition of rhenium can significantly improve the strength and resistance to high temperature deformation of superalloy materials including iron series, nickel series and cobalt series.

The manufacturing process of cemented carbide is to mix the raw materials (hardening phase and bonding phase) according to the specified proportion of composition, add alcohol or other media, wet mill in a wet ball mill, in order to pulverize them and fully mix, dry, screen, and then add the mixture such as paraffin or gum, and then dry and improve the flow of processing through screening, granulation, etc. The hardening phase forms a eutectic alloy with the bonding phase metal when it is pressed by pressure and die using powder metallurgy and heated to a temperature close to the melting point of the bonding phase metal (1300 to 1500 ° C) (this process is called sintering). After cooling, the hardened phases are distributed in bonded metal grids that are tightly connected to each other to form a solid whole. The hardness of cemented carbide depends on the content of hardened phase and grain size, that is, the higher the content of hardened phase, the finer the grain, the greater the hardness of cemented carbide; The toughness of cemented carbide is determined by the binding metal, the higher the binding metal content, the higher the stiffness, the greater the bending strength of cemented carbide.

In terms of raw material composition ratio alone, in cemented carbide, the content of the hard phase and the bonding phase is the relationship between this and that, in other words, if the content of the hard phase is increased, the hardness and wear resistance are improved, it is bound to reduce the content of the bonding phase metal, thereby reducing the overall strength of cemented carbide; On the contrary, if the content of the bonding phase metal is increased, so that the strength is improved, it is bound to reduce the content of the hard phase, thereby reducing the hardness and wear resistance of the cemented carbide. Therefore, under normal circumstances, the two important performance indicators of cemented carbide: hardness and strength, just like the seesaw, it is difficult to get both.

The introduction of the concept of "super cemented carbide" materials has solved this problem well. It is the use of the common characteristics of the "rhenium effect", the advantages of super alloy high strength, high temperature resistance, high heat resistance corrosion creatively transplanted to the traditional cemented carbide material inside, the original cemented carbide in the bonding phase metal (iron, cobalt, nickel) to strengthen the "rhenium effect", so as to achieve the hardness and strength indicators of cemented carbide "double high double fly" effect.

Application scenarios of super cemented carbide

Super carbide currently has two main application directions: ①For the manufacture of high-speed cutting and specialized processing of hard, tough and other difficult to process materials cutting tools, cutting tools; ② It is used to make high-speed and high-precision stamping dies, cold heading dies, drawing dies, measuring tools and high-wear parts that can withstand impact and vibration;

Application scenarios of super cemented carbide

In the machining industry, the main types of metal materials to be machined are P, M, K, N, S, H six categories, respectively representing: P class ordinary steel, M class stainless steel and cast steel, K class cast iron, N class copper aluminum and other non-ferrous metals, S class refractory stainless steel, special steel, titanium alloy, refractory alloy, superalloy as a typical representative of difficult materials, H class hardened steel, hard materials. Among them, the most difficult to process is S class difficult to process materials, which put forward higher and more demanding requirements for the wear resistance and red hardness of the tool material, and ordinary cemented carbide tools are difficult to meet their processing requirements.

Rhenium uses the advantages of high strength and high temperature resistance of super alloy and high hardness and high wear resistance of cemented carbide to carry out clever transplantation and fusion, thus creating a new material concept of "super cemented carbide" in China. By adding rhenium, ruthenium, osmium and other rare refractory elements to the traditional tungsten-cobalt and tungsten-cobalt titanium cemented carbide, rhenium researchers found that not only significantly inhibited the abnormal growth of WC grains, but also significantly strengthened the solid solution for the cobalt bonding phase, which increased the hardness of the alloy to a certain extent. At the same time, it also greatly improves the strength and red hardness of the alloy, and reduces the thermal conductivity of the cemented carbide to ensure the minimum heat transfer to the cutter body in the cutting. In addition, the addition of ruthenium, rhenium, osmium and other elements can also improve the corrosion resistance of WC-Co cemented carbide in acidic solutions and reduce the corrosion rate.

Under the comprehensive influence of the above comparative advantages, rhenium has been proved by a large number of cutting application experiments: Generally for M, H class, especially S class difficult to process materials of the workpiece, with the addition of rhenium, ruthenium, osmiium and other rare refractory elements of the super carbide material made of high-performance cutting tools, its cutting efficiency, service life and cost performance, compared with ordinary conventional material grades of carbide tools, can significantly improve 2 to 3 times.