Cemented carbide is also known as the "teeth" of the industry. Since its inception, as an efficient tool material and structural material, its application field has been continuously expanded, which has played an important role in promoting industrial development and scientific and technological progress. In the past 20 years, tungsten-cobalt-base
d cemented carbides have been widely used in metal cutting, metal forming tools, mining drilling, and wear parts because of their high hardness, toughness and excellent wear resistance compared to other hard alloys. .
Cemented carbide has a series of excellent performance characteristics: it has high hardness and wear resistance, especially valuable, it has good red hardness, exceeds the normal temperature hardness of high speed steel at 600 °C, and exceeds carbon steel at 1000 °C. Normal temperature hardness; has good elastic modulus, usually (4~7)×104kg/mm2, good rigidity at normal temperature; high compressive strength, up to 600kg/mm2; good chemical stability, Some grades of cemented carbide are resistant to acid and alkali corrosion and do not undergo significant oxidation even at high temperatures; low coefficient of thermal expansion. The thermal conductivity and conductivity are close to those of iron and iron alloys.
According to the average grain size of WC in cemented carbide, cemented carbide can be divided into: nanocrystalline cemented carbide, ultrafine grained cemented carbide, submicron grained cemented carbide, fine grained cemented carbide, Medium grain cemented carbide, coarse grained cemented carbide, super coarse grained cemented carbide.
Sub-micron and ultra-fine grained carbides have high hardness and wear resistance and are widely used in cutting tools, saw blade, milling cutters, stampers, valve stem components, nozzles for sand blasting equipment, etc.
Ultra-thick grained carbide has better toughness and thermal fatigue resistance, and its application in mining and excavation tools has developed rapidly. Gradient alloys and carbide-diamond composites can be used to highlight certain specific properties according to different application requirements, so the application of tools and mining tools has developed rapidly.
The properties of tungsten-cobalt-based cemented carbides mainly depend on the content of Co and the grain size of WC. The typical cobalt-cobalt cemented carbide has a cobalt content of 3 to 30 wt%, and the WC grain size ranges from submicron to several. Micron. The development of nano-scale particle synthesis technology, especially nano-scale WC and Co particles, greatly improved the mechanical properties of nano-WC-Co cemented carbide.
When the WC grain is smaller than the submicron size, the strength, hardness, toughness, and wear properties of the alloy are greatly improved, and the alloy having a high density can be obtained while lowering the sintering temperature. Therefore, in the field of cemented carbide, the conversion of traditional types to ultra-fine and nano-scale has become its development trend.
However, WC grain growth has always been a bottleneck in the development and production of ultra-fine WC-Co alloys. Adding certain additives to cemented carbide is one of the effective ways to improve the properties of the alloy. There are two main types of additives added to the cemented carbide: one is a refractory metal carbide and the other is a metal additive. The role of the additive is to reduce the sensitivity of the alloy to sintering temperature fluctuations and sensitivity to changes in carbon content, to prevent uneven growth of carbide grains, to change the phase composition of the alloy, thereby improving the structure and properties of the alloy.
The most commonly used carbide additives include chrome carbide (Cr3C2), vanadium carbide (VC), molybdenum carbide (Mo2C or Mo C), cobalt carbide, tantalum carbide, and the like. The choice of inhibitor depends on the total inhibitory effect, and the inhibitory effects are as follows: VC>Cr3C2>Nb C>Ta C>Ti C>Zr/Hf C. Commonly used metal additives are chromium, molybdenum, tungsten, rhenium, ruthenium, copper, aluminum and rare earth elements. The addition of rare earth elements in cemented carbide not only inhibits the growth of WC grains during sintering, but also improves the mechanical properties and wear resistance of the alloy, thereby further improving the service life of the products. In the field of cemented carbides, research on rare earth additives has been a hot topic, but the general idea is to add non-nano-scale rare earth additives to modify hard alloys, but the addition of nano-rare earth additives has rarely been reported.
The use of the nano rare earth additive is lower than that of the ordinary rare earth additive, and the gap with the WC grain (large circle) is small, and the arrangement is more dense. The size of the ordinary rare earth additive is almost the same as that of WC, so it is easy to form a crack source. Therefore, this experiment uses nano rare earth as an additive to achieve the purpose of not improving the cost and improving the performance. China is rich in rare earth resources. If we use this kind of thinking to develop new technology, make full use of China's tungsten ore and rare earth resources, research and develop hard alloy rare earth modified materials, improve the production level and development of China's cemented carbide industry. High-quality and high value-added deep-processed carbide products, improving competitiveness, reversing the unfavorable situation in the international market, and achieving a virtuous cycle of raw materials are of great significance.
2. Rare earth hard alloy
The rare earth element is 15 lanthanides of the third subgroup of Mendeleev's periodic table with atomic numbers ranging from 57 to 71, plus a total of 17 elements, which are similar to those of electronic structures and chemical properties. Rare earth is known as the “treasure house” of new materials, and is a group of elements that scientists at home and abroad, especially material experts, are most concerned about. Due to its special properties, rare earths have been widely used in metallurgical materials, optics, magnetism, electronics, machinery, chemicals, atomic energy, agriculture and light industry. Although rare earths are used as additives and modifiers, their direct output value and profit are not high, but the secondary economic benefits can be increased by tens or even hundreds of times. China's rare earth resources are abundant, and its reserves rank first in the world, and its comprehensive production capacity ranks second in the world. At home and abroad, the application of rare earths and their compounds is almost everywhere in the national economy. Rare earth has obvious improvement on the performance of cemented carbide. A large number of studies have shown that the addition of rare earth can improve the strength and toughness of cemented carbide to a large extent, so that rare earth-added cemented carbide can be widely used in tool materials and mining tools. , molds, top hammers, etc., have excellent development prospects. The rare earths commonly used as additives are Ce, Y, Pr, La, Sc, Dy, Gd, Nd, Sm, and the like. The addition form is generally an oxide, a pure metal, a nitride, a hydride, a carbide, a rare earth-cobalt intermediate alloy, a carbonate, a nitrate, and the like. The type and morphology of the added rare earth affect the physical and mechanical properties of the cemented carbide.
3. Mechanism of strengthening and toughening of rare earth
The addition of trace rare earth elements in the cemented carbide not only inhibits the grain growth of the alloy during the sintering process, but also improves the mechanical properties of the alloy, thereby further improving the service life of the product. The strengthening mechanism of rare earth on cemented carbide is as follows:
(1) Zhang Fenglin et al. believe that when the γ phase is cooled from high temperature to room temperature, fcc→hcp is a diffusion type (assisted by Ms mechanism) phase transition. Among them, γfcc and γhcp phase account for about 10%. Since the addition of rare earth can inhibit the martensitic transformation, the content of γhcp in the binder phase can be reduced. The mechanism of its inhibition of martensite transformation may be due to two reasons: one is the rare earth oxide pinning dislocation, which hinders the dislocation motion; on the other hand, the rare earth oxide is pinned at the defect location, making the potential ε nucleation nucleus The embryo is reduced. Thereby, the brittle ε phase is reduced and the toughness α phase is increased.
Wang Ruikun and others believe that the addition of trace rare earths in cemented carbides can inhibit the expansion of stacking faults in the Co binder phase, thereby inhibiting the conversion of fcc α-Co→hcp ε-Co (layered nucleation), making fcc α-Co in the alloy. The volume fraction increases. α-Co has 12 slip systems, while ε-Co has only 3 slip systems. Rare earth cemented carbide is mainly composed of fcc α-Co, which will improve its ability to coordinate strain and relax stress, thereby improving its toughness.
(2) Effect on W solid solubility.
The segregation of rare earths at the WC/Co phase interface affects the desolvation of elements such as W and Ti from Co. It is possible to increase the content of W and Ti in the binder phase, thereby functioning as a solid solution strengthening. But the mechanism is not fully recognized.
(3) Refine the organization.
The rare earth in the cemented carbide is distributed at the interface of WC/Co and WC/WC. The adsorption of rare earth elements at the interface will definitely reduce the interfacial energy of the solid-liquid phase interface. This can suppress the coarsening process of WC grains during sintering.
(4) Strengthening and toughening of grain boundaries and phase boundaries.
In the fracture of cemented carbide, it is mainly along the Co bond phase fracture, and there are some cracks along the WC grain. Therefore, its fracture behavior has an important relationship with the behavior of the WC/Co interface. The presence of rare earths in cemented carbides is mainly due to oxides or intermetallic compounds. The distribution is mainly at the interface of WC/Co and WC/WC. A small amount of rare earth oxides can also be found in the binder phase. Its shape is mainly spherical or polyhedral. Due to the role of rare earth in purifying grain boundaries and phase boundaries, and the improvement of the strength of the phase interface, the fracture toughness of rare earth cemented carbides will be greatly improved.
Due to the different ways, forms, types of rare earths, and research methods, the research conclusions are different, and the proposed mechanism will be different and even contradictory. The research on rare earth toughened cemented carbides needs further study.