Features And Preparation Of Tungsten Carbide Mining Tool

Cemented carbide is a composite material composed of high-hardness refractory metal carbides and cemented metals. Because of its high hardness, wear resistance, and stable chemical properties, it is used in modern tool materials and wear-resistant materials. High temperature and corrosion resistant materials occupy an important position. At present, tungsten carbide-based hard alloys are the most widely used among the carbides produced in the world, with the largest output and the most extensive use. Among them, the WC hard alloy used in mines has been regarded as the “tooth” of the mine development, oil drilling and geological exploration industries, and has received extensive attention.
Mine rock drilling tools are composed of a metal base body and different geometric shapes embedded therein and different grades of WC hard alloy drill teeth according to different working conditions. Take pick-axle picks as an example, the working environment of the picks is harsh, and in addition to the abrasive wear under compression, bending, and high stress, it also bears an indefinite impact force, so carbides often occur during coal mining. The head is broken and falls off, which leads to premature wear and failure of the pick-up matrix, which makes the life of the pick-shaped picks much lower than the design life. Therefore, an excellent hard alloy for mining should have high strength, high hardness required for abrasion resistance and high toughness required for resistance to impact fracture.

Tungsten Carbide Mining Tool Features

1.1 Wear resistance of WC alloy

The shearer of the shearer is in direct contact with the coal seam during the working process. The abrasive wear characteristics of the shearer are closely related to the coal seam structure and hardness. The hardness of the coal is low, generally 100 to 420 HV, but the coal often contains different hardness. Impurities such as quartz and pyrite (900 to 1100 HV) have high hardness and have a great influence on the abrasive wear characteristics of picks.
In most of the operating examples, wear resistance is a basic function of material hardness. The higher the hardness, the higher the abrasive wear resistance. Pure WC is very hard and similar to diamond. In cemented carbide, WC particles form a strong skeleton, so WC cemented carbides exhibit very high hardness. In addition, WC belongs to the hexagonal crystal system and has anisotropy in hardness. The Vickers hardness of the bottom surface {0001} and the edge surface {1010} is 2 100 HV and 1 080 HV, respectively. In the coarse-grained cemented carbide, the proportion of WC grains on the {0001} plane is high, and thus the carbide containing the coarse-grained WC shows higher hardness. At the same time, at a high temperature of 1 000°C, coarse-grained WC hard alloys have higher hardness than ordinary hard alloys and show good red hardness.
In the coal cutting process, WC particles are exposed on the surface of the cemented carbide after cemented phases of the cemented carbide in the tool nose protected by the built-up edge have been squeezed away or are carried away by abrasive scraping. Bonded phase-supported WC particles are easily crushed, destroyed and released. Due to coarse WC grains, the cemented carbide has a strong holding force with respect to the WC, and the WC grains are difficult to pull out and exhibit excellent wear resistance.

1.2 Toughness of WC Alloy

When the cutter bit cuts the coal rock, the cutter head is subjected to high-stress stress, tensile stress and shear stress under the action of the impact load. When the stress exceeds the strength limit of the alloy, the alloy cutter head will be fragmented. Even if the generated stress does not reach the strength limit of the cemented carbide, the fatigue cracking of the cemented carbide will occur under the repeated action of the impact load, and the expansion of the fatigue crack may cause the tool head to fall off or chipping. At the same time, when cutting the coal seam, the shearer pick produces high temperature of 600-800°C on the cutting surface, and the cutting cutting coal seam is a periodic rotary motion. The temperature rise is alternating, and the temperature increases when the cutter head contacts the coal rock. , cool down when leaving the coal rock. Due to the constant change of the surface temperature, the dislocation density increases and concentrates, and the surface of the serpentine pattern appears.
The depth of cracks and the rate of propagation decrease with increasing carbide grain size, and the morphology, direction, and depth of cracks also vary with WC grain size. The cracks in fine-grained alloys are mostly straight and small and long; coarse-grained alloy cracks are irregular and short. The cracks mainly extend at the weak grain boundary. In the coarse-grained cemented carbide, if the micro-cracks bypass the coarse-grained WC grains, they are zigzag-shaped and must have energy that matches the fracture area; if they pass through When WC grains are expanded, they must have considerable fracture energy. As a result, the coarse-grained WC grains have enhanced deflection and bifurcation of cracks, which can prevent the further propagation of micro-cracks and increase the toughness of the cemented carbide. With the same content of cementitious phase, the coarse-grained alloy has a thicker bonding phase, which is beneficial to the plastic deformation of the bonding phase, inhibits the extension of cracks, and shows good toughness.
Studies on the strength and structure of WC-Co cemented carbide also show that there is a certain rule between the strength of cemented carbide and the grain size of WC. When the cobalt content is constant, the strength of conventional low-cobalt alloys always increases as the grain size of WC in the cemented carbide becomes coarser, and the strength of the alloy with higher cobalt content peaks with WC grain coarsening.

2 Research Progress on Preparation Process of Mining WC Alloy

At present, tungsten carbide powders are generally prepared by the process of reducing tungsten oxide to obtain coarse tungsten powder, tungsten powder obtained by high-temperature carbonization to obtain coarse WC powder, and WC powder and Co powder through mixing, wet grinding and sintering. Among them, the choice of coarse WC powder preparation, sintering process and equipment directly affects the performance of the mine WC alloy.

2.1 Preparation of WC Powder

(1) Preparation of coarse tungsten powder

Luo Binhui’s test results show that the oxygen content of tungsten oxide raw material directly affects the particle size of tungsten powder. To produce ultra-fine tungsten powder, tungsten oxide with lower oxygen content should be selected as raw material (usually purple tungsten), and coarser tungsten powder should be selected for oxygen production. A high content of tungsten oxide (yellow tungsten or blue tungsten) is used as a raw material. The results of Zhang Li et al. showed that compared with yellow tungsten, the use of blue tungsten to obtain coarse-grain tungsten powder has no advantage in particle size and distribution. However, the surface micropores are less tungsten powders made from yellow tungsten, and the overall performance of cemented carbides is better. It is known that the addition of an alkali metal to tungsten oxide contributes to the long coarseness of the tungsten powder, but the residual alkali metal in the tungsten powder suppresses the growth of WC crystal grains. Sun Baoqi et al. used lithium-activated tungsten oxide for hydrogen reduction to prepare coarse tungsten powder. Based on the experimental results, he explored the mechanism of activation and grain growth. He believed that by adding volatile lithium salt, the volatile deposition rate during the reduction of tungsten oxide was accelerated, resulting in Tungsten grows at lower temperatures. Huang Xin added Na salt in WO 3 for intermediate temperature reduction. The particle size of tungsten powder is proportional to the amount of Na added. With the increase of Na addition, the number of large crystal grains increased from 50 to 100 μm.

(2) Classification of tungsten powder

Gao Hui believes that the classification of tungsten powder can effectively change the properties of powder and solve the problem of uneven thickness of powder. Reduce the difference between the minimum, maximum, and average particle diameters to produce a coarser, more uniform WC powder; due to the characteristics of tungsten, it is not easily broken, and moderate crushing is performed prior to classification to separate the agglomerated particles in the powder. , more effective separation of powder, improve the uniformity.

(3) Coarse WC Powder Preparation

Preparation of coarse-grained WC powders by high-temperature carbonization of coarse-grained tungsten powders is a classical and classical method. The coarse-grain tungsten powders are mixed with carbon black and then mixed into a carbon tube furnace. The carbonization temperature of coarse tungsten powders is generally about 1 600°C, and the carbonization time is 1 ~ 2 h. Due to carbonization at a high temperature for a long time, this method minimizes the lattice defects of WC and minimizes the microscopic strain, thereby improving the plasticity of WC. In recent years, the tungsten powder carbonization process has been continuously developed. Some cemented carbide production plants have begun to adopt advanced intermediate frequency induction furnaces for vacuum carbonization and hydrogenation.
Due to the phenomenon of sintering and growth of WC powder particles, WC particles grow thicker and thicker at high temperatures. In addition, the finer the original tungsten powder, the more obvious the phenomenon of high temperature and WC grain growth. It is based on this principle that the use of medium-grained tungsten powder and even fine-grained tungsten powders for high-temperature carbonization to obtain coarse-grained tungsten carbide. The use of tungsten powder (Fisher sub-sieve sixer, Fsss 5.61 to 9.45 μm) was reported in the literature. The carbonization temperature was 1 800 to 1 900 °C, and WC powder with Fsss 7.5 to 11.80 μm was obtained. Fine tungsten powder was used. (Fsss < 2.5 μm), carbonization temperature 2 000°C, WC powder with Fsss of 7 to 8 μm was prepared. Due to the large density difference between tungsten and WC, the tungsten particles convert to WC particles during the conversion from tungsten to WC.
The resulting WC particles contain large strain energy, and some of the WC particles burst as a result, and the WC particles become smaller after blasting. Huang Xin et al. adopted a two-step carbonization method. Since the first time was incomplete carbonization, the particle core part remained pure tungsten, and the surface layer of the particles had been carbonized completely. Pure tungsten could be recrystallized to consume part of the strain energy, thereby reducing grain cracking. The probability. Compared with the conventional one-step WC powder, the coarse-grained WC powder produced by the two-step method has a single phase composition and almost no W 2 C, WC (1-x) and other miscellaneous phases. Zhang Li et al. studied the effect of Co doping on the grain size and micro morphology of coarse and coarse WC powders. The results show that Co doping is beneficial to the increase of grain size and free carbon of WC powder and is beneficial to single crystals. WC powder. When the doping content of Co is 0.035%, the crystal integrity of the WC grains is significantly improved, showing a distinct growth step and growth plane.

(4) Coarse-crystal aluminum thermal process

The distinctive feature is that tungsten carbide can be used to directly produce tungsten carbide, and the tungsten carbide powder produced is particularly thick and carbonized. A mixture of tungsten ore and iron oxide is reduced with aluminum, while carbide is used for calcium carbide. As long as the charge is ignited, the reaction proceeds spontaneously, resulting in an exothermic reaction with a self-heating temperature of up to 2500°C. After the reaction is over, the reaction kiln and material are allowed to cool down. The lower part of the kiln will produce a WC-based block layer, and the rest will be metal iron, manganese, excess metal aluminum, and a small amount of slag. The upper slag layer was separated, the lower ingot was crushed, excess calcium carbide was removed by washing with water, iron, manganese, and aluminum were removed by acid treatment, and finally, WC crystals were sorted by gravity dressing. The WC produced by this process is ground to a micron level for use with a variety of different cemented carbides.

2.2 Sintering of WC Carbide

(1) Vacuum sintering

In the vacuum sintering, the wettability of the bonding metal to the hard phase is significantly improved, and the product is not easily carburized and decarburized. Therefore, many of the world’s famous cemented carbide manufacturers use vacuum sintering, and vacuum sintering in China’s industrial production has gradually replaced hydrogen sintering. Mo Shengqiu studied the preparation of WC-Co cemented carbide with low cobalt content by vacuum sintering, and pointed out that the process system in the pre-firing stage is the key to vacuum sintering of WC-Co cemented carbide with low cobalt content. At this stage, the impurities and oxygen in the alloy are eliminated, the volumetric shrinkage is relatively intense, and the density increases rapidly. The pre-burning vacuum in the 0.11 ~ 0.21 MPa alloy has better final performance. For coarse-grained WC-Co cemented carbides with cobalt content between 4% and 6%, for high strength, the pre-sintering temperature should be between 1 320 and 1 370 °C.

(2) Low pressure hot isostatic pressing

Vacuum sintered cemented carbide has a small amount of pores and defects. These pores and defects not only affect the performance of the material, but also tend to be the source of the fracture during use. Hot isostatic pressing technology is an effective method to solve this problem. From the early 1990s, low-pressure hot isostatic pressing sintering furnaces were introduced in some large enterprises in China, such as Jianghan Bit Factory, Zhuzhou Cemented Carbide Factory, and Zigong Cemented Carbide Factory; Low-pressure sintering furnaces independently developed by Beijing Iron and Steel Research Institute have been put into operation. use. The application of low pressure hot isostatic pressing reduces the porosity of the cemented carbide and the structure is dense, and improves the impact toughness of the alloy and improves the life of the cemented carbide.
Jia Zuocheng and other experimental results show that the low pressure hot isostatic pressing process is beneficial to the elimination of voids in the alloy and WC grain growth, and increases the flexural strength of coarse-grained WC-15Co and WC-22Co alloys. Xie Hong et al. studied the effects of vacuum sintering and low-pressure sintering on the properties of WC-6Co cemented carbides. The results show that the vacuum sintering material Vickers hardness 1 690kg / mm 2, the transverse rupture strength is 1 830 MPa, while the low-pressure sintered material Vickers hardness is increased to 1 720 kg / mm 2, the transverse rupture strength is 2140 MPa. Wang Yimin also produced WC-8Co alloys by vacuum sintering and low pressure sintering. The results show that the vacuum sintered material has a hardness of 89.5 HRA and a transverse rupture strength of 2270 MPa; and the low-pressure sintered material has an increased hardness of 89.9 HRA and transverse fracture. The strength is 2 520 MPa. The temperature uniformity of the sintering furnace is an important factor affecting the quality of high-performance carbide products. A large number of studies have simulated and optimized the temperature field in the sintering furnace. The literature proposes a piecewise simulation method that is consistent with the experimental results. The temperature distribution in the graphite tube is not uniform, which is mainly due to the unreasonable arrangement of the graphite boat and the sintered product and the structure of the graphite tube. In the test, optimization measures were proposed to reduce the surface temperature deviation of sintered products by approximately 10 K during the vacuum phase and within ±7 K during the gas heating phase, thereby improving the sintering quality.

(3) Spark Plasma Sintering (SPS)

A method of sintering under pressurized conditions using instantaneous and intermittent discharge energy. The mechanism of SPS sintering is still controversial. Scholars at home and abroad have conducted extensive research on this topic. It is generally believed that a discharge plasma is instantaneously generated when a direct current pulse is applied to an electrode, so that heat generated uniformly by each particle in the sintered body activates the surface of the particle, and sintering is performed by the self-heating effect of the inside of the powder. Liu Xuemei et al used XRD, EBSD and other test methods to compare the phase composition, microstructure and properties of the hard alloy materials obtained by hot press and spark plasma sintering. The results show that the SPS sintered materials have high fracture toughness. Xia Yanghua, etc. using SPS technology with an initial pressure of 30 MPa, sintering temperature 1 350 °C, holding 8 min, the temperature of 200 °C / min prepared carbide hardness of 91 HRA, transverse fracture strength of 1 269 MPa. The literature uses SPS technology to sinter WC-Co cemented carbides. It can produce WC- with relative density of 99%, HRA ≥ 93 and good phase formation and uniform microstructure under sintering temperature of 1270°C and sintering pressure of 90 MPa. Co Carbide. Zhao et al. of the University of California, USA prepared the binder-free cemented carbide by SPS method. The sintering pressure was 126 MPa, the sintering temperature was 1 750°C, and no holding time was obtained. A fully dense alloy was obtained but a small amount of W 2 C phase was contained. In order to remove impurities, an excess of carbon was added. The sintering temperature was 1 550°C and the holding temperature was 5 μm. The material density remained unchanged and the Vickers hardness was 2 500 kg / mm 2.
Spark plasma sintering as a new type of rapid sintering technology has broad application prospects. However, the research at home and abroad is still limited to the laboratory research stage. The sintering mechanism and sintering equipment are the main obstacles to its development. SPS sintering mechanism is still controversial, especially the intermediate processes and phenomena of sintering have yet to be further studied. In addition, the SPS equipment uses graphite as a mold. Because of its high brittleness and low strength, it is not conducive to high-temperature and high-pressure sintering. Therefore, the mold utilization rate is low. For actual production, it is necessary to develop new mold materials with higher strength and reusability than the currently used mold materials (graphite) in order to increase the bearing capacity of the mold and reduce the cost of the mold. In the process, it is necessary to establish the temperature difference between the mold temperature and the actual temperature of the workpiece in order to better control the product quality.

(4) Microwave sintering

A method in which microwave energy is converted into heat energy for sintering by using the dielectric loss of a dielectric in a high-frequency electric field, and the entire material is uniformly heated to a certain temperature to achieve densification and sintering. The heat is generated from the coupling of the material itself with the microwave, rather than from the external heat source. The Monika team studied the microwave sintering and traditional sintering densification of WC-6Co cemented carbides. The experimental results show that the degree of densification of microwave sintering is faster than that of traditional sintering. Researchers at the University of Pennsylvania studied the production of tungsten carbide products in the microwave sintering industry. They have higher mechanical properties than conventional products, and have good microstructure uniformity and low porosity. The microwave sintering process of WC-10Co cemented carbide by microwave sintering was studied in the omni-peak system. The interaction of microwave electric field, magnetic field and microwave electromagnetic field on WC-10Co cemented carbide was analyzed.
The lack of material properties data and equipment are two major obstacles to the development of microwave sintering technology. Without the data on the material properties of materials, one cannot know the mechanism of action with microwaves. Due to the strong selectivity of microwave sintering furnaces for products, the parameters of microwave ovens required for different products are very different. It is difficult to manufacture microwave sintering equipment with a high degree of automation, with variable frequency and automatic tuning functions, which is a bottleneck restricting its development.

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