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Applications of Tungsten Carbide Coatings in 3 Fields

joshua — Fri, 21 Feb 2025 08:38:58 +0000

Tungsten carbide coatings are high-hardness protective layers formed by depositing tungsten carbide (WC) on the surface of the substrate. Their basic composition can be understood from three aspects: chemical composition, microstructure, and functional characteristics.

Chemical Composition

Tungsten carbide is a compound formed by tungsten (W) and carbon (C) atoms in a 1:1 ratio, with the chemical formula WC. This compound has very stable chemical bonds and can maintain its chemical stability in high temperatures and harsh chemical environments. Tungsten is a metal element with very high density and melting point. When carbon is combined with tungsten to form carbides, the material not only possesses the high strength characteristics of tungsten but also greatly increases in hardness due to the addition of carbon. This makes tungsten carbide a material that combines high hardness with high-temperature stability, making it very suitable for preparing coatings that require extremely high wear resistance and corrosion resistance.

Microstructure

The microstructure of tungsten carbide coatings mainly includes grain size, porosity, and coating thickness. These microscopic characteristics have a direct impact on the performance of the coating. Typically, tungsten carbide coatings have a fine and uniform grain structure, which provides higher hardness and wear resistance. The density of the coating is also a key factor; an ideal tungsten carbide coating should have an extremely low porosity to prevent the penetration of corrosive media and enhance the mechanical strength and adhesion of the coating.

In industrial applications, by optimizing the preparation process, the grain size and thickness of the coating can be controlled, thereby adjusting the hardness, toughness, and wear resistance of the coating. For example, tungsten carbide coatings used under high-temperature conditions typically require a larger grain structure to improve high-temperature stability, while in severely abrasive environments, finer grains help to enhance the coating’s wear resistance.

Performance and Characteristics of Tungsten Carbide Coatings

With their outstanding hardness, wear resistance, corrosion resistance, and high-temperature stability, tungsten carbide coatings have become one of the widely used surface engineering technologies in industrial applications. To better understand the practical application effects of tungsten carbide coatings, this section will discuss their performance and characteristics in detail from four aspects: wear resistance, corrosion resistance, high-temperature stability, and mechanical properties.

Abrasive Property

The High Hardness Characteristics of Tungsten Carbide

The wear resistance of tungsten carbide coatings is derived from their ultra-high hardness, which excels in resisting mechanical wear. The hardness of tungsten carbide is close to 9 on the Mohs scale, making it one of the hardest compounds known in materials. This high hardness enables tungsten carbide coatings to effectively resist wear when in contact with other hard materials, reducing the loss of the material surface.

The Wear Mechanisms of Coatings

The wear mechanisms of tungsten carbide coatings mainly include abrasive wear, adhesive wear, and fatigue wear. In abrasive wear, hard particles or debris slide or roll between two contact surfaces, leading to the scraping or cutting of the coating material. Due to the high hardness of tungsten carbide, it can effectively resist this type of wear. In adhesive wear, when two surfaces move relative to each other under high pressure, the coating material may be torn or transferred, but the high hardness and low coefficient of friction of the tungsten carbide coating can significantly slow down this process. Fatigue wear occurs under repeated mechanical stress, where micro-cracks form on the coating surface and gradually expand, eventually leading to the flaking off of the material.

Performance of Wear Resistance in Different Application Scenarios

Tungsten carbide coatings exhibit excellent wear resistance across various application scenarios. For instance, in cutting tools, tungsten carbide coatings can maintain the sharpness of the cutting edge and extend the service life of the tool; in mining and oil drilling equipment, tungsten carbide coatings can effectively resist wear from hard rock and gravel, reducing the maintenance frequency of the equipment; in the automotive industry, tungsten carbide coatings can be used in engine components to significantly improve the wear resistance and service life of the parts. These applications demonstrate that tungsten carbide coatings can maintain their integrity and functionality in extreme wear environments.

Anti-corrosive Property

The Stability of Tungsten Carbide in Corrosive Environments

Tungsten carbide coatings not only possess excellent wear resistance but also demonstrate good corrosion resistance, particularly in acidic and alkaline environments. The chemical inertness of tungsten carbide allows it to remain stable in a variety of corrosive media, with a low propensity for chemical reactions. This characteristic enables tungsten carbide coatings to protect the substrate from chemical corrosion in many industrial applications, thereby extending the service life of the equipment.

Tolerance of Coatings under Acidic and Alkaline Conditions

Under acidic conditions, such as in strong acid environments like sulfuric acid and hydrochloric acid, a dense oxide film forms on the surface of the tungsten carbide coating. This film effectively prevents further erosion by acidic media, thereby protecting the substrate. In alkaline environments, tungsten carbide coatings also exhibit good tolerance. The chemical stability of the coating resists erosion by strong alkalis, preventing corrosive damage to the substrate. This excellent corrosion resistance has led to the widespread application of tungsten carbide coatings in fields with stringent corrosion requirements, such as chemical engineering and marine engineering.

Comparison with Other Corrosion-Resistant Materials

Compared to other corrosion-resistant materials, such as stainless steel and nickel-based alloys, tungsten carbide coatings have higher hardness and a lower corrosion rate. In environments with dual challenges of high wear and high corrosion, tungsten carbide coatings often provide longer periods of protection, reducing maintenance and replacement costs. For instance, in applications in marine environments, tungsten carbide coatings can resist the corrosion of seawater for extended periods, far outperforming traditional metal coatings.

High-temp Stability

Physical and Chemical Changes of Coatings under High-Temperature Conditions

The stability of tungsten carbide coatings in high-temperature environments makes them an ideal choice for high-temperature applications. At high temperatures, the chemical structure of tungsten carbide remains stable without decomposition or phase transformation. The high-temperature stability of the coating is not only reflected in the maintenance of its hardness but also in its oxidation resistance. Tungsten carbide can form a stable oxide film at high temperatures, which effectively prevents further oxidation, thereby protecting both the coating and the substrate.

Case Analysis of High-Temperature Applications

Tungsten carbide coatings are widely used in various equipment and components in high-temperature environments. For example, on the blades of gas turbines, tungsten carbide coatings can resist the erosion and oxidation caused by high-temperature combustion gases, extending the service life of the blades. In metal cutting tools, tungsten carbide coatings can maintain the sharpness of the tools under high-temperature cutting conditions, reducing tool wear and replacement frequency. Additionally, in oil drilling, tungsten carbide coatings can maintain their structural integrity in high-temperature and high-pressure environments, preventing premature equipment failure.

Factors Affecting High-Temperature Stability

The high-temperature stability of tungsten carbide coatings is influenced by various factors, including the thickness of the coating, its microstructure, and the nature of the substrate. A thicker coating generally provides better thermal insulation, slowing down the impact of high temperatures on the substrate. The microstructure of the coating, such as grain size and porosity, also affects its performance at high temperatures. Optimizing these factors can further enhance the high-temperature stability of tungsten carbide coatings, meeting the needs of different industrial applications.

Mechanic Property

Elastic Modulus and Hardness of Tungsten Carbide Coatings

The mechanical properties of tungsten carbide coatings are largely determined by their elastic modulus and hardness. The elastic modulus is a measure of a material’s ability to resist elastic deformation; tungsten carbide coatings have a high elastic modulus, which allows them to maintain stability in shape and size under high stress conditions. Hardness, on the other hand, is a measure of a material’s ability to resist plastic deformation; the high hardness of tungsten carbide coatings makes them less prone to deformation or wear when in contact with hard materials.

Impact and Fatigue Performance of Coatings

The impact performance of tungsten carbide coatings is demonstrated by their ability to maintain structural integrity under high-impact conditions, with a lower likelihood of spalling or cracking. This is particularly important in applications that require protection against high kinetic energy impacts, such as mining equipment and tool surface protection. Fatigue performance refers to a material’s ability to resist the formation and propagation of fatigue cracks under repeated cyclic stresses. Tungsten carbide coatings exhibit excellent fatigue resistance in fatigue tests, maintaining their mechanical properties over long periods of cyclic stress, thereby extending the service life of the coating.

Mechanical Properties Testing and Result Analysis

When testing the mechanical properties of tungsten carbide coatings, methods such as nanoindentation testing, microhardness testing, and fatigue testing are commonly used. Nanoindentation testing measures the coating’s elastic modulus and hardness, with results showing that the hardness of tungsten carbide coatings is significantly higher than that of most metal coatings. Microhardness testing further verifies the uniformity of the hardness distribution of the coating at the microscale. Fatigue testing assesses the fatigue life of the coating through cyclic stress tests of repeated loading and unloading, and the results indicate that tungsten carbide coatings exhibit outstanding durability under high-stress cycling conditions.

Application Fields of Tungsten Carburo Coatings

Thanks to their outstanding physical and chemical properties, tungsten carbide coatings have been widely used in various industrial fields. Whether it’s the demand for wear resistance in extreme environments or the requirement for stability under high temperature and pressure conditions, tungsten carbide coatings can provide reliable solutions. The following will discuss in detail the role and impact of tungsten carbide coatings in four main application fields: aerospace, tool manufacturing, oil and gas, and the automotive industry.

Aerospace

The application of tungsten carbide coatings in the aerospace field mainly focuses on turbine blades, gas turbine components, landing gears, and other key components. Turbine blades are prone to thermal fatigue and high-temperature oxidation due to long-term exposure to high-temperature and high-pressure environments. Tungsten carbide coatings can effectively enhance the wear resistance and oxidation resistance of the blade surfaces, extending the service life of the blades while reducing maintenance frequency. Additionally, tungsten carbide coatings are widely used in aircraft landing gear components, which endure significant friction and impact during takeoff and landing. The application of the coatings significantly improves the durability and safety of these components.

Future Development of Coating Technology in Aerospace

With the continuous advancement of aerospace technology, the requirements for material performance are also increasing. In the future, tungsten carbide coatings will continue to play a significant role in material lightweighting, high-temperature oxidation resistance, and adaptability to extreme environments. Particularly in the development of new hypersonic aircraft and space exploration vehicles, the application prospects of tungsten carbide coatings will be even broader. By combining with other high-performance materials, such as composites and nanomaterials, tungsten carbide coatings will demonstrate greater potential in meeting the increasingly stringent technical requirements of the aerospace industry.

Tools Manufacture

Application of Tungsten Carbide Coatings in Cutting Tools

Cutting tools are the core equipment in industrial manufacturing, widely used in metal processing, automotive manufacturing, aerospace, and electronics, among other fields. Since cutting tools need to operate under high-intensity and high-speed machining conditions, the surface materials must possess extremely high hardness and wear resistance. Tungsten carbide coatings, with their ultra-high hardness and excellent wear resistance, have become the ideal choice for the manufacturing of cutting tools.

The Impact of Coatings on Tool Life

During the cutting process, tool wear is a key factor affecting machining accuracy and efficiency. Tungsten carbide coatings can significantly extend the service life of cutting tools, reducing wear and the frequency of tool replacement. The high hardness of the coating allows the tool to maintain its sharpness during cutting and resist the heat generated by high temperatures and friction, preventing edge chipping and blunting. Experimental data show that cutting tools coated with tungsten carbide can have their service life extended by 2 to 3 times or more, while also improving the surface finish and precision of the machined parts.

Outlook for Coating Technology in Tool Manufacturing

As the demand for high-precision and high-efficiency machining increases in the manufacturing industry, the future of tool manufacturing will rely more heavily on advanced coating technologies. The application of tungsten carbide coatings will be further expanded, especially in the fields of ultra-high-speed cutting, dry cutting, and micro-machining. Future coating technologies will place more emphasis on the development of nanostructured coatings, as well as the application of multi-layer coatings, to further enhance the wear resistance, thermal stability, and impact resistance of tools. At the same time, the composite use of tungsten carbide coatings with other hard coating materials will also become an important direction for improving tool performance.

Application in Oil and Gas Fields

The Wear-Resistant Application of Coatings in Perforación Equipo

Drilling equipment in the oil and gas industry operates in extreme and harsh environments, typically facing challenges such as high pressure, high temperature, and severe wear. Drill bits and drill pipes are the most wear-prone components in drilling equipment. Due to its outstanding wear resistance, tungsten carbide coatings are widely applied to these key components. The application of these coatings significantly extends the service life of drilling equipment, reducing downtime and maintenance costs.

The Corrosion Resistance Performance of Tungsten Carbide in Harsh Environments

In addition to wear resistance, drilling equipment must also cope with the complex chemical environment underground, such as high salinity formation water and corrosive gases. Tungsten carbide coatings have good chemical stability and can resist erosion from acids, alkalis, and salts, preventing corrosive damage to the equipment during prolonged operation. The high-temperature stability of the coating also allows it to maintain structural integrity in high-temperature and high-pressure drilling environments, preventing equipment failure under extreme conditions.

Prospects for Coating Technology in the Oil and Gas Industry

As the depth and difficulty of oil and gas extraction increase, the requirements for equipment materials become ever more stringent. Tungsten carbide coating technology will continue to play a key role in deep-sea drilling, high-pressure gas wells, and the extraction of unconventional oil and gas resources. In the future, the technology combining tungsten carbide coatings with other composite materials will be further developed to meet the performance demands of equipment in extreme environments. At the same time, the development of environmentally friendly coatings and repairable coating technologies will further promote the application and popularization of tungsten carbide coatings in the oil and gas industry.

Automotive Industry

Demand for Wear-Resistant Coatings in Automotive Parts

With the development of the automotive industry, especially the rise of new energy vehicles and high-performance cars, the demand for wear-resistant coatings in automotive parts is increasingly growing. Key components such as engine parts, transmission systems, suspension systems, and brake systems all require the application of high-performance wear-resistant coatings to enhance durability and reliability, and reduce maintenance costs.

Application of Tungsten Carbide in Engines and Transmission Systems

In automotive engines and transmission systems, tungsten carbide coatings are primarily used for components such as piston rings, camshafts, crankshafts, and gears. These components operate under high loads and high temperatures, making them prone to wear and fatigue failure. The application of tungsten carbide coatings can effectively reduce the coefficient of friction, enhance the wear resistance, and increase the service life of these components. Additionally, the coatings can improve the thermal efficiency of the engine, reduce fuel consumption, and lower exhaust emissions, which is of great significance for enhancing the environmental performance of automobiles.

Future Directions of Coating Technology in the Automotive Industry

Looking ahead, as the automotive industry moves towards intelligence, lightweight, and electrification, coating technology will continue to play a significant role. Tungsten carbide coatings will continue to work on improving component performance, extending service life, and reducing energy consumption. The development of nanoscale coatings, composite coatings, and self-healing coatings will further promote technological progress in the automotive industry. At the same time, the environmental friendliness of tungsten carbide coating technology will receive more attention. Future coating processes will place greater emphasis on reducing environmental impact, promoting the development of green manufacturing.

Study on the Preparation of Ultrafine Cemented Carbide by High-Energy Ball Milling

joshua — Mon, 20 Jan 2025 03:21:41 +0000

High-energy ball milling is the simplest and most effective to produce ultrafine cemented carbide materials, mainly due to two aspects: one is the production of ultrafine or even nanoscale powders, and the other is the inhibition of grain growth during sintering. To prepare high-quality cemented carbide, not only must the particle size of the powder raw material be controlled at the micro-nano level, but the particle size distribution must also be narrow. Moreover, control of the phase structure of the alloy, especially carbon control, is also crucial.. Therefore, this paper will study the use of high-energy ball milling to prepare raw material powders for cemented carbide, and then obtain ultrafine cemented carburo through pressing and sintering methods. This paper will investigate the role of high-energy ball milling and the influence of ball milling conditions on powder particle size.

Main Preparation Techniques for Micro-Nano Cemented Carbide Powders:

1.Low-Temperature Reduction Decomposition Method

This method is an improved version of the conventional reduction carburization method to prepare micro-nano cemented carbide powders. It uses a low-temperature hydrogen process to reduce tungsten acid, tungsten oxide, or tungstic acid to micro-nano-scale tungsten powder, followed by carburization of the tungsten powder to micro-nano-scale WC powder at low temperatures.

2.Mechanical Alloying (MA)

This is a traditional method using mechanical force for the chemical synthesis of micro-nano cemented carbide powders. It involves placing a certain proportion of elemental powder mixtures in a ball mill jar and subjecting them to high-energy ball milling under an inert atmosphere for a long time. The powder particles undergo repeated grinding, breaking, extrusion, cold welding, and low-temperature solid-state chemical processes under the action of mechanical force, resulting in alloy powders with uniform composition and structure.

3.Spray Drying Method

Also known as thermochemical synthesis, this is currently the main method for industrial mass production of WC-Co composite powders. The process was developed by L.E. McCandlish and B.E. Kear of Rutgers University and has been patented. The Nanodyne Company in the United States uses this process to produce nanoscale WC-Co powder with a particle size of 20～40nm. The process involves mixing ammonium metatungstate [(NH?)?(H?WO??O?o)·4H?O] aqueous solution with cobalt chloride (CoCl·nH?O) to form an original solution, which is then atomized and dried to form a uniformly composed, fine mixture of tungsten and cobalt salts, followed by reduction and carburization in a fluidized bed to obtain nanophase WC-Co powder.

4.Gas Phase Reaction Method

This method uses the principle of gas-phase chemical reaction deposition to produce powders. It involves evaporating and vaporizing metals or alloys in equipment and reacting with active gases at certain temperatures to produce metal compounds, which are then condensed to obtain micro-nano-scale compound powders.

Experimental Method

High-purity cobalt powder and 0.8μm tungsten carbide powder were used as raw materials, mixed in a WC-10%Co ratio, and subjected to high-energy ball milling with a ball-to-material ratio of 9:1 using a QM-IF type planetary ball mill. The ball milling time was set at 24 hours per interval, with milling times of 24, 48, 72, and 96 hours. The particle size of the milled powder was measured using the Fsss method and X-ray diffraction. The powder samples with different ball milling times were then pressed and sintered under the same conditions to prepare cemented carburo specimens. Subsequently, strength and hardness tests were conducted, along with metallographic analysis.

Results and Analysis

The Influence of Ball Milling Time on Powder Particle Size and Grain Size

Table 1 shows the measured Fsss particle sizes. It can be seen that the powder particle size decreases with the extension of ball milling time but becomes coarser after decreasing to a certain extent. However, the grain size continuously decreases. Powder particles and grains are different concepts. Particles consist of multiple grains encapsulated by cobalt powder. As ball milling progresses, while powder particles break, cold welding can also occur between the cobalt on the particle surfaces. Therefore, when ball milling reaches a certain degree, particle agglomeration exceeds breaking, leading to an increase in particle size, which eventually maintains a certain equilibrium state. The measured grain size is usually WC grains, which are brittle phases and easily broken during ball milling. Due to the encapsulation by cobalt powder, it is difficult for WC grains to cold weld, so the grain size continuously decreases.

Microstructure After Sintering

Figure 3 shows the metallographic structure obtained after sintering, clearly indicating the influence of ball milling time on the grain size, shape, and distribution. As the ball milling time extends, the grains in the sintered body are significantly refined, tend to be uniform in size, and the WC grains are more dispersed. This is due to two factors: first, the WC grains themselves are refined and homogenized through ball milling, indicating that ball milling not only breaks the WC grains but also homogenizes them; second, it is formed during the sintering process. The sintering process only coarsens the particles and causes non-uniformity due to abnormal grain growth, which typically becomes more severe with longer ball milling time, as ball milling can cause lattice distortion, promoting abnormal grain growth. However, the results of this experiment do not show this; instead, the grain size tends to be more uniform with the extension of ball milling time. Clearly, the homogenization of WC grains is due to the effect of ball milling. This demonstrates that high-energy ball milling can refine and homogenize WC grains.

The Effect of Ball Milling Time on the Mechanical Properties of Ultrafine Cemented Carbide

Tables 1 and 2 show the measured bending strength and hardness, respectively. It can be seen from the tables that both strength and hardness increase with the extension of ball milling time. As previously analyzed, the longer the ball milling time, the finer the WC grains in the sintered samples and the more uniform their distribution. This indicates that bending strength and hardness increase simultaneously with the refinement of grains. In the grain size range above the micron level, the strength and hardness of cemented carbide typically have an inverse relationship; that is, as bending strength increases, hardness decreases, and vice versa. However, in this case, both have improved simultaneously, which clearly shows that after high-energy ball milling, the obtained cemented carbide has reached the micro-nano scale grain size range. The hardness improvement of the cemented carbide in this study is very significant; generally, WC-10%Co has a hardness of about HRA91, but here it has reached as high as 92.8. This indicates that grain refinement has a very strong strengthening effect on cemented carbide.

Conclusión

This paper has conducted a preliminary study on the relationship between ball milling time and grain size in WC-Co ultrafine cemented carbide, as well as the relationship between grain size and strength, hardness, and the following conclusions are drawn:

High-energy ball milling has a very strong breaking effect on WC grains, and the WC grain size refines with the increase of ball milling time. However, there is a critical point for the powder particle size during ball milling. Upon reaching this critical point, the grain size of the cemented carbide is the smallest, after which, with the increase of ball milling time, the grains may become coarser instead.

Micro-nano grain size cemented carbide can be obtained through high-energy ball milling, and the WC grains are more uniform and dispersed.

The refinement of the grain size of cemented carbide to the micro-nano scale can simultaneously increase the bending strength and hardness.

What is the Effect of Different Abrasive Shapes on the Performance of Cemented Carbide Powder Abrasives?

joshua — Sat, 11 Jan 2025 02:05:56 +0000

Grinding is an important step in the preparation of cemented carbide mixtures and is a process that directly controls the alloy grain size, which has a significant impact on the performance of cemented carbide. The use of appropriate ball milling equipment and the setting of reasonable grinding process parameters are necessary to produce mixtures that meet the requirements.

Research on the grinding process of WC-based cemented carbide mainly focuses on grinding speed, grinding time, and ball-to-material ratio, with less research on the shape of the grinding media. Therefore, this paper selects grinding media of different shapes, prepares WC-10%Co cemented carbide, and studies the influence of the shape of the grinding media on the micro-morphology of the powder, the morphology, and performance of the alloy, thereby providing a reference for the development of a reasonable grinding process.

Experimental Methods and Procedures

The technical parameters of the WC powder used are shown in Table 1. Wet grinding was carried out using a cemented carbide wet mill jar. WC powder, Co powder, and Cr?C? were proportioned according to the experimental design plan in Table 2, with a ball-to-material mass ratio of 4:1. Different WC-Co grinding media (spherical 06.5 and rod-shaped 07×14, as shown in Figure 1) were selected for wet grinding and mixing in a drum mill at 90 r/min. The addition of wet grinding medium alcohol was 280 mL-kg1, the binder was PEG4000 (2.0 wt.%), and the grinding time was 25h and 40h. After grinding, the slurry was placed in a vacuum drying oven to dry. After screening, the mixture was pressed into green bodies of 6 mm×10 mm×15 mm specification at a pressure of 150 MPa. The green bodies were dewaxed and sintered in a sintering furnace. Dewaxing was carried out using a slight positive pressure, with a temperature range of 180-500°C. The sintering temperature was 1450°C, with a holding time of 2h, and finally, the cemented carbide samples were obtained.

The micro-morphology of the samples was observed using a German Zeiss EVO 18 scanning electron microscope, and the average grain size of the alloy was measured using the intercept method. The specific surface area of the powder was determined using an American Conta Monosorb MS specific surface area meter. The density of the samples was obtained by the Archimedes method, and the relative density was calculated. The coercive force (Hc) of the samples was tested using a coercive force analyzer (Zhongda-ZDHC40). The transverse rupture strength (TRS) of the samples was tested according to the GBT 3851-1983 B standard using a Sansi UTM5105 electronic universal testing machine. The fracture toughness of the samples was tested according to the ASTM B771 standard using a Sansi UTM5105 electronic universal testing machine. The hardness of the samples was tested using a Rockwell hardness tester (Wilson-RS74).

Analysis and Discussion of Results

Figure 2 shows the SEM images of powders prepared with different shapes of grinding media and different grinding times. As shown in the figure, with the increase of grinding time, the average particle size of the powder gradually decreased whether using grinding balls or rods. This is because as the grinding time increases, the breaking and extruding effects of the grinding media and the mill jar on the powder also deepen continuously. The more energy produced during grinding, the more intense the impact and shear the powder receives, leading to the generation of a large number of dislocations. The particles continuously break along the particle interfaces and grain boundaries, resulting in the continuous refinement and homogenization of the powder.

The state of cemented carbide powder during time ball milling less than 40 hours

When the grinding time reached 40h, the specific surface area of the powder obtained by rod grinding was 2.01 m2·g1, which was higher than the 1.85 m2·g1 obtained by ball grinding. The finer the powder, the larger the specific surface area, and the higher the powder activity, with greater surface energy, making it easier to agglomerate together and adsorb oxygen. This is beneficial for pore shrinkage and the disappearance of vacancy clusters during sintering, achieving densification.

Powder status after ball milling time more than 40h

After the grinding time reached 40h, the powder produced using grinding balls had some coarse particles and generated more broken powder, which adhered to the surface of larger particles or agglomerated together. Additionally, the powder produced using grinding rods appeared rounder in appearance, while the powder produced using grinding balls had an irregular shape. This is due to the different contact methods of the two shapes of grinding media, as shown in Figure 3 for a schematic of the contact between grinding balls and rods. The contact method between grinding balls is point contact, which easily produces a larger force at the points of contact, leading to a higher likelihood of breakage. Moreover, during the movement of the grinding balls, the contact with the powder is non-selective, resulting in low precision of the breakage. This leads to the powder ground with balls being irregularly broken, producing a large amount of broken powder. In contrast, the contact method between grinding rods is a combination of line contact and point contact. During the grinding process, the force applied at the points of contact is more dispersed, avoiding the generation of large forces and thus preventing over-grinding. The grinding rods have a selective breaking action that breaks coarse particles while protecting fine particles. In the grinding process, the coarse particles are necessarily the first to be ground, making the probability of coarse particle powder being ground higher than that of fine particle powder. This results in the powder ground with rods being more uniform.

Figure 4 shows the SEM images of alloys prepared with different shapes of grinding media and different grinding times. It can be observed from the figure that WC grains are randomly distributed in the Co phase, with shapes typical of irregular rectangles and triangles . As the grinding time reaches 40h, the size of the WC grains is reduced to varying degrees, and the cemented carbide grains obtained with grinding rods are finer and more uniform. In contrast, the alloy grains obtained with grinding balls exhibit a significant issue of coarse grain inclusion, with obviously large grains present. It can also be found in Figure 4(b) that there are a large number of pores near the large grains, and the presence of large grains and numerous pores will inevitably affect the mechanical properties of the alloy. Figure 5 shows the particle size distribution of alloys prepared with different shapes of grinding media and different grinding times. After the grinding time extends from 25h to 40h, the average grain size (D) of the spherical medium decreases from 1.530 μm to 0.618 μm, and the average grain size of the rod-shaped medium decreases from 1.847 μm to 0.538 μm. The distribution curve in the graph becomes narrower, and the standard deviation significantly decreases, indicating that the grains become more uniformly distributed as the grinding time increases. Combined with Figure 4, it can be found that after 40h of grinding, the alloy grains obtained with grinding rods are finer and more uniform, while the cemented carbide grains obtained with grinding balls have coarse grain inclusion, with obviously large grains observable. This is because the grinding intensity of the grinding balls is higher than that of the grinding rods, which easily produces broken powder and irregular large particles, leading to abnormal grain growth during sintering.

Table 3 shows the properties of cemented carbides prepared with different shapes of grinding media and different grinding times. After 25h of grinding, the relative density of the alloy ground with grinding balls is 97.4%, which is higher than the 97.1% of the alloy ground with grinding rods; when the grinding time is increased to 40h, the relative density of the alloy ground with grinding rods is 99.6%, higher than the 99.0% of the alloy ground with grinding balls. The main factors affecting the relative density of cemented carbide include pores, specific surface area, carbon content, and composition. As the grinding time is extended, the relative density improves, which could be due to the alloy grains becoming finer and more uniform with the extended grinding time, resulting in reduced pores and increased density. After 40h of grinding, the alloy ground with grinding balls has large grains, and as observed in Figure 4(b), there are a large number of pores near the large grains, which weakens the density. The alloy ground with grinding rods has finer and more uniform grains, and the powder has a larger specific surface area and higher activity, which is beneficial for pore shrinkage and the disappearance of vacancy clusters during sintering, promoting densification and thus achieving a higher relative density.

It can be seen from Table 3 that as the grinding time extends, the hardness and bending strength of the alloys ground with both types of grinding media increase. At 40h of grinding time, the hardness and bending strength of the alloy ground with grinding rods are higher. First, due to the dispersion and mixing of agglomerated mixtures during grinding, as the grinding time increases, the WC grains become finer, and the finer WC grains will reduce the contact between each other, prompting an increase in the average free path of the Co phase, with a more uniform distribution, increasing the effective deformation range, and thereby increasing the hardness and bending strength of the cemented carbide . The alloy ground with grinding rods for a longer time has finer and more uniform grains, with a better fine-grain strengthening effect, so at 40h of grinding time, the hardness and bending strength of the alloy ground with grinding rods are higher. Secondly, hardness and bending strength are also closely related to density and pores; the presence of pores will weaken the alloy’s ability to resist damage, according to the empirical formula:

In the formula: σ represents the strength of the cemented carbide corresponding to the porosity P; σ0 represents the strength of the alloy when the porosity is zero; b is a constant; P is the porosity. Under the same conditions, the higher the porosity of the material, the smaller the effective area that bears the load, resulting in a lower corresponding material strength. Therefore, as shown in Figures 4 and 3, the alloy ground with grinding balls has large grains, and there are a large number of pores near the large grains, which reduces its density. The alloy ground with grinding rods has finer and more uniform grains, fewer pores, and greater density. Hence, as the grinding time increases, the hardness and bending strength of the alloy are enhanced, with the alloy ground with grinding rods being higher.

Finally, according to the Hall-Patch formula, the relationship between the alloy strength and grain size is as follows:

In the formula, σ represents the strength of the cemented carbide ; d represents the grain size. Combining Figures 2 and 4, when the grinding time reaches 40h, the grains ground with rod-shaped media are finer and more uniform, and the density is also higher. Therefore, the more refined the alloy grains become, the stronger the fine-grain strengthening effect will be, leading to a higher bending strength and hardness of the alloy. In contrast, the alloy prepared at a grinding time of 25h, especially when using grinding rods, results in coarser grains, lower density, and more pores. This leads to a lower average free path of the Co phase, uneven distribution of the binder phase, and a shorter effective deformation range. Consequently, the bending strength and hardness of the alloy are both low.

For the fracture toughness of the samples, it can be observed from Table 3 that at a grinding time of 40h, the fracture toughness of the alloy ground with rod-shaped media is 9.5 MPam1/2, which is lower than that of the cemented carbide ground with spherical media at 10.3 MPam1/2, and this is inversely proportional to the hardness relationship between the two. Combining with Figure 4, it can be found that the alloy ground with grinding balls may have coarse grains, which hinder crack propagation, making transgranular fracture more likely to occur at large grains. Moreover, the larger the grain size, the stronger the ability to accommodate moving dislocations, and the greater the resistance to crack propagation. The cemented carbide ground with grinding rods has finer and more uniform grains, mainly exhibiting intergranular fracture, making crack propagation easier and leading to a decrease in fracture toughness. Therefore, the fracture toughness of the alloy ground with grinding balls is higher than that of the alloy ground with grinding rods.

It can also be known from Table 3 that as the grinding time increases, the coercive force of the alloy ground with grinding balls increases from 103 kA m-1 to 123 kA m-1, and the coercive force of the alloy ground with grinding rods increases from 97 kA m-1 to 129 kA m-1. According to literature reports, the coercive force of the alloy has the following relationship with the WC grain size and Co content:

In the formula, Hc represents the coercive force of the alloy; dwc represents the grain size of WC; Wc represents the mass fraction of Co in the alloy. In this experiment, the Co content is the same for all samples. Therefore, it can be understood that the thinner the grain size of the alloy, the smaller the thickness of the magnetic bonding phase will be, and the more evenly it will be dispersed, leading to an increase in coercive force. When the grinding time reaches 40h, whether using grinding balls or rods, the coercive force of the cemented carbide is significantly improved due to the refinement of the grains with increased grinding time. Since the alloy ground with grinding rods has finer and more uniform grains, whereas the alloy ground with grinding balls has larger grains and poorer uniformity, the coercive force of the alloy ground with grinding rods is higher.

Conclusions

WC-10%Co cemented carbide with the same composition was prepared using grinding media of different shapes. The micro-morphology of the powder and the morphology and properties of the alloy were studied and analyzed, and the following conclusions were drawn:

Compared to grinding for 25h, the powder ground for 40h with both shapes of grinding media is more refined. However, the grinding intensity of the grinding balls is higher than that of the grinding rods, leading to the presence of coarse grains and broken powder in the ground material, which affects the properties of the alloy.

The grain size of the alloy ground for 40h with both shapes of grinding media is finer and more uniform than that ground for 25h. Compared to the alloy ground with grinding balls, which has abnormally large grains deteriorating the grain distribution and properties of the alloy, the alloy ground with grinding rods has finer and more uniform grains, enhancing the properties of the alloy. The alloy ground with grinding rods for 40h can achieve better properties: relative density of 99.6%, coercive force of 129 kA·m-1, hardness of HRA 91.5, fracture toughness of 9.5 MPa·m1/2, and bending strength of 3565 MPa.

La influencia del tama?o de las partículas de negro de carbón en el contenido de carbono del polvo de WC

joshua — Sat, 07 Dec 2024 02:36:38 +0000

In recent years, the production technology of ultra-fine grain cemented carbide has developed rapidly, especially in the key technology of preparing ultra-fine grain WC powder, where the competition is very fierce. Many promising methods have emerged, some of which have reached the level of practical application. However, there are few reports in domestic literature, especially on basic theories or regularity research. In the process of developing the preparation technology of ultra-fine grain WC powder, this paper deeply investigates the influence of the particle size of carbon black powder on the carbon content of WC powder under different carbonization temperatures when using ultra-fine (0.35 μm) tungsten powder.

Experimental Content and Method

Test Materials of WC powder

(1) Ultra-fine tungsten powder: The ultra-fine tungsten powder is prepared by the ultrasonic spray pyrolysis method from ultra-fine WO? powder, which is generally amorphous with an average particle size of 25~30 nm. It is reduced by hydrogen at a medium temperature (750~780℃) to obtain tungsten powder with an average particle size of ≤0.35μm (BET particle size).

(2) Carbon black powder: Carbon black powder is produced by two methods: one is by cracking ethane and propane (at 850℃) and then subjected to high-energy ball milling for different periods to produce carbon black powders with average particle sizes of 0.1 and 0.3 μm, respectively; the other is from activated carbon powder with an original particle size of 100~200μm, which is subjected to high-energy ball milling for different periods to produce carbon black powders with average particle sizes of 0.8 and 4.5 μm, respectively.

Experimental Method

Using ultra-fine tungsten powder with the same particle size (0.35 μm) and four different particle sizes (0.1, 0.3, 0.8, 4.5 μm) of carbon black powder, the carbon is blended according to the reaction formula W+C=WC (with an additional loss of 0.1%). The mixture is ball milled for 1.5 hours in a conventional ball mill with a ball-to-material ratio of 1:1. The heat-resistant stainless steel boat containing the mixed material is placed in a stainless steel tube furnace and carbonized under a hydrogen atmosphere. The holding time is 40 minutes for all. In the temperature range of 830~1300℃, the carbonized material is taken out at different temperatures, cooled, and then removed from the furnace. Subsequently, XRD phase analysis and chemical analysis are conducted to determine the total carbon and free carbon, and the amount of combined carbon in WC is calculated. Finally, the quantitative relationship between the particle size of carbon black powder and the phase composition and combined carbon content of WC powder under different carbonization temperatures can be plotted.

Test Results and Discussion

Figure 1 shows the effect of carbon black powder with different particle sizes carbonized at different temperatures on the combined carbon content of WC powder. Curve 1 in Figure 1 represents the relationship between carbonization temperature and the combined carbon content of WC powder when using ultra-fine carbon black powder with a particle size of 0.1 μm. As can be seen from Figure 1, when using 0.1 μm ultra-fine carbon black, the combined carbon content of WC powder can reach 5.8% (by mass), which is equivalent to 95% of the theoretical content, at a very low carbonization temperature (850℃). When the temperature is greater than 950℃, the carbon content of WC powder can reach the theoretical content. This result indicates that the carbonization reaction can be completed at a low temperature when using ultra-fine tungsten powder in conjunction with ultra-fine carbon black powder. This phenomenon suggests that the ultra-fine W powder particles are carbonized before they undergo significant aggregation and growth.

At this temperature, as the particle size of the carbon black used increases, the combined carbon content in WC powder rapidly decreases. For example, as shown in Curve 2 of Figure 1, when using 0.3 μm carbon black at 950℃ with the same holding time of 40 minutes, the combined carbon content of WC powder only reaches 5.2% (by mass). XRD analysis indicates that there is still significant W?C in the WC powder, as shown in Figure 2. The free carbon phase was not detected due to its low content. The combined carbon content in WC powder can only reach the theoretical value when the temperature is increased to 1060℃. Its complete carbonization temperature is about 130℃ higher than that of the 0.1 μm carbon black.

When using 0.8 μm carbon black, as shown in Curve 3 of Figure 1, at 950℃ with a holding time of 40 minutes, the combined carbon content of WC is only 3.18% (by mass). XRD analysis indicates that in addition to the obvious W?C phase, there is also significant free carbon present in the WC powder, as shown in Figure 3. The combined carbon content in WC powder can only reach the theoretical value when the carbonization temperature is increased to 1230℃. The temperature required for complete carbonization has increased significantly.
The reason for this phenomenon is that when the carbon content ratio is constant, the finer the carbon black particle size, the easier it is to mix uniformly with W powder particles, increasing the contact area between them. Undoubtedly, fine-grained carbon black will accelerate the carbonization rate. Moreover, the finer the carbon black particle size, the more its specific surface area and the number of active carbon atoms on the surface increase dramatically. Thus, the carbonization reaction rate between carbon and tungsten accelerates, and the complete carbonization temperature decreases with the decrease of carbon black particle size.

Conclusions

(1) When the particle size of ultra-fine W powder is the same (≤0.35μm), the combined carbon content in WC powder decreases significantly with the increase of the carbon black particle size at a certain temperature.
(2) When the particle size of carbon black is constant, the content of combined carbon increases with the rise of carbonization temperature.

Cómo el tama?o del grano transforma la fricción y el desgaste del carburo cementado

joshua — Sat, 30 Nov 2024 06:14:56 +0000

The size of WC grains is of great significance to the friction and wear properties of cemented carbide. It is generally believed that coarse-grained cemented carbide has poorer wear resistance than fine-grained cemented carbide. After the grain refinement of cemented carbide, the size of the hard phase decreases, increasing the surface area of the hard phase grains and the bonding force between the grains, and the binder phase is more evenly distributed around them, which can improve the hardness and wear resistance of the cemented carbide. In this paper, the author conducts friction and wear experiments on cemented carbide to analyze the friction and wear properties under different parameters and the material removal mechanism, providing experimental evidence for the optimization design of high-speed cutting tool materials, reasonable material selection, and the study of high-speed cutting wear mechanisms.

Experiment

Test Materials

Three types of WC-6Co cemented carbide with different grain sizes were selected for the test. The size of the cemented carbide disc was φ55mm×4mm, and the surface was rough ground, finely ground, and polished. The mating material used was Al?O? balls with a diameter of 9.5mm. Both samples were ultrasonically cleaned in acetone for 20 minutes and dried for use. The material properties are shown in Table 1.

Friction and Wear Test

The test was conducted on a UMT-2 multi-functional friction and wear testing machine produced by CETR Corporation in the United States, using a ball-on-disc contact method. The structure of the testing machine is shown in Figure 1. The cemented carbide friction disc was attached to the working table with double-sided tape, and the Al?O? ball was placed in the fixture. The two types of mating materials produced mutual movement and force of action. The friction force generated was detected by the sensor, and the curves of friction force, normal force, and friction coefficient were automatically generated by the related software according to Coulomb’s law.

The test was conducted at room temperature, with normal forces of 10N and 20N respectively, and the linear velocities of the friction pair sliding were 40m/min, 80m/min, 120m/min, and 160m/min. The sliding distance was 500m. After the test, a scanning electron microscope (SEM) was used to observe the wear scar surface morphology of the upper and lower samples, and an X-ray energy dispersive spectrometer (EDS) was used to detect the elemental composition of the worn surfaces. All samples were analyzed for the elemental composition of the friction and wear surfaces under the same conditions.

Results and Analysis

Friction and Wear Performance

Figure 2 shows the friction coefficient curve of ZH cemented carbide drawn by the testing machine’s accompanying software (load 20N, sliding speed 160m/min). The experiment found that each friction process follows a similar pattern, that is, the initial dynamic friction coefficient undergoes a rapid increase from the initial value during the transition period, and then remains relatively stable, showing a fluctuating characteristic in the stable phase. In the beginning, under the action of the normal load, only local micro-convex bodies on the friction surface are in contact, the adhesive area is small, and the molecular attraction on the contact surface is weak, so the friction coefficient is small; as the friction process progresses, the micro-convex bodies interfere with each other, gradually get worn down, the adhesive area increases, and the molecular attraction also increases, leading to a gradual increase in the friction coefficient. The entire friction process is a continuous process of the contact surface adhering and then being sheared under the action of shear stress. Due to the peeling and breaking of the Co phase on the surface, the wear of the sample surface occurs, and the local adhesion on the surface quickly reaches a dynamic equilibrium, resulting in the friction coefficient of the surface being maintained within a relatively stable range, which is called the stable period.

Most scholars use the average value of the friction coefficient over a period of time (distance) as a characterization parameter of friction behavior. Therefore, this experiment selects the average value during the stable friction phase as the friction coefficient of the cemented carbide under the corresponding parameters. Figure 3 shows the friction coefficients of three types of cemented carbide under different loads and speeds.

It can be seen that with the increase of friction speed and load, the friction coefficient of the cemented carbide generally shows a decreasing trend, and the decrease is most obvious in the transition from relatively low speed (40m/min and 80m/min) to high speed (120m/min and 160m/min). From the perspective of material, the friction coefficient of ZH cemented carbide is smaller than that of the other two materials, and the friction coefficients of ZHX and HG cemented carbides are not significantly different, with the friction coefficient of HG cemented carbide being slightly larger.

Wear Mechanism

After the friction and wear tests, the microstructure of the worn surfaces of each sample was observed using a scanning electron microscope (SEM), and SEM images were taken, along with an analysis of the surface composition. The friction and wear mechanisms of the cemented carbide under different friction parameters are similar, as shown in Figure 4 (sliding speed 160m/min, load 20N).

In the initial stage of cemented carbide wear, the binder phase Co undergoes plastic deformation, and the surface layer of Co is extruded by the WC grains. Due to the low hardness and good ductility of Co, under certain conditions, a micron-scale friction film can form on the surface, while the harder WC particles gradually protrude from the friction surface, preventing further rapid wear of the surface and allowing the friction process to enter a relatively stable stage. As the binder phase Co continues to be lost, the WC framework of the material is damaged, and the dislocation density within the WC particles significantly increases. When the dislocation density accumulates to a certain extent, microcracks will form on the WC particles, causing the WC particles to begin to pull out from the cemented carbide matrix. The detached WC particles remain in the wear area, transforming into abrasive particles, which, under the action of the load, compress against the matrix, resulting in new plastic deformation and grain damage.

As can be seen from Figure 4, as the grain size of the cemented carbide decreases, the grain density increases, and the degree of surface wear decreases. The surface of the ZHX cemented carbide shows no obvious shedding of WC particles, while the surface density of HG is very good, with almost no obvious shedding of WC particles. Therefore, for the traditional grain size cemented carbide ZH, the main wear mechanism is abrasive wear caused by the extrusion of the binder phase Co and the shedding of WC grains. As the grain size decreases and the density of the fine-grained cemented carbide increases, the phenomenon of WC grain spalling decreases, Co still wraps around the WC, the microstructure of the material remains intact, and most grains only undergo a certain degree of plastic deformation.

Conclusión

The size of WC grains has an important effect on the friction and wear properties of cemented carbide. As the grain size decreases, the friction coefficient slightly increases, and the wear resistance is enhanced.

The friction coefficient of cemented carbide is influenced by speed and load, and it shows a decreasing trend with the increase of speed and load.

The wear mechanism of traditional grain size cemented carbide is mainly characterized by the extrusion of the binder phase Co and the fracture and spalling of the hard phase WC grains; the grain spalling phenomenon of fine-grained cemented carbide is not obvious, and the main wear mechanism is plastic deformation.

Análisis del rendimiento de tres ligantes Cabide habituales

joshua — Sat, 23 Nov 2024 05:47:38 +0000

Carbides are composed of refractory carbides with high compressive strength, hardness, and elastic modulus, which are difficult to plastically deform during the pressing process. To improve the formability of the powder and increase the strength of the compact, a binder must be added to the powder material before shaping.
As an intermediate auxiliary material, the binder must be completely removed during the degumming stage, as any residue can pose a quality risk to the product. The total carbon content in the alloy must be strictly controlled to produce high-quality carbide products. Although many factors can affect the total carbon content in carbide products, the application of the binder is a crucial aspect, especially when the quality of the tungsten carbide raw material is stable.

Therefore, the performance of the binder is a key factor directly affecting the properties of the blank and the final sintered product.

Research Status and Issues

According to surveys, some carburo manufacturers have used synthetic resins, dextrin, starch, methyl alcohol, and cellulose as binders in the past. For example, East Germany used a mixture of ceresin, paraffin wax, and mineral oil with an addition rate of 48%-59%; General Electric in the United States used starch, rubber, and synthetic resins; the United Kingdom applied water-soluble fibers and polyacrylamide; and some manufacturers even added surfactants.

Foreign carbide manufacturers, equipped with advanced production equipment and high automation levels, use pipeline conveying for mixed material preparation equipment, fully automatic high-precision presses, and multi-atmosphere pressure degumming and sintering integrated furnaces. The binders used in foreign carbide production are primarily paraffin and PEG, with paraffin acetone as the ball milling medium, and rubber as a binder is very rare.

Currently, the widely used binders in domestic carbide manufacturers are rubber, paraffin, and polyethylene glycol (PEG). Depending on the foreign manufacturer from which the technology was introduced and the time of introduction, each manufacturers usage may vary. Manufacturers that have introduced Sandvik technology generally use PEG as a binder and adopt a spray drying process. Some use paraffin as a binder and also adopt a spray drying process. Some enterprises use a combination of binders, and there are also mixtures of rubber and paraffin. SMEs basically use the rubber process, with each type of binder having its own advantages and disadvantages.

Rubber Binders

In the late 1950s and early 1960s, the carbide industry in China used butadiene sodium rubber imported from the Soviet Union, which had stable rubber quality. Later, due to changes in the situation, domestically produced synthetic butadiene sodium rubber from Lanzhou was used. Due to manufacturing process technology and equipment issues, the quality stability of the rubber was poor. The butadiene sodium rubber dissolved in gasoline had a high gel content, and the solution was suspended, making filtration difficult, with high ash and impurity content, which affected the normal production of the alloy.

Rubber solvents have good formability and can press out products with complex shapes and larger volumes, and the compact is less likely to crack. However, the disadvantages include high ash content, high residual carbon, difficulty in precise carbon control, vacuum removal, and unstable product quality, and it is not suitable for the spray drying process.

Paraffin-Type Binders

Paraffin is derived from petroleum refining and is a mixture of various hydrocarbons, with a small amount of liquid “impurities” present as oil, and the solid component is saturated alkanes. The properties of paraffin are ultimately determined by its chemical composition, whether they are straight-chain, branched, or cyclic structures. Paraffins can be classified into: paraffin, microcrystalline wax, montan wax, vegetable wax, animal wax, and synthetic wax. There are dozens to hundreds of different varieties, each with different molecular weight, structure, performance, and uses.
The paraffin used for carbides is mainly composed of normal alkanes, with few straight-chain molecules and aromatic hydrocarbons. The molecular weight range is 360-540, with a melting point of 42-70 degrees and slight solubility in ethanol. Microcrystalline wax has a molecular weight of 580-700, mostly branched molecules, with more cyclic hydrocarbon compounds. Paraffin is brittle, while microcrystalline wax is stronger and more flexible, with higher tensile strength and melting point, greater adhesiveness, and is a saturated straight-chain hydrocarbon that can completely volatilize at high temperatures without leaving any residue and is easily removed under vacuum. This reduces the difficulty in controlling the carbon content and improves the precision of the carbon content in the alloy, but it has a low viscosity, resulting in low compaction strength and large elastic after-effect, which makes it prone to cracking at stress concentration areas, difficult to produce complex-shaped products, and the compacts are brittle and prone to chipping.

Water-Soluble Polymer Binders

PEG (Polyethylene Glycol) is a water-soluble polymer, and according to foreign literature, PEG is classified as a synthetic wax. It is prepared by stepwise addition of ethylene oxide to water or ethylene glycol, with a molecular weight range of 200-20000. PEG is completely soluble in water and has very low solubility in ethanol at room temperature (less than 1%). It is compatible with many substances and shows the greatest compatibility with substances with high polarity. It is non-toxic and non-irritating. The formability of PEG is equivalent to that of paraffin, and it has less residual carbon. Therefore, it can be considered a safe and environmentally friendly binder suitable for spray drying. However, PEG has a serious tendency to absorb moisture, and its moisture absorption capacity decreases with increasing molecular weight. It has very strict requirements for humidity and temperature in the working environment. After absorbing moisture, the powder becomes hard, the pressing pressure increases, and higher requirements are placed on the press. Additionally, it is more difficult to form some complex products.

Comparison in Actual Production

To compare the performance of the three binders, three batches of mixed materials were prepared using sodium butadiene rubber, paraffin, and PEG as binders. The basic composition of the mixture was WC-8%Co, and the blanks were compressed to the same weight and then sintered in a vacuum degassing integrated furnace to obtain metallographic and physical properties for comparison.

Experimental Section

The WC particle size used in this experiment was 6.5 m. The rubber used was sodium butadiene rubber, paraffin, and PEG.
The rubber and paraffin materials used aviation gasoline as the wet milling medium, while the PEG material used anhydrous alcohol as the ball milling medium. After ball milling, all materials were dried in a vacuum, screened, and granulated before pressing the compacts. They were then sintered under vacuum and pressure at a temperature of 1430°C.

From a direct analysis of the physical and mechanical performance data, it can be observed that the samples using paraffin and PEG as binders have increased strength and reduced magnetism, which is a significant advantage for mining carbides. Additionally, the metallographic photographs indicate that the microstructure using paraffin and PEG binders is more uniform compared to rubber binders. This is because paraffin and PEG have less residual carbon, while rubber binders are difficult to remove, leading to the growth of local grains due to the presence of a large amount of residual carbon.
Due to the lack of spray granulation equipment, the mixed materials using paraffin and PEG as binders were dried in a vacuum and then granulated using a manual screen. This had a significant impact on the pressing performance of the mixed materials, such as the accumulation of PEG in the drying process causing uneven distribution within the material, leading to agglomeration in the alloy phase. The poor effect of manually screening paraffin also posed a problem. However, from the perspective of the physical performance of the samples, it is still evident that PEG and paraffin have advantages over the rubber process.
During the experiment, the poor formability of paraffin due to manual screening was addressed by using manual weighing and pressing methods. However, in actual production, to accommodate large-scale production with self-pressing machines, increasing the pressing pressure and extending the holding time were necessary to avoid cracks or chipping of the paraffin material, which would reduce labor efficiency. Using a spray drying system to obtain a well-flowing mixture can effectively solve this problem.
The above discussion is a preliminary exploration of three commonly used binders in China. The research on binders is a systemic project involving a wide range of knowledge. To conduct in-depth research, one must possess knowledge in organic chemistry, polymer chemistry, and combine it with practical production knowledge of powder metallurgy to apply it to the production process of carbides. This will be a long-term and challenging task.

Conclusión

With the continuous expansion of research and application fields of carbide materials, such as the emergence of ultra-fine and nano-carbides, and the extensive use of metal ceramics and ceramic materials, the raw materials for these products have undergone significant changes compared to the previous ordinary carbides. They have smaller particle sizes, lower bulk densities, poorer fluidity, and much worse forming performance than ordinary carbides. Therefore, a more excellent binder is needed. Specifically, research can be initiated in the following three aspects:
1.Studying the interaction between different types of powder materials and binders to understand the impact on forming performance.
2.Developing new polymer binders with different characteristics by combining different components.

3.Researching the thermal cracking characteristics of binders to meet the requirements of carbide production processes in terms of process characteristics and residual carbon content.
Through the above three aspects of research, it is expected to obtain a new generation of binders with good forming performance, environmental friendliness, stable performance, no toxicity, and no residue at the molecular level.

?Qué es el prensado isostático en caliente a baja presión de carburo de minería reciclado?

joshua — Fri, 15 Nov 2024 07:09:40 +0000

Low-pressure hot isostatic pressing (HIP) is a new sintering process developed in Western developed countries in the 1980s, which combines vacuum sintering and hot isostatic pressing in a single device to complete the process in one step. We have utilized low-pressure hot isostatic pressing technology to manufacture recycled mining carbide, which effectively improves the mechanical and physical properties of the alloys, resulting in a virtually pore-free microstructure and excellent rock drilling performance on-site.

Experimental Method

Recycled WC powder with a Fisher particle size of 3.00～10.00 μm and normal WC powder with a Fisher particle size of 10.00～18.00 μm were mixed with Co powder or Ni powder with a loose packing density of 0.5～0.7g/cm3 to prepare mixtures of grades YJ1, YJ2, N309, etc. The mixtures were shaped, degummed, and then sintered in a domestically produced horizontal vacuum furnace and a low-pressure hot isostatic pressing furnace manufactured by a German specialized equipment company. The low-pressure hot isostatic pressing process is as follows: loading → vacuum pumping → heating → maintaining sintering temperature → charging argon and pressurizing → maintaining pressure and temperature → cooling and depressurizing → unloading. Electron microscopy was used for metallographic analysis, and the linear shrinkage and shrinkage rate of the samples during the sintering process were measured by the low-pressure hot isostatic pressing sintering furnace to analyze the densification process. The test alloys were made into D43×22 straight horseshoe bits for calibration tests in mining operations.

Experimental Results

Comparison of Properties

Between Low-Pressure Hot Isostatic Pressing Treatment of Recycled Material and Vacuum Sintering Treatment of Normal Material. The two types of tungsten carbide powders, recycled and normal, were processed using the same manufacturing process, undergoing vacuum sintering and low-pressure hot isostatic pressing treatment, respectively. The results are listed in Table 1.

As can be seen from Table 1, the porosity of the alloy treated with low-pressure hot isostatic pressing using recycled WC powder is even lower than that of the normal alloy, and its performance has been significantly improved, with an increase in the transverse rupture strength value; moreover, the elimination of type B pores ranging from 10 to 25 μm indicates the intrinsic relationship between the reduction in porosity and the increase in transverse rupture strength, while also confirming the capability of low-pressure hot isostatic pressing sintering to eliminate pores in recycled alloys.

Low-Pressure Hot Isostatic Pressing Alloy Linear Shrinkage Test

The linear shrinkage and shrinkage rate of the samples during the sintering process in the low-pressure hot isostatic pressing furnace were measured as shown in the attached figure. The alloy undergoes two stages: vacuum sintering and hot isostatic pressing. The macroscopic pores are eliminated during the vacuum sintering stage, and the microscopic pores are eliminated during the hot isostatic pressing stage to achieve the final densification level.

Comparison of On-site Rock Drilling Effects

The two types of tungsten carbide?powders, recycled and normal, were made into alloys of grades YJ1, YJ2, N309, etc., and calibration tests were conducted at the Taolin Lead-Zinc Mine. The results are listed in Table 2.

The rock drilling calibration indicates that high-quality mining carbide?can be produced from recycled WC powder through low-pressure hot isostatic pressing treatment, and their performance is comparable to that of mining carbide?made from normal tungsten carbide.

Result Analysis

Process Characteristics of Low-Pressure Hot Isostatic Pressing for Eliminating Pores in Recycled carbide

The densification of carbide?primarily occurs during sintering, where the plastic flow of the binder phase and the rearrangement of WC grains are driven by surface tension. However, under atmospheric or vacuum sintering, a certain amount of porosity always remains after shrinkage densification is complete; this is because when pores are sealed, the stress inside the pores reaches equilibrium with the surface tension of the pores. Additionally, due to the mixed composition of recycled materials and the presence of more harmful impurities, large pores and voids are easily formed during vacuum sintering, leading to issues such as low alloy density, low fracture strength, significant hardness variations, and severe contamination of the alloy. Applying a certain pressure can promote further flow of the binder phase and rearrangement of WC grains, thereby greatly reducing or even completely eliminating these pores or voids.

Study on the Densification Mechanism of Low-Pressure Hot Isostatic Pressing

The change curve of the linear shrinkage rate of recycled carbide?samples during low-pressure hot isostatic pressing sintering is shown in the attached figure. There are three peaks on the shrinkage rate curve: Peak A appears at a sintering temperature of 1200°C, which is solid-phase sintering. Due to the low yield point of the binder phase, plastic flow occurs under a small external force. The flow of the binder metal changes the contact situation between powder particles, causing the carbide?particles to move and come closer together. Peak B appears during the liquid-phase sintering process at 1340°C, where WC particle rearrangement, solution precipitation, and skeleton formation result in significant shrinkage of the sintered body, and macroscopic pores are eliminated during the vacuum sintering process of low-pressure hot isostatic pressing. Peak C appears at the beginning of the pressurization stage, where the rise in pressure eliminates the micro-pores in the product. However, with the extension of the pressure maintenance time, no new shrinkage peak appears in the product.

Conclusión

(1) The physical and mechanical properties of the recycled alloy treated by low-pressure hot isostatic pressing are superior to those of alloys manufactured by conventional processes, with a significant reduction in porosity and the elimination of type B pores.

(2) The recycled alloy treated by low-pressure hot isostatic pressing does not fall short of normal alloys in on-site rock drilling tests, and its wear resistance is even improved.

(3) The mechanism by which low-pressure hot isostatic pressing improves the performance of the alloy is mainly the elimination of large-sized pores and the reduction in porosity.

Estudio de las propiedades de los polvos esféricos de carburo de tungsteno fundidos preparados por diferentes métodos

joshua — Sat, 09 Nov 2024 03:20:06 +0000

Many structures made of WC-Co carbide are subject to thermo-mechanical loading and have to conduct heat in order to function properly in industrial application. The current work provides results on a significant drop in thermal conductivity of WC-Co carbides as a function of material volume damage that accumulates during cyclic high-temperature loading of the materials depending on material microstructure. Average WC grain size and Co binder metal content of the investigated grades ranged from submicron to medium and from 10 to 12 wt%, respectively. The carbides were subjected to uniaxial cyclic loading in a vacuum for different numbers of load cycles at 700 °C and 800 °C. Damage features accumulated in the material volume were documented by means of scanning electron microscopy. Thermal conductivity properties of virgin and damaged materials were determined via laser flash analysis. The results indicated a significant decrease depending on the materials’ microstructure, i.e. the defects’ predominant location within the microstructure. The damage features that occurred mainly between WC grains in the coarser-grained grade led to larger drops in thermal conductivity with rising temperature compared to damage features that occurred within the Co binder metal in the finer-grained grade. The presented results are of high relevance to the thermo-mechanical load situation of e.g. milling tools since the heat conduction away from their cutting edges is hindered by the documented effect and deemed to lead to a self-acceleration of the damage accumulation.

Spherical cast tungsten carbide powder is a new type of ultra-wear-resistant ceramic particle material. Compared with traditional tungsten carbide, spherical cast tungsten carbide has two significant advantages: first, it has a regular spherical appearance, good powder flowability, and wettability, which results in good integration with the surrounding tissue when added as particles, reducing the likelihood of stress concentration; second, the internal structure of the tungsten carbide particles is dense, with good toughness, fine grains, high hardness, and the coating has excellent wear resistance and is less likely to break under load. Due to its outstanding performance, spherical cast tungsten carbide powder is gradually replacing traditional tungsten carbide powder in the surface protection of components in mining machinery, oil machinery, construction industry, and foundries, significantly improving the wear resistance, corrosion resistance, and oxidation resistance of workpieces, and extending the service life of workpieces.

Introduction to the Methods of Spherical Cast Tungsten Carbide

Currently, the spherical cast tungsten carbide powders available in the market are mainly prepared by the following methods: induction remelting spheroidization, plasma remelting spheroidization, and plasma rotating electrode atomization.

The induction remelting spheroidization method involves heating the material in a reactor to the spheroidization temperature through induction heating, and the material moves forward slowly the vibration of the furnace tube. If the dispersion of the material is not well controlled, the molten droplets will grow due to collision and adhesion, making particle size control difficult. Moreover, during the operation, the powder must not come into contact with the reactor, otherwise it will affect the entire spheroidization process and cause material waste.

The plasma remelting spheroidization method uses casting tungsten carbide powder as the raw material and employs radiofrequency plasma flame to heat argon gas to a high temperature of 3000 to 10000 ℃, melting the casting tungsten carbide particles into a liquid state and directly quickly condensing them into spherical particles. This method can easily obtain fine-grained spherical tungsten carbide powder by controlling the particle size and composition of the raw material.

The plasma rotating electrode atomization method uses a tungsten carbide rod as the electrode, fixed within the rod material bin, and then subjected to plasma atomization under inert gas protection. The plasma arc melts the end face of the high-speed rotating rod, and under the action of centrifugal force, the molten droplets separate from the edge of the molten pool and solidify in the form of spherical particles. This technology avoids the difficulty of material dispersion at ultra-high temperatures during remelting spheroidization, and the obtained spherical tungsten carbide powder has a narrow particle size distribution range and is easy to control.

The following will study the chemical composition, micro-morphology, microstructure, microhardness, and other powder properties of spherical cast tungsten carbide powders prepared by different methods.

Chemical composition

The table above shows the chemical composition of spherical cast carburo de tungsteno powder samples prepared by different methods. It can be observed that the main components of the spherical cast tungsten carbide powder are W and C elements, and all contain trace amounts of Fe, V, Cr, and Nb elements. The ideal spherical cast tungsten carbide should be a eutectic of WC and W2C, with an eutectic temperature of 2525 ℃ and a carbon content of 3.840% (by mass) at the eutectic point. From the data in the table, it can be seen that the total carbon content of the spherical cast tungsten carbide prepared by the plasma rotating electrode atomization method has the smallest deviation from the theoretical eutectic carbon content, with the lowest free carbon content; the powder obtained by the induction remelting spheroidization method has the largest difference in total carbon content from the theoretical value, with a difference of 0.170% (by mass). This is due to the carbon content increase caused by the graphite tube heating method used in the induction remelting spheroidization process. In addition, by comparing samples 2#, 3#, and 4# with similar particle sizes, it can be determined that the powder prepared by the plasma rotating electrode atomization method has the relatively lowest impurity content. However, the impurity content of sample 1# prepared by the plasma rotating electrode atomization method is relatively high, which may be related to the quality of the cast tungsten carbide raw material rod. This suggests that, compared to other methods, the plasma rotating electrode atomization method can more accurately control the carbon content of spherical cast tungsten carbide powder, preventing overeutectic and hypoeutectic reactions caused by carburization and decarburization, and obtaining a nearly complete eutectic structure, which is crucial for improving the microstructure and properties of spherical cast tungsten carbide.

Microscopic morphology

The Microscopic Morphology of Spherical Cast Tungsten Carbide Powder Samples

The image above shows the microscopic morphology of spherical cast tungsten carbide powders prepared by different methods. It can be observed that the spherical cast tungsten carbide powders prepared by the three methods are all regular and smooth, nearly spherical in shape.

Cross-sectional Photos of Spherical Cast Tungsten Carbide Powder

The image above shows the cross-sectional photos of spherical cast tungsten carbide powders prepared by different methods. As can be seen from (a) and (b), the spherical tungsten carbide powder particles prepared by the plasma rotating electrode atomization method are dense with almost no defects. However, as seen in (c) and (d), there are some obvious pores within the spherical tungsten carbide powder particles prepared by the plasma remelting spheroidization method and the induction remelting spheroidization method, resulting in some hollow powders. The main reason for this is that the crushed tungsten carbide powder material used in the above methods is likely to contain residual pores from the casting process. During the short plasma or induction heating process, the interior of the crushed tungsten carbide powder is difficult to completely melt, leading to some residual pores within the particles.

Microstructure

Microstructure Photos of Spherical Cast Tungsten Carbide Powder Samples After Corrosion

The image above shows the microstructure photos of spherical cast tungsten carbide powder particles after corrosion. It can be observed that the internal structure of the spherical tungsten carbide powder particles prepared by the three methods mainly consists of a typical fine acicular WC and W2C eutectic structure. Compared to the plasma remelting spheroidization method and the induction remelting spheroidization method, the spherical cast tungsten carbide powder prepared by the plasma rotating electrode atomization method has a denser eutectic structure. This is because, unlike the plasma remelting spheroidization method and the induction remelting spheroidization method, the plasma rotating electrode atomization method completely melts the cast tungsten carbide feedstock rod and then solidifies by being thrown out under the action of centrifugal force. During the crystallization of the molten cast tungsten carbide, the degree of undercooling is greater, nucleation is more rapid, and a larger number of crystal nuclei are generated, resulting in a finer and denser eutectic structure.

Microhardness

The table below shows the average microhardness of spherical cast tungsten carbide powders prepared by different methods. It can be seen that the microhardness of the spherical cast tungsten carbide powders prepared by the three methods is all above 2800 HV0.1, with the powder prepared by the plasma rotating electrode atomization method having the highest microhardness, reaching 3045 HV0.1. This is mainly due to the finer eutectic structure within the spherical cast tungsten carbide prepared by the plasma rotating electrode atomization method.

Other Physical Properties of spherical cast tungsten carbide

The table below shows the flowability and apparent density values of spherical cast tungsten carbide powders prepared by different methods. It can be seen that the powder prepared by the plasma rotating electrode atomization method has the worst flowability and the smallest apparent density; whereas the powder prepared by the induction remelting spheroidization method has the best flowability and the largest apparent density.

Conclusión

(1) The spherical cast tungsten carbide prepared by the plasma rotating electrode atomization method has the smallest deviation from the theoretical eutectic carbon content, the lowest free carbon content, and relatively low impurity content.

(2) The spherical tungsten carbide powder particles prepared by the plasma rotating electrode atomization method are dense with almost no defects, and the eutectic structure is finer. The spherical tungsten carbide powder particles prepared by the plasma remelting spheroidization method and the induction remelting spheroidization method both have some obvious pores, resulting in some hollow powders.

(3) The spherical cast tungsten carbide powders prepared by the three methods mainly consist of WC and W2C phases.

(4) The microhardness of the spherical cast tungsten carbide powders prepared by the three methods is all above 2800 HV0.1, with the powder prepared by the plasma rotating electrode atomization method having the highest microhardness, reaching 3045 HV0.1. The powder prepared by the induction remelting spheroidization method has the best flowability and the largest apparent density.

El efecto de los aditivos sobre las propiedades de los carburos cementados

joshua — Mon, 04 Nov 2024 00:17:57 +0000

Carbides contain mainly two types of additives: one is refractory metal carbides, and the other is metal additives. The functions of additives are as follows:

(1) To reduce the alloy’s sensitivity to sintering temperature fluctuations and carbon content changes, and to prevent the uneven growth of carbide grains;

(2) To change the phase composition of the alloy, thereby improving the structure and properties of the alloy.

This paper reviews the effects of adding rare earth elements, metals, and metal carbides to cemented carbides on their properties.

The Effect of Adding Rare Earth Elements on Carbide Properties

Rare earth elements are the 15 lanthanide elements with atomic numbers ranging from 57 to 71 in the third subgroup of the Mendeleev periodic table, plus scandium and yttrium, which have similar electronic structures and chemical properties, totaling 17 elements. Rare earths are known as the “treasure trove” of new materials and are a group of elements of particular concern to scientists worldwide, especially material experts. The following sections discuss the effects of adding rare earth elements on the hardness, bending strength, and grain size of cemented carbides.

1.1 Hardness

Whether the addition of rare earths has a significant effect on the hardness of the alloy is an issue of concern. The influence of yttrium and lanthanum on WC-TiC-Co cemented carbides is not significant, but different rare earth elements have different trends; however, the hardness of alloys with Nd or Ce added, regardless of the content, is slightly higher than that of the untreated alloys, with an average increase of 0.3 HRA units. For YG6 alloys, the addition of mixed rare earths results in a decrease in hardness to varying degrees when the content reaches 1%; for YT? alloys, the hardness remains largely unchanged or slightly increased with the addition of La or Y.

1.2 Bending Strength

Data shows that adding a certain amount of rare earth elements to the alloy can increase its bending strength. After the addition of rare earth oxides, the strength of the alloy is improved due to the dispersion strengthening of nickel by the rare earth oxides. When the content of rare earths is 1.2% to 1.6% of the binder metal content, the bending strength of the alloy reaches its maximum value; after adding mixed rare earth oxides equivalent to 0.25% to 1.00% of the binder mass fraction, the bending strength of the WC-8%Co alloy is improved to some extent. When the addition amount is 0.25% to 0.50%, the bending strength can be increased by 1.5%, but excessive addition of rare earths will lead to a decrease in bending strength.

1.3 Grain Size of Carbides

A large number of literature reports have been published on the effect of rare earths on the WC grain size in cemented carbides, but there is no unified conclusion to date. Regardless of the type of rare earth element added, the carbide grains in the alloy are finer than those without additives, and as the amount added increases, the refinement becomes more pronounced, and the grain size of the rare earth element-added alloy appears more uniform than that of the untreated alloy; studies have shown that the addition of trace rare earth elements does not affect the particle size of tungsten carbide and the binder phase.

Through extensive observation of the WC grain size and microstructure of WC-Co-TiC-TaC with rare earths and WC-Co with rare earths alloys, it is believed that the effect of rare earths on the WC grain size of cemented carbides is determined by two refinement effects and one growth effect. Table 1 shows the comparison of properties between rare earth alloys and alloys without rare earths.

2 The Effect of Adding Metals on the Properties of Cemented Carbides

Commonly used metal additives include chromium, molybdenum, tungsten, tantalum, niobium, copper, aluminum, and others. Except for copper and aluminum, all of these can form carbides. Therefore, the change in the carbon content of the alloy must be considered when adding these metals.

2.1 Adding Noble Metals

Sintered cemented carbide products with added noble metals such as Ru, Rh, Pd, and Re exhibit high wear resistance and corrosion resistance and can be used in corrosive and abrasive media. Noble metals do not form carbide phases and exist in the binder metal as solid solutions. Ru and Re cause the formation of a substructure in the binder phase of the cemented carbide. Alloying sintered cemented carbides with noble metals can increase the microhardness and elastic modulus of the binder phase, while also improving the bending strength, compressive strength limit, and yield point of the sintered cemented carbide as a whole.

2.2 Adding Copper

The addition of a small amount of copper to alloys used in mining can both increase the strength of the alloy and improve its impact toughness. Research results indicate that after adding a small amount of copper to the WC-13% Fe/Co/Ni alloy, the hardness of the alloy slightly decreases, but the bending strength is significantly improved. When the copper content is around 0.8%, the alloy exhibits the best performance. Moreover, copper also has the effect of refining and spheroidizing WC grains.

2.3 Adding Alkali Metals

Alkali metals can promote the growth of ba?o grains, but their effect is limited by other factors. For instance, in the presence of silicon, sodium actually refines the WC grains; whereas if sodium is present during the carbonization process, the WC grains will become finer. Adding industrial-grade Li?CO? with a purity of 98% to 99% to the alloy results in a cemented carbide with coarser average grains, clear and well-defined grain edges, and high bending strength.

2.4 Adding Aluminum

The effect of adding a small amount of Al on the properties and structure of the WC-13% Fe/Co/Ni cemented carbide shows that the addition of a small amount of aluminum can refine the WC grains. While the hardness of the alloy increases by 2 to 3 HRA, the bending strength of the alloy can be improved by 100 to 200 MPa. When the amount of Al added exceeds 0.8%, the bending strength of the alloy decreases, which is due to reasons such as the enrichment of martensite at the phase interface and the relative change in the amount of γ phase. Table 2 shows the effect of metal additives on the properties of the alloy.

?Cuál es la distribución del plano cristalino de los carburos cementados WC-Co?

joshua — Fri, 25 Oct 2024 03:41:36 +0000

The research and development of crystal plane in cemented carbides is one of the hotspots in the field of cemented carbides both domestically and internationally. The microstructure of the alloy determines its mechanical properties; in WC-Co cemented carbides, the grain size and distribution of the WC phase, as well as the substructure within the grains, are important factors affecting the material’s mechanical properties [10-13]. WC grains have a close-packed hexagonal structure (crystal system P-6m2, a=0.2906nm, c=0.2837nm)24, and the habit planes of WC grains are two basal planes and three prismatic planes. WC grains are prismatic in three-dimensional space, with the basal plane being parallel to the (0001) crystal plane and the prism plane being parallel to the (1-100) crystal plane 5. On the basis of this basic shape, some of the edges and corners are replaced by new crystal planes?6-21. Given that cemented carbides with specific orientations can produce excellent mechanical properties to meet special application requirements, we believe that it is necessary to conduct in-depth research.

This paper takes WC-12Co cemented carbide as the research object. Firstly, nano WC-Co composite powders were prepared by in-situ reduction carbonization reaction at different temperatures, and the composite powders were rapidly densified by discharge plasma sintering technology. The sintered blocks were systematically studied to analyze the effect of the in-situ reduction carbonization reaction temperature on the distribution characteristics of special crystal planes.

Experiment

The raw materials used in the experiment were carbon black, blue tungsten (purity 99.5wt.%, average particle size 50μm) and cobalt oxide (purity 98.5wt.%, average particle size 35μm). Carbon black, blue tungsten, and cobalt oxide were weighed according to the proportion for generating WC-12Co powder, and high-energy ball milling was carried out in a hard alloy ball mill jar with ethanol as the ball milling medium. The mixed powders were subjected to in-situ reduction carbonization reaction at 850°C, 900°C, and 1000°C in a vacuum environment, with a holding time of 1 hour. 2wt% of grain growth inhibitor was added to the composite powders prepared by in-situ reaction, and discharge plasma sintering densification was carried out. The sintering temperature was 1080°C, the holding time was 5 minutes, and the sintering pressure was 60MPa.

Material phase analysis was carried out on a Rigaku D/max-3c X-ray diffractometer with an acceleration voltage of 35kV and a current of 30mA, and a scanning rate of 2°/min. The micro-morphology of the powder was observed on a FEI-NovaNano SEM field emission scanning electron microscope.

Results and of Crystal Plane Distribution

Figure 1 shows the thermogravimetric (TG-DSC) curves of the nano WC-Co composite powder synthesized by in-situ reaction when heated to 1000°C at a heating rate of 10°C/min under Ar gas protection. From the change in the TG curve, it can be seen that as the temperature increases, the mass of the in-situ synthesized WC-Co composite powder decreases. When the temperature reaches 1000°C, the reaction ends, and the powder weight loss reaches nearly 6%. Throughout the heating process, the mass loss of the composite powder is divided into two stages: the first stage is from room temperature to 190°C, where there is a significant decrease in powder mass due to the ease of gas adsorption by nanoscale powders, and the gas desorbs and releases during the heating process, causing powder weight loss; the second stage is from 628°C to 1000°C, where the weight loss rate decreases from high to low. This is because the composite powder did not fully react during the low-temperature in-situ reduction carbonization reaction, and a secondary reaction occurred during the heating process, resulting in rapid weight loss of the powder.

Figure 2 shows the nanoscale WC-Co composite powder prepared by in-situ reduction carbonization reaction at 850°C. The powder particle size is mainly distributed between 30~120nm, with an average particle size of ~83.4nm. The particles have a good sphericity, and spherical or quasi-spherical powders exhibit good dispersibility and flowability, which can effectively avoid hard agglomeration of the powder. This is beneficial for the uniform dispersion of the powder in the sintering mold, thereby ensuring the uniformity and density of the sintered bulk structure. The composite powder prepared by the in-situ reduction carbonization reaction is subjected to rapid sintering densification in an SPS system to obtain nearly fully dense WC-Co cemented carbide blocks. XRD tests are conducted on the alloy specimens in the direction perpendicular to the sintering pressure and in the direction parallel to the sintering pressure. The relative integral intensities (i.e., integral area) of the diffraction peaks for each crystal plane in the VD (vertical to the sintering pressure) and PD (parallel to the sintering pressure) planes of the specimens are obtained through fitting calculations and compared with the relative intensities (integral area) of the diffraction peaks for each crystal plane in the PDF card (which represents the diffraction peak intensity distribution characteristics of traditional sintered cemented carbides), as shown in Table 1. The results indicate that the sintered WC-Co cemented carbide block specimens exhibit a significant characteristic of high anisotropic distribution for specific crystal planes.

Figure 3 shows the XRD patterns of the VD (vertical to the sintering pressure) and PD (parallel to the sintering pressure) planes of the specimens obtained by SPS sintering of the nanoscale WC-Co composite powder prepared by in-situ reaction at 850°C. From the figure, it can be observed that on the PD plane, the intensity of the (0001) plane peak is lower, while the intensity of the (10-10) plane peak is relatively higher. By fitting and calculating the integral area of each diffraction peak, the relative integral area of each crystal plane and the proportion of each crystal plane’s integral area in the total integral area are obtained, as shown in Figures 5 and 6.

crystal plane distribution

From Figure 5, it can be seen that on the PD plane, the integral area of the (0001) plane accounts for only 10.72% of the total integral area, which is a decrease of 3.3% from the 14.02% in the PDF card (as shown in Figure 4). The proportion of the (10-10) plane reaches 35.73%, which is higher than the 31.87% in the PDF card, and the proportion of the (10-11) plane decreases to 21.76% compared to the 28.11% in the PDF card. Compared to conventional cemented carbide samples, in the PD direction of the composite powder sintered block prepared by in-situ reaction at 850°C, the distribution proportion of the main characteristic planes (0001) and (10-11) decreases, while the distribution proportion of (10-10) increases.

Discussion

On the VD direction of the sample, as shown in Figure 6, the proportion of the (0001) crystal plane has significantly increased to 40%, compared to the 14.02% in the PDF card, while the proportion of the (10-10) crystal plane has decreased to 11.68%, a reduction of 20.19% from the 31.87% in the PDF card. From the above analysis, it can be understood that in the sintered block of composite powder prepared by in-situ reaction at 850°C, there is an orientation distribution of characteristic crystal planes. In the direction perpendicular to the pressure, WC grains rotate, causing the (0001) plane to become perpendicular to the pressure direction, thereby reducing the interfacial energy between WC grains. The sintered block of low-temperature in-situ reaction synthesis powder exhibits an orientation distribution characteristic of WC grain characteristic crystal planes, which is believed to be due to the formation of WC grains through secondary reactions during sintering, with atoms preferentially aligning along the (0001) plane and the rotation of WC grains under sintering pressure causing the (0001) plane to tend towards being perpendicular to the pressure direction.

When the temperature is raised to 900°C for in-situ reaction, the prepared composite powder is mainly WC with only a small amount of carbon-deficient phase. When the temperature is raised to 1000°C for in-situ reaction, pure WC-Co composite powder can be obtained. Since the crystal plane distribution of the grains in the powder is isotropic, the crystal plane distribution of WC grains does not change during the subsequent sintering densification process at 900°C and 1000°C, and they still exhibit a randomly distributed isotropic characteristic.

Conclusión

1) The nanoscale WC-Co composite powder prepared by low-temperature in-situ reduction carbonization reaction at 850°C can be sintered into cemented carbide block materials with a highly oriented distribution characteristic of WC grain crystal planes using SPS. The sintered blocks of in-situ reaction powder at 900°C and 1000°C maintain an isotropic distribution of WC grain characteristic crystal planes.

2) In the WC-Co cemented carbide with highly oriented characteristic crystal planes, the basal plane (0001) occupies the largest area fraction in the direction perpendicular to the sintering pressure, reaching 40.0%; the prism plane (10-10) occupies the largest area fraction in the direction parallel to the pressure, reaching 35.7%.

3) The phase purity of the composite powder generated by the in-situ reaction plays an important role in the crystal plane orientation of the sintered block. When the main phase of the composite powder is WC, the crystal plane distribution of the sintered block does not exhibit oriented characteristics. However, when the main phase of the composite powder is the carbon-deficient phase, the crystal plane of the sintered block presents an oriented distribution characteristic.