Requirements for Seal Face Materials in Mechanical Seals

To ensure the proper operation of sealing rings in mechanical seal devices, they are typically configured as a pair consisting of a hard ring and a soft ring with different hardness levels, considering aspects such as wear reduction, corrosion resistance, and prevention of galling. During operation, the sealing rings may come into contact and generate friction when starting, stopping, or experiencing fluctuations in working conditions. Therefore, the material for the hard ring needs to have sufficient strength, rigidity, wear resistance, and thermal conductivity. Friction and fluid shear forces can elevate the temperature of the sealing rings, so the sealing material must exhibit good thermal conductivity, heat resistance, and thermal shock resistance. To ensure a long service life, the sealing ring must also have good corrosion resistance. Additionally, the hard ring should possess good formability and machinability, low density and permeability, and excellent self-lubricating properties. No single material can fully meet all these requirements, so typically, the main performance criteria for sealing materials are defined based on the operating environment, and suitable materials are selected accordingly.

Application of WC-Ni carbide as Mechanical Seal Materials

To ensure the longevity and stable operation of mechanical seal materials, the sliding materials should have appropriate thermal compatibility and thermal conductivity, as well as suitable coefficients of thermal expansion, elastic modulus, and friction factors. WC-Ni carbides are known for their excellent performance in mechanical seals, making them suitable for applications in high-pressure, high-speed, high-temperature, corrosive environments, and media containing solid particles.

Characteristics of WC-Ni carbide

Since the introduction of carbides in the 1920s, cobalt has been regarded as the best binder phase and continues to play a significant role in the preparation of carbides. With the rapid advancement of science and technology, the applications of carbides have expanded, leading to a surge in demand. Due to the scarcity of cobalt resources, scientists worldwide have prioritized cobalt as a strategic material and have been researching ways to reduce or substitute cobalt in carbides. Nickel, being close to cobalt in the periodic table, with similar density, melting point, and atomic radius, can effectively wet and support the hard phase and has lower radioactivity compared to cobalt, making it a common substitute.

The characteristics of WC-Ni and WC-Co carbides during the sintering process are similar. However, due to the different strengthening effects of Ni and Co on the hard phase, WC-Ni may exhibit slightly lower performance in certain aspects compared to WC-Co carbides. By adding a small amount of metal elements to enhance the binder phase, using fine low-carbon WC particles, and employing vacuum sintering processes, WC-Ni carbides with lower porosity and a uniform, fine-grained structure can be achieved. Their hardness, bending strength, and tribological properties can meet or exceed those of WC-Co carbides, while their corrosion resistance is also significantly improved. Additionally, as Ni replaces the radioactive element Co, it provides good radiation protection when used under radioactive conditions. By carefully controlling the total carbon content and grain size of the alloy, and adding appropriate amounts of Mo and Cr, WC-Ni carbides can be produced with non-magnetic properties and excellent physical and mechanical performance, thereby mitigating the effects of special working conditions and environmental factors.

Physical and Mechanical Properties of WC-Ni carbide for Mechanical Seals

WC-Ni carbides are made by mixing WC and Ni powders in a specific ratio, adding a binder, and then pressing and sintering the mixture. With a melting point of approximately 2700°C, WC particles are primarily bonded together during the sintering process through the melting of Ni. At high temperatures, some WC dissolves into Ni, forming a WC-Ni eutectic with a lower melting point than Ni. Consequently, the sintering temperature varies with changes in Ni content and WC grain size. For composite materials, physical parameters such as elastic modulus, coefficient of thermal expansion, Poisson’s ratio, thermal diffusivity, and thermal conductivity can vary based on the proportion and distribution of each phase.

Application of WC-Ni carbide Materials in Mechanical Seals

Due to their exceptional toughness, rigidity, high hardness, good wear resistance, high bending strength, and high thermal conductivity, both WC-Ni and WC-Co carbides are notable. WC-Ni carbides offer superior corrosion resistance compared to WC-Co alloys and do not emit radiation under neutron exposure, making them suitable for use in mechanical seals operating under high pressure, high speed, high temperature, corrosive media, media containing solid particles, and radioactive environments. Currently, WC-Ni carbides have significant application value in vehicle transmission shaft seals, power shift transmissions, pumps in special operating conditions, and rotary seals for aircraft, as well as in the petrochemical industry and nuclear power seals.

Influence of Microstructure on the Properties of WC-Ni carbides

The unevenness in microstructure can adversely affect the strength of carbides. Minor variations in the binder phase content and distribution, WC grain size, carbon content, and any form of impurity contamination can lead to an uneven microstructure that negatively impacts the mechanical properties of WC-Ni carbides.

Influence of WC Grain Size on the Properties of WC-Ni carbides

WC-Ni carbides use Ni as the binder metal. During the sintering process, Ni melts at the sintering temperature and bonds the WC particles together into a solid mass. These alloys exhibit very high hardness, are difficult to machine, and possess excellent wear resistance. Variations in the processing methods can lead to significant differences in the alloy’s composition and properties, and the morphology of the WC grains can also affect the performance of WC-Ni carbides.

3.2Changes in WC Grain Morphology During Liquid Phase Sintering of WC-Ni carbides

The coarseness of WC grains can significantly affect the bending strength of carbides, while uneven distribution of Ni can lead to brittle fracture of the alloy. To improve the fracture toughness of the product, it is essential to strengthen the interface between WC and the binder phase or to enhance the strength of the binder phase. Therefore, controlling the sintering process and conditions will impact the mechanical properties of WC-Ni carbides.

During sintering, the shape of WC grains in the carbide?is also influenced by shape relaxation and the grain growth process. The higher the ratio of the average intercept length of the binder phase (the average length of each grain intersected by any testing line on a cross-section) to the WC grain size, the less impact it has on the shape of the WC grains, resulting in a more equiaxed grain morphology.

Effect of Binder on Temperature Residual Stress in WC-Ni carbides

When the content of the metallic binder Ni in WC-Ni alloy materials is relatively high, the compressive stress in fine WC grains is greater than that in coarse grains. This is because, with a constant WC content, the average free path of the binder in fine powder is shorter than in coarse powder. When the WC-Ni alloy contains less binder, the difference in the average free path of the binder is minimal, and the variation in residual stress with temperature is not significant. Therefore, if conditions allow, the Ni content in WC-Ni alloy sealing rings should be reduced to minimize the uneven distribution of residual stress due to temperature changes, thus reducing or even preventing thermal cracking in the sealing rings.

Current Research Status on the Application Performance of WC-Ni carbides

?Corrosion Resistance of WC-Ni Alloys

Compared to WC-Co carbides, WC-Ni carbides exhibit superior wear resistance. This is due to the binder in WC-Ni alloys having excellent corrosion resistance, with both passivation and electrochemical corrosion rates for WC-Ni carbides being significantly lower than those for WC-Co carbides. Under acidic conditions in practical production processes, WC alloys with Ni as the binder show better acid resistance than those with Co as the binder.

Table 2 compares the corrosion resistance of WC-Ni and WC-Co carbides. The results show that substituting Co with Ni significantly enhances the corrosion resistance of WC carbides. However, the corrosion resistance of a material is specific to its alloy composition, grain size, and the corrosive conditions (including temperature, concentration, time, and corrosion state). For example, the corrosion resistance of YWN8 in 68%-90% HNO? is not significantly different from, and even slightly lower than, that of YG6 alloy. This is primarily due to the poor resistance of metallic Ni to strong oxidizing acids like HNO?; as the concentration of HNO? and the Ni content in the alloy increase, its corrosion resistance decreases.

Table 2 Corosion resistance performance comparison between WC-Ni and WC-Co cemented carbides

Tribological Performance of WC-Ni Alloy Mechanical Seal Rings

WC-Ni carbide?sealing rings exhibit excellent wear resistance. This is because WC-Ni carbides possess strong oxidation resistance and corrosion resistance in fluid sealing media, which contributes to their superior wear resistance. The friction coefficient of WC-Ni carbides is related to the content, grain size, and distribution of the binder phase. A softer binder phase can lead to adhesion during friction. Additionally, the content and composition of the binder phase can affect the hardness of WC-Ni, thereby influencing the wear resistance of the WC-Ni carbide.

結(jié)論

Mechanical seal materials are a crucial area of research in sealing technology. With the advancement of modern science and technology and the increasing demands of production and daily life, the requirements for sealing technology have become more stringent. However, research in this field in our country is still relatively behind. Strengthening interdisciplinary collaboration and continually improving experimental and theoretical research are key to overcoming the technological barriers in mechanical seals imposed by foreign countries.

Overview

In recent years, research on coarse grain carbide grades and materials has been advancing in two different directions: ultra-coarse and ultra-fine grains. Ultra-coarse grain cemented carbides have been widely applied in mining rock drilling tools, roll mills, and stamping molds.

Studies have revealed several primary forms of carbide failure during drilling: impact fatigue, abrasive wear, and thermal fatigue. For hard rock formations, such as granite (drilled with impact or rotary impact drills), abrasive wear is relatively lower, and carbide failure is primarily caused by impact and impact fatigue. The compressive strength and bending strength of the carbide are directly related to its impact fatigue resistance; additionally, this resistance is associated with the carbide’s purity, WC grain size, and Co phase’s average free path. Furthermore, the carbide’s impact fatigue resistance is directly related to the impact energy.

Reasons for Issues in Cemented Carbide Rock Drilling

For medium-hard rock formations, such as quartzite (drilled with impact drills), abrasive wear dominates. Abrasive wear generally consists of two aspects: micro-cracks at the contact points of abrasive particles and premature wear of the Co phase. The former primarily occurs on hard and brittle carbides, especially when abrasives have high fracture strength; the latter occurs on softer carbides with higher Co content, particularly when abrasives are very brittle. Figure 1 shows a scanning electron microscope (SEM) image of the wear surface of a GF20D grade drill tooth, produced by Xiamen Jinlu Special Carbide Co., Ltd., after drilling about 500 meters into quartzite. The YG6 grade carbide, composed of 94% WC with a grain size of 2-3 μm and 6% Co, has a hardness of HV30:1430. The image illustrates typical abrasive wear, characterized by premature Co phase wear and cracking and spalling of the WC phase.

For soft rock formations, such as sandstone, thermal fatigue is the primary cause of carbide failure, accompanied by abrasive wear. For ultra-soft rock formations, such as calcite and limestone, thermal fatigue is the main cause of carbide failure. The propagation of cracks and premature wear of the Co phase directly impact the drill tooth’s lifespan. Especially when drilling magnetite, thermal fatigue cracks, also known as creep cracks, dominate. Figure 2 shows a typical undulating cracking morphology of cemented carbide drill teeth formed while drilling magnetite. Figure 3 is an SEM image of a traditional polished cross-section of a carbide drill tooth that drilled about 5 meters into magnetite, composed of 94% WC with a grain size of 5 μm and 6% Co, with a hardness of 1230 HV. The image reveals that the thermal fatigue cracks on the carbide surface have extended into the carbide’s interior.

Figure 1: SEM photo of the wear surface of the quartzite at a depth of about 500 meters on the YG6 drill tips inserted in drill bits

Figure 2. Typical ups and downs of crack morphology formed when carbide drill teeth drill magnetite

Figure 3. The carbide drill teeth drill a conventional polished cross-section of about 5m into the magnetite. The grade consists of 94% WC with a grain size of 5um and 6% Co with a hardness of 1230HV(SEM).

Reasons for Developing New Rock Drilling Cemented Carbides

The fundamental reason for developing new rock drilling carbides lies in the continuous advancement of mining and drilling technology both domestically and internationally. As drilling equipment becomes more advanced and drilling efficiency improves, there is a growing use of fully hydraulic, high-power, and high-efficiency rock drilling rigs and rotary-percussion drills. This advancement has raised higher demands for the quality and lifespan of rock drilling cemented carbides. When drilling tools penetrate rock, the pressure rises from 0 to 10 tons within 1/10 of a second, and the temperature increases from 20°C to 1000°C. During impact and rotation, drilling carbides generate extremely high temperatures. Especially when drilling magnetite, rapid formation of thermal cracks, commonly referred to as “snake skin” or “tortoise shell” cracks, occurs.

To meet the requirements of modern rock drilling technology, the performance of rock drilling cemented carbides needs to be improved and optimized in several key areas: the thermal conductivity (the ability of the material to conduct heat) should be as high as possible; the thermal expansion coefficient (the linear expansion of the material when heated) should be as low as possible to ensure minimal growth rate of thermal cracks; high-temperature hardness should be further enhanced to guarantee good wear resistance at high temperatures; in addition, the transverse rupture strength (TRS) and fracture toughness (Kic, the material’s ability to resist sudden fractures caused by micro-cracks) should also be improved.

Table 1 lists the thermal performance data of pure WC, pure Co, three commonly used WC-Co carbide grades, and three types of rock. These three grades, with varying Co content and WC grain sizes, are suitable for different rock drilling teeth, hot-rolled rolls, and multi-purpose applications.

Directions for Developing New Rock Drilling Cemented Carbides

It is well known that Co has low thermal conductivity and a high thermal expansion coefficient. Therefore, the Co content should be minimized as much as possible. On the other hand, cemented carbides with high Co content exhibit better strength and fracture toughness. From a mechanical perspective, especially when carbide drill bits penetrate rock surfaces at high speeds, the drill bits endure high impact and loads, or mechanical vibrations under hard cutting conditions, necessitating improved strength and fracture toughness in the carbide. Additionally, compared to fine-grained carbides, coarse WC grain sizes contribute to greater strength and fracture toughness of the cemented carbide.

As a result, the preparation of rock drilling cemented carbides tends to use lower cobalt content and increase WC grain size to achieve good mechanical properties and the required high-temperature wear resistance. This approach results in ultra-coarse grain carbides. Traditionally, the production of ultra-coarse grain cemented carbides involves high-temperature reduction of coarse grain tungsten powder followed by high-temperature carburization to produce coarse grain WC powder. This powder is then mixed with Co powder and ball-milled to form a mixture, which is subsequently pressed and sintered to create the cemented carbide. However, coarse grain WC powder produced from tungsten powder via high-temperature carburization generally consists of polycrystalline particles, where each WC particle is composed of multiple WC single crystals.

Figure 4 shows a scanning electron microscope image of coarse grain carbide powder with a Feret diameter of 23.20 μm. The image reveals that each WC particle contains multiple WC single crystals. Although the original powder has a coarse grain size, after grinding, the polycrystalline particles easily break down into fine single crystal particles. Consequently, the ground WC powder has a Feret diameter of only 4.85 μm. Figure 5 shows the metallographic photo of a cobalt-containing carbide with 6% Co produced using conventional carbide production processes. The average grain size of this carbide is approximately 4.0 μm.

Figure 4: SEM image of coarse grain WC powder with a particle size of 23.20 μm.

Figure 5: Metallographic photo of WC-6% Co alloy produced from coarse grain WC powder with a particle size of 23.20 μm using conventional processing methods.

U.S. Patents 5505902 and 5529804 disclose methods for producing ultra-coarse grain cemented carbides. The methods outlined in these patents involve the dispersion and classification of coarse grain WC powder through jet milling and sieving to remove fine WC particles, selecting only the coarse-grained carbide, and then coating these WC particles with Co. Patent 5505902 utilizes the sol-gel method, where WC, methanol, and triethanolamine are mixed in a reactor. During heating, methanol evaporates, and Co precipitates onto the WC grains, forming a sol-gel.

Patent 5529804 employs the polyol method, where Co acetate, water, and WC are mixed and then spray-dried. The mixing process is optimized to prevent the breaking of coarse WC particles. The mixture produced using these patented methods is then subjected to conventional pressing and sintering processes to create cemented carbides with 6% Co and an average grain size of 13-14 μm, with porosity easily controlled between A02 and B02. This new carbide shows better WC matrix adjacency compared to carbides produced by traditional ball milling. Consequently, this new carbide has been successful in specific applications where conventional carbides fall short, such as in hard rock layers like granite and hard sandstone. In these cases, conventional column teeth fail due to Co dissolution at high temperatures, leading to spalling of elongated or hexagonal WC grains, and eventually, complete spalling of the drill bit within minutes, causing rapid crack propagation and subsequent fracture. In contrast, carbides produced with new technology can be used for extended periods in hard rock layers, displaying stable wear resistance without deep cracks. Due to the high adjacency of the WC matrix, the thermal conductivity of the 6% Co carbide with a WC average grain size of 14 μm can reach 134 W/m°C, which is 20% higher than that of coarse-grained carbides with the same Co content produced by traditional methods and comparable to the thermal conductivity of pure WC.

Application Examples of New Ultra-Coarse Grain Cemented Carbide Production Technologies

Two types of impact drilling cemented carbides were simultaneously produced using both traditional and new methods and tested in iron ore. Both samples had a WC average grain size of 8 μm, 6% Co, and 94% WC content.

Sample A: Produced using traditional ball milling, drying, pressing, and sintering processes. This carbide has a wide distribution of crystal sizes.

Sample B: The WC powder was subjected to jet dispersion and classification to remove coarser and finer WC particles, selecting 6.5-9 μm WC powder. The WC grains were pre-coated with 2% Co, and then 4% pure Co was added to achieve a 6% Co content. After wet mixing (without ball milling) to obtain the desired slurry, a thickening agent was added if necessary to prevent coarse grain WC sedimentation. The slurry was dried, shaped, and sintered, resulting in a narrower particle size distribution, with over 95% of the grains ranging from 6.5 to 9 μm. The adjacency of these carbides was measured: Sample A had an adjacency of 0.41, while Sample B had an adjacency of 0.61.

Testing was conducted in magnetite, which is prone to generating high heat and thermal fatigue. After drilling 100 μm, Sample A exhibited thermal cracking. Cross-sectional observation of the used carbide revealed small cracks extending into the carbide, damaging its microstructure and reducing its lifespan. With regrinding after every 100 μm of drilling, the carbide’s drilling lifespan was 530 meters. Sample B showed no or only minimal thermal cracking after drilling 100 meters. Cross-sectional observation showed no internal cracks, only some fractured surface grains. With regrinding after every 200 meters, the average drilling lifespan was 720 meters.

Carbide?Valve Seat

carbide?valve seats can also be called tungsten steel petroleum valve heads. carbide?valve seats, valve assemblies, or carbide?ball valves evolved from traditional plug valves, with the opening and closing component being a sphere, achieving opening and closing purposes by rotating the sphere around the valve stem axis. The main function of carbide?valve balls in pipelines is to cut off, distribute, and change the direction of fluid flow.

Production Process of Valve Seat

carbide?valve seats are made using the carbide?cold extrusion process. Cold extrusion is a metal fine forming process with minimal or no cutting, and very low power consumption. Using cold extrusion to produce metal formed parts has unparalleled advantages in machining, especially suitable for the production of large batches of metal parts, among which carbide?valve seats are one of the applicable parts. It can also serve as a forming process for products.
There are many advantages of the carbide?cold extrusion process. Firstly, it saves raw materials. Cold extrusion uses the plastic deformation of carbides to produce parts of the required shape, thus greatly reducing machining and improving the utilization rate of raw materials. The material utilization rate of cold extrusion generally exceeds 80%, which is advantageous for the carbide?industry production.

Carbide Ball Valve

The ball valve emerged in the 1950s. With the rapid development of science and technology and continuous improvement in production processes and product structures, within a short span of 40 years, it has rapidly developed into a major type of valve. In countries with developed industries in the West, the use of ball valves is increasing year by year. In China, ball valves are widely used in industries such as petroleum refining, long-distance pipelines, chemicals, papermaking, pharmaceuticals, water conservancy, electricity, municipal engineering, and steel, occupying a pivotal position in the national economy.

Carbide?ball valves are mainly used for cutting off or connecting the medium in pipelines and can also be used for fluid regulation and control. Among them, hard-sealed V-shaped carbide?ball valves have strong shear force between the V-shaped ball core and the carbide?metal seat, especially suitable for media containing fibers, tiny solid particles, etc. Multi-way carbide?ball valves can flexibly control the confluence, diversion, and flow direction switching of the medium in pipelines, while closing any one channel to connect the other two channels. Such valves should generally be installed horizontally in pipelines. Classification of carbide?ball valves: pneumatic carbide?ball valves, electric carbide?ball valves, manual carbide?ball valves.
It has a 90-degree rotational action, with the closure body being a sphere with a circular through-hole or passage along its axis. carbide?ball valves are mainly used in pipelines for cutting off, distributing, and changing the flow direction of the medium. They can be tightly closed with just a 90-degree rotation and a small turning torque. carbide?ball valves are most suitable for use as on-off and cut-off valves, especially V-shaped carbide?ball valves.They are also widely used in vacuum systems.

Features of carbide ball valves

Carbide?ball valves not only have simple structure and good sealing performance but also within a certain nominal diameter range, they are small in size, lightweight, consume less material, have small installation dimensions, and require low driving torque. They are easy to operate and achieve quick opening and closing, making them one of the fastest-growing valve types in recent decades. carbide?ball valves evolved from plug valves, with their closing member being a sphere, which rotates 90° around the valve stem axis to achieve opening and closing. carbide?ball valves are mainly used in pipelines for cutting off, distributing, and changing the flow direction of the medium, and carbide?ball valves with V-shaped openings also have good flow regulation functions.

Applications of Several Common Carbide Ball Valves

Especially in industrialized countries such as the United States, Japan, Germany, France, Italy, Spain, and the United Kingdom, carbide?ball valves are widely used, with the variety and quantity of use still expanding. They are moving towards high temperature, high pressure, large diameter, high sealing, long life, excellent regulatory performance, and multifunctional direction. Their reliability and other performance indicators have reached a high level, and they have partially replaced gate valves, globe valves, and throttle valves. With the technological progress of carbide?ball valves, they will have broader applications in the foreseeable future, especially in oil and natural gas pipelines, refining and cracking units, and nuclear industry. In addition, in medium and large-caliber, medium and low-pressure fields in other industries, carbide?ball valves will also become one of the dominant types of valves.

carbide?ball valves are suitable for double-position adjustment, high sealing performance requirements, fast opening and closing (1/4 turn), high pressure drop, small operating torque, low flow resistance, and erosion in pipeline systems or vaporization.
In pipelines with certain corrosive media.
In low-temperature devices or high-temperature and high-pressure pipeline systems.
Full-bore welded carbide?ball valves can be used in petroleum pipelines and natural gas pipelines buried underground.
Specially designed V-shaped carbide?ball valves also have certain adjustment functions.

Assembly of carbide?Ball Valves and Valve Seats

1.Ball Processing

The core component of a ball valve is the sphere, so ball processing is the most critical part of ball valve manufacturing. Common ball processing materials include copper, iron, stainless steel, etc., and processing techniques include forging, casting, etc. The production of ball valve balls requires multiple processes such as rough machining, finishing, surface treatment, and carbidespraying to meet production requirements.

2.Valve Body Casting

The valve body of a ball valve is usually made of cast steel or forged steel. Casting methods include sand casting, pneumatic mold casting, etc. Before casting, mold design, casting process, and procedures need to be formulated, and strict material selection and quality inspection are required to ensure the reliability and durability of the valve body.

3.Stem Processing

The stem is the connecting part between the ball and the actuator of the ball valve, so the processing and production of the stem are very important. Chromium-molybdenum alloy steel is commonly used for stem materials, and processing techniques include turning, grinding, etc. During stem processing, attention should be paid to accuracy and smoothness, and heat treatment and surface treatment should be carried out to improve corrosion resistance.

4.Seal Installation

The seals of a ball valve include valve seats, sealing gaskets, etc., which must be installed in the appropriate positions to ensure the sealing performance of the ball valve. When installing seals, correct assembly and installation operations should be performed according to the structure and requirements of the ball valve.

5.Testing

After the manufacturing of the ball valve is completed, testing is required to verify its good performance and reliability. Testing includes hydrostatic testing, airtightness testing, pressure resistance testing, etc. Only ball valves that pass the test can be put into use.
In summary, the manufacturing process of ball valves includes multiple steps such as ball processing, valve body casting, stem processing, seal installation, and testing. Each step requires strict manufacturing standards and control to ensure the quality and performance of ball valves.

結(jié)論

High-precision customized production and processing of carbide valve seats, valve assemblies, and carbide ball valves are used in oil drilling, deep-sea drilling pump valve balls, and seats, which are critical supporting parts in oil pump equipment. The use of valve seats and petroleum valve heads is mainly for petroleum equipment production purposes.Meteou specializes in the technical and professional production of carbide products, providing customized carbide valve seats, petroleum valve balls, petroleum valve assemblies, and other series of wear-resistant and corrosion-resistant carbide parts tungsten steel fittings that meet the requirements of petroleum equipment usage.

Selection of carbide?Brazing Materials

The brazing materials for tungsten carbide and steel brazing are divided into three categories: high-temperature brazing materials, room-temperature brazing materials, and low-temperature brazing materials according to their melting points and brazing temperatures.

High-temperature Brazing Materials

Brazing materials with a brazing temperature above 1000°C are called high-temperature brazing materials, such as purple copper and 106 brazing materials, etc. Medium-temperature brazing materials have a brazing temperature between 850 and 1000°C, such as H62 and H68 brass brazing materials, etc. Low-temperature brazing materials refer to brazing materials with a brazing temperature between 650 and 850°C, such as B-Ag-1 and L-Ag-49 silver-containing brazing materials.

Room-temperature Brazing Materials

Purple copper brazing materials have a high brazing temperature and low weld seam strength, and are mostly used for vacuum brazing. Pure copper brazing materials have a single-phase structure, are relatively easy to control the brazing temperature, have good wetting ability for various types of carbides, good plasticity, and are the cheapest. The shear stress of purple copper brazing weld seam is about 150MPa, and it can be used below 400°C. The brazing temperature of H68 brass is much lower than that of purple copper, but because of its low weld seam strength, it is not commonly used. The melting point and brazing temperature of H62 brass are relatively low, and the weld seam has certain room temperature strength, making it a commonly used carbide?brazing material. It is generally used for carbide?tools under medium and small loads. When high temperature strength of the weld seam or small welding area is required, 105 brazing material should be used.

Low-temperature Brazing Materials

L-Ag-49 low-temperature silver brazing material is widely used abroad because of its low melting point (690-710°C), good wetting ability for tungsten carbides, and advantages such as convenient brazing and low stress. When necessary, purple copper sheets can be used as compensating shims, which can almost completely eliminate brazing stress and prevent brazing cracks. It can be used for brazing some easily cracked carbides or some large brazing surface carbide tools. Since the workpiece brazed with L-Ag-49 silver brazing material will rapidly decrease in weld seam strength as the operating temperature increases, the working temperature of the workpiece brazed with L-Ag-49 brazing material should be limited to below 200°C. The brazing fluxes used in conjunction with L-Ag-49 brazing materials contain more fluorides and chlorides, requiring higher cleaning requirements after welding, otherwise, surface corrosion of the workpiece may occur due to inadequate cleaning.

Ultra-low-temperature Brazing Materials

B-Ag-1 brazing material is an ultra-low-temperature silver brazing material with a melting point around 600-610°C, which can further reduce the residual stress of tungsten carbide joint and can also be used for brazing some easily cracked workpieces with purple copper sheets as compensating shims. Due to the ultra-low melting point of B-Ag-1 silver brazing material and its good wetting ability to tungsten carbide, it is also suitable for brazing certain diamond tools such as large diamond saw blades. However, the price of B-Ag-1 silver brazing material is high, and its high-temperature strength is low, so it is only suitable for use at temperatures below 150°C. The cadmium content in this brazing material is 24%, which is easy to evaporate at high brazing temperatures, harmful to human health. In addition to controlling the brazing temperature during brazing, exhaust devices should also be installed at the brazing operation site. After brazing, attention should also be paid to cleaning the workpiece thoroughly to prevent corrosion of the workpiece.

Selection of Brazing Flux

The function of brazing flux is to reduce the oxides on the surface of the shank and tungsten carbide, enabling the brazing material to wet the metal surface to be brazed effectively. Generally, the melting point of brazing flux is at least 100°C lower than that of the brazing material, with good fluidity and low viscosity. The melted brazing flux during brazing serves to protect the brazing material and brazing surface, while also acting to reduce oxides.

The main requirements for carbide?brazing flux are as follows:

1.The flux should exhibit good wetting ability on both the carbide?to be brazed and the steel substrate, ensuring that it possesses good fluidity and penetrability.

2.One of the characteristics of carbide?use is its high red hardness, so it’s crucial to ensure that the brazed weld seam has sufficient strength at both room temperature and high temperatures.

3.The melting point of the flux should be as low as possible to minimize brazing stress and prevent cracking. However, the melting point of the flux should be at least 300°C higher than the working temperature of the weld seam to ensure that the tool can function normally at high cutting speeds.

4.The flux should not contain elements with low evaporation points to avoid affecting joint quality during brazing heating or posing health risks.

Preparation Before Vacuum Brazing

1.Prior to vacuum brazing, check whether the tungsten carbide has cracks, bends, or uneven surfaces. The brazing surface must be flat. If the brazing surface is spherical or rectangular, it should conform to certain geometric shapes to ensure good contact between the alloy and the substrate, thus guaranteeing brazing quality.

2.Perform sandblasting treatment on the carbide. If sandblasting equipment is not available, the oxide layer and black grade letters on the brazing surface can be removed by holding the tungsten carbide by hand and grinding it on a rotating green silicon carbide wheel. If the oxide layer on the brazing surface of the carbide is not removed, the brazing material will not wet the carbide easily. It has been verified that if there is an oxide layer or black grade letters on the brazing surface, sandblasting treatment should be carried out. Otherwise, the brazing material will not wet the carbide easily, and black letters will still appear in the brazing seam, reducing the brazing area and causing brazing defects.

3.When cleaning the brazing surface of the carbide, it is best not to use chemical mechanical grinding or electrochemical grinding methods. Instead, wire cutting by electric spark erosion should be used. The processed tungsten carbide can then be sandblasted again or ground with a green silicon carbide wheel to remove the surface layer. The sandblasted carbide can be cleaned with gasoline or alcohol to remove oil stains.

4.Before vacuum brazing, carefully check whether the groove shape on the steel substrate is reasonable, especially for carbide grades prone to cracking and tungsten carbide workpieces with large brazing surfaces. Stricter requirements should be imposed. The grooves should also be sandblasted and cleaned to remove oil stains. When the cleaning volume is large, an alkaline solution can be boiled for 10-15 minutes. For multi-blade tools and complex gauges brazed with high frequency or immersed copper, it is best to boil them in a saturated borax water solution for 20-30 minutes, take them out to dry, and then carry out welding.

5.Before using the brazing material, wipe it clean with alcohol or gasoline and cut it into shape according to the brazing surface. When brazing general carbide cutting tools or molds, a brazing material thickness of about 0.4-0.5mm is suitable, and its size should be similar to the brazing surface. When using continuous nitrogen protection for brazing furnace heating, the brazing material can be appropriately increased. When brazing tungsten carbide multi-blade cutting tools, gauges, etc., the brazing area should be minimized. Generally, the brazing material can be cut to about 1/2 of the brazing surface. When the brazing technology is proficient, the brazing material can be reduced to 1/3 of the brazing surface or even smaller. Reducing the amount of brazing material can make the appearance of the welded workpiece more beautiful and facilitate grinding.

Vacuum Brazing Process

Uniform Heating of the Shank and carbide Inserts The correct vacuum brazing process for tungsten carbide tools plays a crucial role in welding quality. The heating rate significantly affects the quality of the weld. Rapid heating can cause cracks and uneven temperatures in the carbide inserts. However, heating too slowly can lead to surface oxidation, reducing joint strength.

During vacuum brazing of carbide tools, uniform heating of the shank and carbide inserts is one of the basic conditions to ensure welding quality. If the temperature of the tungsten carbide inserts is higher than that of the shank, the melted brazing material wets the carbide inserts but not the shank, leading to decreased joint strength. In this case, when shearing the alloy insert along the weld layer, the brazing material remains intact, detaching along with the alloy insert. Milling marks from the shank support surface can also be seen on the weld layer. Conversely, if the heating rate is too fast and the temperature of the shank is higher than that of the alloy inserts, the opposite phenomenon occurs.

Arrangement of Flux, Brazing Material, and tungsten carbide Inserts

The sequence and positions of flux, brazing material, and carbide inserts directly affect brazing quality. The correct arrangement method is as follows: place the brazing material on the groove, sprinkle flux, then place the carbide inserts, and sprinkle another layer of flux along the side weld seam on the top surface of the carbide inserts. This facilitates temperature control during brazing, reducing excess brazing material adhering outside the weld seam. It’s crucial to control the vacuum brazing temperature of the workpiece correctly. Excessive vacuum brazing temperature can cause weld seam oxidation and zinc evaporation from zinc-containing brazing materials.

Brazing Parameters

During vacuum brazing, maintain a vacuum degree of about 5 X 10-2 Pa. The heating rate is an important parameter during vacuum brazing, as excessive heating rate leads to a sharp drop in vacuum degree, which can cause oxidation of carbide and brazing materials. In production, the vacuum brazing process is as follows: increase the temperature from room temperature to 800°C at a rate of 10°C/min, hold for 30 minutes, then increase the temperature to the set temperature at a rate of 9°C/min, hold for 10 minutes, and then cool down with the furnace. The 800°C holding temperature ensures uniform heating of the base material. Increasing the temperature to the set temperature at a rate of 9°C/min ensures that the vacuum degree does not decrease significantly, and the short-term holding at the set temperature allows the brazing material to melt fully and prevents excessive evaporation of particulate magnesium in high vacuum.

Cooling Speed After Vacuum Brazing

The cooling speed after vacuum brazing is one of the main factors affecting brazing cracks. During cooling, instantaneous tensile stress occurs on the surface of carbide?inserts, and the tensile stress of carbide?is much lower than the compressive stress. Especially for carbide?grades such as YT60, YT30, and YG3X, which have large brazing areas and small matrices but large carbide?inserts, attention should be paid to the cooling speed after brazing. Usually, the post-welded workpiece is immediately immersed in lime or charcoal powder to cool slowly. This method is simple but does not control the tempering temperature. If conditions permit, the workpiece can be immediately placed in a furnace at 220-250°C for tempering for 6-8 hours after brazing. Low-temperature tempering can eliminate some brazing stress, reduce cracks, and prolong the service life of carbide?tools.

Cleaning After Vacuum Brazing

After vacuum brazing, the product can be polished with sandpaper and then polished with polishing paste. Post-brazing cleaning of well-brazed carbide?parts is necessary to remove residual flux around the weld seam. Otherwise, excess flux will clog the grinding wheel during tool sharpening, making grinding difficult. Residual flux after brazing can also corrode the weld seam and base material. Common cleaning methods include boiling the cooled workpiece in boiling water for about 1-2 hours, followed by sandblasting to remove residual flux and oxides adhering to the weld seam surroundings. Alternatively, the workpiece can be immersed in an acid bath for pickling (nitric acid concentration of 3%-5%) and washed with NaOH solution. The pickling time is about 1-4 minutes, followed by thorough rinsing in cold and hot water tanks.

Quality Inspection of Vacuum Brazed Products

The main inspection focuses on the quality of the brazed joint between the carbide?and the steel shank, as well as the presence of cracks in the carbide. A normal weld seam should be uniform without black spots, and the unfilled portion of the weld seam should not exceed 10% of the total length of the weld seam. The width of the weld seam should be less than 0.15mm. If the brazed blade is skewed and does not meet the drawing requirements, it should be re-brazed. The tendency for cracks in carbide?blades can be checked using the following methods:

1.After cleaning the tool with sandblasting, wash it with kerosene and then observe it with the naked eye or a magnifying glass. When there are cracks on the blade, obvious black lines will appear on the surface.

2.Prepare a solution by mixing 65% kerosene, 30% transformer oil, and 5% pine oil, and add a small amount of Sudan red. Immerse the pre-checked tools in this solution for about 10-15 minutes, then remove and rinse them with clean water. Apply a layer of kaolin clay, dry it, and then inspect the surface. If there are cracks on the tool, the color of the solution will be displayed on the white clay, which can be clearly seen with the naked eye.

概要

The characteristics of using carbide?tools in mining are large impact loads and vibrations, requiring the carbide?inserts to be firmly brazed with minimal brazing stress. The surface roughness of groove machining has a significant impact on the strength of the weld seam. The lower the roughness, the higher the strength of the weld seam (although lower roughness makes machining more difficult). Generally, a roughness of around Ra6.3 is sufficient. To obtain weld seams that are thin and uniform in appearance, the groove machining accuracy of carbide?gauges should be higher.

In conclusion, the vacuum brazing and quenching of cutter teeth are becoming increasingly common due to their high production efficiency, stable quality of brazed joints, and complete heating, brazing, and quenching in a vacuum environment, which avoids oxidation and decarburization of the cutter teeth. Therefore, the vacuum brazing and quenching process for cutter teeth will be more widely adopted.

The wear forms on valves

Based on the conditions and mechanisms of friction, wear can take various forms, with common types including abrasive wear, adhesive wear, fatigue wear, corrosive wear, and erosive wear.

Common parameters used to characterize material wear performance include wear volume, wear rate, wear depth, wear resistance, and relative wear resistance. The fundamental requirement for abrasion resistance is that the surface of the object must have high hardness (surface hardness should exceed the hardness of the abrasive). Additionally, it should exhibit good oxidation resistance at the operating temperature. The most effective way to control or minimize wear is to enhance material hardness and wear resistance.

Sintered tungsten carbide has high strength, a smooth surface finish, and a lower coefficient of friction compared to steel when used in conjunction with other materials. This significantly reduces contact surface frictional forces, effectively lowering operating torque.

Material features of carbide

Whole sintered carbide?is produced by high-temperature heating of a mixture of tungsten and carbon. The hardness of most tungsten carbides is very high, with microhardness second only to diamond. It has a melting point of 2870°C and a boiling point of 6000°C, with a relative density of 15.63 (at 18°C). It is resistant to decomposition at high temperatures and exhibits excellent oxidation resistance.

Field investigations indicate that tungsten carbide demonstrates wear resistance in situations such as abrasive wear, erosive wear, and abrasion, which is about 100 times higher than that of tool steel, stainless steel, iron, and brass. It has 2-3 times the rigidity of steel and 4-6 times the rigidity of cast iron and brass, with impact resistance similar to that of quenched tool steel.

The reason carbide is needed in valves

In conditions involving high temperature, high pressure, strong corrosion, and slurries or powders with solid particles such as in gasification and polycrystalline silicon, the sealing surfaces of conventional hard-sealed ball valves, V-port control valves, coal powder control valves, butterfly valves, and slide valves use carbides as the sealing materials for the valve disc and seat. However, due to the limitations of the sprayed tungsten carbide coating—thickness <2mm, hardness <60HRC, and coating adhesion to the substrate <1000psi—the spraying process is typically conducted under harsh conditions at temperatures as high as 10000°C. Valve lifespans are challenging to guarantee for 10,000 cycles, making it difficult to meet the long-term stable production requirements of systems handling coal chemical slurries and polycrystalline silicon powders.

On the other hand, the strength of sprayed tungsten carbide mainly relies on the base material, and when the coefficients of thermal expansion of the two materials are significantly different, the usage is limited by temperature and cannot exceed 450°C. The valve performance has seen significant improvements with the adoption of integral sintered carbides for the valve disc and seat in new types of hard-sealed ball valves (Figure 1), V-port control valves (Figure 2), coal powder control valves (Figure 3), butterfly valves (Figure 4), and slide valves, addressing these challenges.

Fig1 Sealing ball valve

Fig2 control valve

(1) High Hardness: With a hardness greater than 80HRC, it can withstand the high-speed scouring of multiphase particle media such as water-coal slurry, coal powder, and silicon powder.

(2) High-Temperature Resistance: Capable of prolonged operation at temperatures up to 750°C, it is not limited by temperature in terms of strength, adhesion, and thermal expansion. This completely meets the requirements of high-temperature conditions, such as those encountered in coal chemical processes.

(3) High Pressure Resistance: The transverse fracture strength of integral sintered tungsten carbide reaches 4000MPa, more than 10 times the strength of conventional steel. It can operate long-term under working pressures up to 25MPa.

(4) Corrosion Resistance: Integral sintered tungsten carbide exhibits stable chemical properties. It is insoluble in water, does not react with hydrochloric acid and sulfuric acid, and is not dissolved even in aqua regia. This corrosion resistance satisfies the special requirements of industries such as coal chemical processing.

Fig3 Coal powder control valve

Fig4.?Butterfly valve

(5) Wear Resistance: The high hardness and stability of integral sintered tungsten carbide ensure excellent anti-wear properties for sealing components. This meets the special wear requirements of media such as coal powder and organosilicon (silicon powder particle hardness is 62HRC).

(6) Erosion Resistance: Conventional valves with sprayed tungsten carbide coatings on sealing surfaces often suffer severe erosion, exhibiting honeycomb patterns within a month under conditions of 250°C and losing functionality completely. In contrast, V-port control valves and coal powder control valves, which use integral sintered tungsten carbide as control components, have a lifespan of 12 months under 450°C (other conditions remaining the same). Disc valves and slide valves, subjected to more than 300,000 switching cycles, fully meet the long-term operational requirements of industries such as coal chemical processing for 8000 hours.

(7) High-Temperature Flexibility: Both the ball and seat are made of integral sintered carbides, with coefficients of thermal expansion ranging from 1/3 to 1/2 of conventional steel. This effectively prevents the common issue of valve sticking at high temperatures, ensuring excellent operational performance under high-temperature differential conditions.

(8) Low Friction: The use of sintered carbide?anti-wear pads not only extends the high-temperature lifespan of the pads but, due to their lower friction coefficient, typically only 1/3 to 1/2 of conventional paired materials. This significantly reduces frictional forces between components, lowering valve operating torque.

結(jié)論

Integral sintered carbide?possesses high strength, high hardness, a high melting point, stability, a low friction coefficient, wear resistance, erosion and cavitation resistance, and corrosion resistance. Manufacturing wear-resistant valve sealing components for demanding operating conditions has enhanced the applicability of valves, expanded their range of use, prolonged their operational lifespan, ensured various performance indicators, and met the development needs of the chemical industry.

As the competition in the steel product quality and price market intensifies, steel companies continuously upgrade their equipment technology to enhance rolling mill speeds. Simultaneously, reducing downtime and further improving the effective operation rate of rolling mills have become crucial concerns for steel engineers. The adoption of rolls made from materials with higher rolling life is one of the essential means to achieve this goal.

Classification of carbide rolls

carbide?rolls can be classified into integral carbide?rolls and composite carbide?rolls based on their structural forms.

Integral carbide?rolls are widely used in the pre-finishing and finishing stands of high-speed wire rolling mills, including fixed reduction stands and pinch roll stands.

Composite carbide?rolls are made by combining carbide?with other materials and can be further divided into carbide?composite roll rings and integral carbide?composite rolls. carbide?composite roll rings are installed on the roll shaft, while integral carbide?composite rolls have the carbide?roll ring directly cast into the roll shaft, forming a single unit. The latter is applied in rolling mills with higher loads.

Performance of carbide?Roll Rings: The performance of carbide?rolls is influenced by the content of the bonding phase metal and the size of the tungsten carbide particles in the matrix. Different levels of bonding agent content and corresponding tungsten carbide particle sizes result in different grades of carbide. Series of carbide?grades have been developed for various rolling mill stands. Tungsten carbide typically constitutes 70% to 90% of the total mass composition in carbide?rolls, with an average particle size ranging from 0.2 to 14 μm. Increasing the metal bonding agent content or enlarging the tungsten carbide particle size can reduce the hardness of the carbide?while enhancing its toughness. The bending strength of carbide?rolls can exceed 2200 MPa, impact toughness can reach (4–6) × 106 J/m2, and the Rockwell hardness (HRA) ranges from 78 to 90.

Quality standard elements for carbide roll rings

The quality standards for tungsten carbide rolls include material porosity, WC grain size, total carbon and free carbon content, density, hardness, magnetic saturation intensity, coercivity, bending strength, impurity content, as well as the processing precision and surface roughness of the rolls. Each indicator reflects the quality of the roll and predicts its performance. Factors influencing the quality of the rolls include the dispersion, particle size, and size distribution of WC powder and Co powder in the mixture, as well as total carbon, free carbon, oxygen, iron content, etc. The type and quantity of adhesive used for molding, the temperature and time for degreasing, and the temperature, time, and atmosphere for sintering are also crucial.

Moreover, the precision of the grinding machine used for roll processing, the quality of the diamond grinding wheel, and other factors can impact the quality of the rolls. In 2009, Meetyou?cemented carbide?Co., Ltd. obtained ISO9001 international quality assurance system certification, ensuring the provision of high-quality and stable tungsten carbide roll ring products to customers.

New tech for WC roll rings

LGM Technology

WC roll rings undergo high temperatures, rolling stresses, thermal corrosion, and impact loads during the hot rolling process. Consequently, they exhibit poor wear resistance and are prone to fracturing during use. Building upon conventional carbide?roll ring materials, we have developed a Lubrication Gradient Material (LGM) roll ring by introducing Lubrication Gradient Material (LGM), a lubrication-resistant gradient material.

This technology involves adding sulfur and oxygen to conventional carbide?materials, forming stable gradient metal oxides and metal sulfides (Co3O4 and CoS, respectively) on the surface of the metal substrate. Both Co3O4 and CoS demonstrate excellent lubrication and wear resistance properties. Industrial tests of LGM roll rings have shown that the sulfides and oxides in the gradient material can reduce the friction coefficient during rolling, significantly improving the lubrication performance of the roll ring under high-temperature and high rolling force conditions. This leads to a reduction in the generation of transverse cracks, and the lifespan of LGM roll rings is 1.5 times that of conventional carbide?roll rings. Moreover, it decreases the amount of grinding and the frequency of roll changes, resulting in significant economic benefits.

Cast In Carbide (CIC) Composite Carbide Roll Ring

To meet the demands of modern rolling production, a new type of carbide?composite roll ring, known as Cast In Carbide (CIC) composite carbide?roll ring, has been developed. This technology involves casting the carbide?ring and ductile iron inner sleeve together. The connection between the roll ring and the roll shaft is key-linked. In this connection method, the outer layer of the composite roll ring, composed of carbide?material with extremely high hardness and excellent wear resistance, bears the rolling force, while the inner layer, made of ductile iron with good strength and toughness, transfers torque.

The development of the Cast In Carbide (CIC) composite roll ring technology represents a new combination of powder metallurgy and casting technology. It signifies a significant advancement in the application of composite wear-resistant materials in roll rings.

Composite WC Roll Ring

This technology involves combining a carbide?ring with a steel matrix containing added Ni and Cr powder through powder metallurgy. The key process involves first pressing and sintering the carbide?powder into a ring, followed by molding and sintering with the selected steel matrix powder. There is a strong metallurgical connection between the carbide?and the steel base. The crucial elements of this process include maintaining sintering temperatures between 1100 to 1200 ℃ and pressure conditions of 100 to 120 MPa. The sintered blanks undergo processes such as rough turning and stress relief, followed by precision turning and grinding for the final shaping.

By selecting appropriate base materials, coupled with advanced processes and proportions, it is possible to minimize residual stress between the carbide?and the steel matrix in the composite roll ring.

Multi-source presses have multiple independent hydraulic sources, meaning they have several independent working cylinders. An example is the hexahedral press, which has six working cylinders connected to each other by hinge beams. On the other hand, a single-source press has only one hydraulic source, commonly referred to as a two-sided press. While the previous discussions have focused more on hexahedral presses, let’s talk about two-sided presses today.

Chinese manufacturers of high-pressure, high-temperature products primarily use hexahedral presses, whereas those in Europe and America predominantly use two-sided presses. The earliest research and production of products such as diamond, CBN, PDC, and gem-grade cultured diamonds were achieved on two-sided presses. Therefore, it can be said that two-sided presses play an irreplaceable role in the production of ultra-high-pressure products.

Different from hexahedral presses, two-sided presses apply pressure on both sides of the ultra-high-pressure mold, such as ring-type devices and concave die devices.

Characteristics of the two-sided press

(1) The two-sided press is a single-source press with cylinders located in the lower part of the frame. This structure uses cylinders with larger diameters, resulting in lower working pressure and relatively longer cylinder lifespan.

(2) The main machine and hydraulic system have a simple structure, low maintenance rate, and are easily scaled up.

(3) Between the hammer and the pressing cylinder of the two-sided press, there are guides for fixtures and sealing bowls, ensuring high precision, simple material loading and unloading, and easy implementation of automation.

(4) During the synthesis in the two-sided press, with the support of the?carbide?pressing cylinder, as the hammer advances, the sealing and pressure-transmitting bowl gradually becomes thinner, and the core rod gradually thickens. Compared to the assembly and high-pressure characteristics of the hexahedral press, there is very little overflow inside the assembly chamber, resulting in minimal deformation of the core rod.

(5) The separation of pressure sealing and heat insulation allows for independent adjustment of pressure sealing, pressure transmission, and chamber insulation functions.

(6) Uniform changes in insulation throughout, consistent deformation of the heater, small variations in the temperature field, and easy adjustment of the temperature field.

(7) Small internal pressure fluctuations, high pressure transmission efficiency, excellent linear characteristics in internal pressure transmission, easy pressure calculation, and strong adjustability in pressure utilization due to simple assembly adjustments.

(8) Under high-temperature and high-pressure conditions, the core rod undergoes simple and uniform thickening. This makes it easy to achieve flatness in products with high requirements for surface finish, such as large-diameter PCD/PCBN composite sheets, after synthesis.

(9) The two-sided press, equipped with characteristics such as moderate high-pressure stroke, good pressure stability, and excellent synthesis repeatability, allows the temperature and pressure fields in the synthesis chamber to remain stable for a long time. It is suitable for growing high-grade diamonds and other products.

Limitations of the two-sided press

(1) The carbide pressing cylinder has a high mass, and its hardness and toughness requirements are demanding, leading to production difficulties and high costs. During the synthesis process, the dimensions of the carbide pressing cylinder change rapidly, necessitating regular inspections of its size.

(2) The carbide pressing cylinder is highly stressed, leading to a short overall lifespan, significant consumption, and higher costs.

(3) The mold system of the two-sided press requires high standards. Multiple layers of steel rings or multiple layers of steel rings combined with flat steel wires are used to protect the carbide pressing cylinder in the mold. This approach must ensure sufficient rigid support for the pressing cylinder while minimizing bending deformation during high-pressure conditions. Otherwise, the lifespan of the pressing cylinder is extremely short. The technical content of mold design, calculation, and manufacturing is high, with stringent requirements for the hardness and dimensions of each component.

(4) During the synthesis process, the two-sided press has a large hammer stroke, long sealing sides, slow pressurization and depressurization speeds, resulting in longer single-synthesis times and lower efficiency.

(5) The design of the sealing and pressure-transmitting sides and the coordination of internal assembly components are crucial. When designed improperly, various issues such as explosions, increased deformation of the pressing cylinder, shortened hammer lifespan, and lower pressure utilization efficiency may occur.

(6) Strict requirements for the dimensions of assembled components, with some parts requiring adjustments based on changes in the pressing cylinder’s size.

(7) Difficulty in temperature and pressure measurements.

Certainly, many of the concerns mentioned above also apply to hexahedral presses, but the emphasis may vary.

結(jié)論

The main machine and hydraulic system of the two-sided press have a simple structure, resulting in a very low maintenance rate, and it is easy to achieve automation and scalability for large-scale operations. Its assembly characteristics determine uniform temperature distribution, easy adjustability, high pressure utilization efficiency, and easy adjustment of the pressure. The deformation of the core rod after synthesis is minimal, making it particularly suitable for the production of high-pressure and high-value-added products such as gem-grade cultured diamonds, high-grade large-diameter PDC, PCBN, PCD drill bits, and other products with stringent pressure requirements.

The structure and working principle of a toothed wheel drill bit

A toothed wheel drill bit primarily consists of three components: the toothed wheel, bearings, and the drill bit body. The toothed wheel is the core component of the drill bit, composed of multiple teeth embedded on the wheel’s surface. These teeth continuously impact and break rock formations as the toothed wheel rotates, enabling the drilling operation.

Milled-tooth wheel drill bit

The teeth of a milled-tooth drill bit are machined from the toothed wheel blank, primarily in the form of wedge-shaped teeth. Depending on their location, these teeth are categorized as gauge teeth, inner-row teeth, and chisel teeth. Chisel teeth are embedded with carbide for enhancing gauge retention. The determination of tooth structure parameters takes into account both their effectiveness in breaking rocks and the teeth’s strength.

Generally, drill bits designed for soft formations have larger tooth height, tooth width, and tooth spacing, while those for hard formations have the opposite characteristics. To enhance the wear resistance of milled teeth, a carbide wear-resistant layer is deposited on the tooth surface.

Feature of milled-tooth wheel drill bit

The teeth of milled-tooth drill bits are directly machined from metal materials, and the tooth shape, width, and height can be designed and processed according to the characteristics of the geological formation. Therefore, they exhibit high mechanical drilling speed in soft formations.

The material of milled teeth is limited by the toothed wheel material. Despite the application of carbide overlay through welding, their wear resistance is still insufficient. In hard and highly abrasive formations, their service life is significantly reduced.

Inset toothed wheel drill bit

Insert toothed wheel drill bits are created by drilling holes into the toothed wheel and then fitting teeth made of carbide materials into these holes.

The carbide teeth used on toothed wheel drill bits are typically composed of tungsten carbide (WC) and cobalt (Co) series carbides. These alloys are fabricated through powder metallurgy methods, with tungsten carbide powder as the skeleton metal and cobalt powder as the binder. Sometimes, a small amount of tantalum or niobium carbides is added. With an increase in cobalt content in the alloy, density decreases while hardness gradually decreases, indicating reduced wear resistance. However, flexural strength increases, and impact toughness also improves. Without altering the tungsten carbide and cobalt content, increasing the grain size of tungsten carbide can enhance the toughness of the carbide, while its hardness and wear resistance remain unchanged. In recent years, diamond composite material insertshave been developed.

Inserts form the cutting structure at the bottom of the well. Depending on the position of the teeth in the cutting structure, teeth can be categorized as inner-row teeth, gauge teeth, and chisel teeth.

Shapes of tooth on insert toothed wheel drill bit

The shape of the teeth on an insert toothed wheel drill bit is designed based on the geological formation conditions. In general, pointed and long teeth are suitable for soft formations, while short and blunt teeth are better suited for hard formations. Representative tooth shapes are described as follows:

Wedge-shaped teeth

These teeth have a “wedge” shape, with tooth tip angles ranging from 65° to 90°. They are suitable for breaking highly plastic soft formations and moderately hard formations. Teeth with smaller tip angles are suitable for soft formations, while those with larger tip angles are suitable for harder formations.

Bucket-shaped teeth

Introduced in the 1980s by the Hughes Tool Company in the United States, this tooth shape is asymmetric and features a concave bucket-shaped cutting face and a slightly convex arc-shaped back. This structure improves tooth stress conditions, enhancing both crushing efficiency and tooth strength. It is effective for breaking very soft to moderately soft formation rocks.

Conical teeth

Conical teeth come in various shapes, including long cone, short cone, single cone, and double cone, with higher strength compared to wedge-shaped teeth. Medium conical teeth with angles of 60 to 70 degrees are used for drilling moderately hard formations such as limestone, dolomite, and sandstone. Teeth with 90° or 120° conical angles are used for drilling highly abrasive hard rocks like hard sandstone, quartzite, and flint.

Ball-shaped teeth

These teeth have a half-spherical top and are suitable for highly abrasive hard formations such as flint, quartzite, basalt, and granite. They offer both high strength and wear resistance.

production of screw

Processing of wire

Fastener screws are typically made from wire materials with diameters ranging from 5 to 19 mm. The main materials used for these screws include carbon steel, stainless steel, and copper.

Rough machining of wire

To remove the oxidation layer that forms on the wire material during production and storage, a coarse drawing process is necessary. After this treatment, the wire surface will exhibit its original metallic luster.

The coarse drawing process consists of two steps:

1.Annealing: This step involves adjusting the crystalline structure, reducing the wire’s hardness, eliminating graininess, and improving the wire’s machinability at room temperature.

2.Acid pickling and phosphating: A layer of phosphate salt film is formed on the metal surface. This step makes the wire more suitable for further shaping and reduces wear and tear on the molds during subsequent processes such as wire drawing and cold forging.

3.finishing process of wire

This step involves drawing the wire to the desired wire diameter thickness based on the specific product requirements and specifications. This is done to facilitate further processing and shaping.

4.Final molding process

This step is divided into three stages:

Cold Heading: Using a cold heading machine to cut the wire into the desired length, create screw heads, and add markings.

End Trimming: Shaping the wire into a dovetail shape for penetration through steel plates.

Threading: Adding threads to the semi-finished product.

5.Hot pressing

Cleaning: Removing grease from the surface of the screw.

High-temperature carburizing: Allowing carbon atoms to penetrate the surface layer of the screw, increasing its hardness.

Quenching: Forming a crystal layer on the surface of the screw to achieve the final hardness requirements.

Cleaning quenching oil: Removing quenching oil from the surface of the screw.

Low-temperature tempering: Reducing the core hardness of the screw to make it more ductile and prevent breakage due to excessive core hardness.

6.Electroplating

The purpose of electroplating is to prevent screws from rusting, extend their lifespan, and improve their appearance. Galvanizing, chromium plating, nickel plating, and copper plating can mitigate corrosion on the screw’s surface.

Brief intro of cold heading

In fact, the manufacturing methods of screws can be divided into cold heading, hot forging, and machining (turning, milling, etc.). Cold heading involves using the metal’s plasticity and applying pressure or cold drawing at room temperature to achieve solid-state deformation of the metal. It is a method for shaping the top of a bar or wire by increasing its thickness.

Applications of cold heading: It is mainly used for manufacturing parts such as bolts, nuts, iron nails, rivets, and steel balls. Forging materials can include copper, aluminum, carbon steel, alloy steel, stainless steel, and titanium alloys, among others.

Advantages of cold heading: Material utilization can reach 80-90%, and it offers high production efficiency, often exceeding 300 pieces per minute.

Procedures of cold heading

Through the interaction between the molds, the wire is cut and upsetting to form a screw blank, forming the head, cross slot (or other head shape), thread blank diameter and rod length, and head fillet.

Selection of material of cold heading molds

During cold heading, the metal blank undergoes plastic deformation due to impact pressure at room temperature, causing a redistribution and transfer of the blank’s volume within the die to achieve the desired shape. Cold heading forming processes are primarily used for the production of fasteners (various types of screws and nuts). Cold heading dies have to withstand high impact forces per unit pressure, which can reach 2000-2500 MPa, and the impact frequency is very high. The die’s cavity surface and the working surface of the punch also experience strong impact friction, and the working temperature can reach around 300°C. The punch also undergoes bending tension.

The choice of steel materials for cold heading dies should be based on factors such as the working conditions of different parts of the die, cross-sectional size, depth of the hardened layer required, and production batch size due to the characteristics of the working conditions of the cold heading dies and the shallow depth of the hardening layer.

1.For lightly loaded small concave dies, it is common to use integral modules with a surface that has a certain hardened layer. When a shallow hardened layer depth is required, materials like T10A steel can be used. If a deeper hardened layer is needed, low-alloy tool steels may be chosen. High-carbon chromium steel, or high-speed steel, or composite materials are not suitable in such cases.

2.For heavily loaded and complex-shaped concave dies, high-carbon medium chromium steel, high-speed steel, or a base steel embedded module may be chosen. These insert modules can be embedded using press-fit or hot-insert methods into a mold liner made of a material with good toughness.

3.When the production quantity exceeds 200,000 pieces, cemented carbide or steel-bonded cemented carbide insert modules may be considered. These materials offer high wear resistance, tight tolerances, and long service life, which can compensate for their higher cost. They are used for cold heading punches and should have good anti-friction and wear resistance properties. Additionally, they should possess sufficient hardness to prevent collapse in areas exposed to impact.

4.For low loads, T10A steel or low-alloy tool steel is often used. For larger mold dimensions and heavy loads, modular assembled die sets using the same material as the concave die are recommended.

carbide?shield tunneling cutter?usually come in three types:

1Soft soil cutter: used for excavation in soft soil, sand layers, and small particle sandstone formations where the stratum is loose and there is no need to break the rock layer with a roller cutter. The scraper is used to scrape and disturb the face only, and all tools are carbide cutting blades.

2Composite cutte : used for excavation in various geological formations such as sand and rock layers. In this case, the rock needs to be broken by the roller cutter, and then the scraper is used to scrape off the part of the rock between the two cutter rings. As the rock has already been compressed and cracked by the roller cutter, it is no longer dense, and the scraper can easily cut it. Therefore, the tools in this case are composed of roller cutters and carbide cutting blades.

3 Hard rock cutter: used for excavation in highly dense pure rock formations. The roller cutter breaks the rock, and as the cutter continues to penetrate, the cracks connect and form slag, which is peeled off from the surface of the cutter head. There is no need for a carbide scraper to scrape the residual rock, so no carbide tools are required. Only several wear-resistant steel plates need to be set at the slag discharge port to collect the slag. Therefore, the tools in this case consist of roller cutters and wear-resistant scraper blades.

Forms of failure for carbide?shield tunneling?cutters

Through a large number of engineering practices, it has been found that the proportion of tool failure caused by normal wear to the limit is only about 45% of all failed tools. The proportion of carbide cracking in the failure of carbide shield tunneling cutters is about 35%, the proportion of carbide shedding in all cutter damage is about 15%, and the proportion of tool failure caused by other reasons is only about 5%.

The following are several common forms of failure for carbide?cutters:

carbide?cracking

Cracking refers to the phenomenon of carbide?fracturing under external impact force. The reasons for the cracking of the shield tunneling cutter?are as follows:

1 Improper selection of carbide?grade. If high wear-resistant and low toughness carbide?is selected in high-impact formations, it is extremely easy to cause carbide?cracking.

2 Problems with welding process. carbide?and steel substrate are brazed with copper-based/silver-based solders. Improper control of the welding process will cause microcracks in the carbide, and subsequent impact will cause crack propagation, ultimately leading to alloy cracking.

3 Poor welding heat treatment.

a.If the carbide is not heated to a certain temperature before welding and directly heated by medium-frequency induction, the welding process will generate excessive thermal stress.

b.If the welded cutteris not immediately quenched in a tempering furnace after welding, the alloy and steel substrate will cool too quickly, and the difference in thermal expansion coefficients between the carbideand steel will cause inconsistent shrinkage during the cooling process, leading to carbide being pulled apart.

c.Improper shield operation when encountering geological mutations or isolated rocks. If the shield encounters geological mutations, uneven hardness of the formation, or isolated rocks during operation, and the shield driver does not notice or take measures, the cutterwill be subjected to a huge impact, causing carbide to crack.

Carbide detachment

carbide?detachment refers to the phenomenon where the carbide?completely separates from the steel substrate. The main reasons for carbide?detachment are as follows:

Poor brazing process between carbide?and steel substrate, resulting in insufficient bonding strength. When cutting through rock and soil, the welding seam between the carbide?and steel is pulled apart, causing the alloy to detach. The industry-recognized bonding strength for carbide?and steel brazing is 245 MPa. Only by improving the brazing strength can the carbide?be prevented from easily detaching.

To make a conclusion, common forms of shield cutter failure include three types: cutter cracking, cutter deformation, and excessive wear. The main reason for cutter cracking is that the heat treatment hardness is too high or the hardness distribution is uneven. The main cause of cutter deformation is improper selection of cutter material or insufficient consideration of stress in the design. The main reason for excessive wear of the cutter body is improper selection of cutter material or insufficient wear-resistant cladding protection.

Ways to improve the lifespan of shiled tunneling cutters

Do a good carbide grade selection

For shield tunneling，due to the variability of geological conditions, a coarser WC particle size is required for the carbide. Therefore, commonly used grades are listed in order of increasing cobalt content: YG8C, YG11C, YG13C, and YG15C. In this order, the wear resistance of these carbides decreases while their impact toughness increases (see fig. 1).

図1

Get a reasonable material?selection of cutter substrate body

Q345 and Q235 are commonly used for cutter body materials in China. These two materials have low alloy elements and carbon content, making it impossible to significantly improve their material hardness and strength through heat treatment. As a result, the wear resistance and strength of the steel matrix are relatively low, and the probability of wear, deformation, and fracture is high. 40Cr, 42CrMo, 35CrMo, and 40CrNiMo alloy structural steel can be used as matrix materials, which contain various metal elements, improve the quenching ability of steel itself, and increase the hardness of the steel matrix to HRC 38-42 after heat treatment. Its impact resistance and fatigue strength are comprehensively improved.

carbide?brazing process?is important

Silver-based brazing is increasingly used in the welding of shiled tunneling cutter. It is known for its low temperature, good fluidity, and small welding stress, which greatly reduces the phenomenon of carbide virtual welding. Additionally, due to the low temperature, the welding process causes less damage to the carbide, and the possibility of forming welding stress and micro-cracks is smaller.

Wear-Resistant Overlaying Protection

In addition to carbide?and steel matrix, wear-resistant overlaying is also required for cutter tools. The main component of wear-resistant overlaying is tungsten-cobalt, which is remelted on the surface of the steel matrix by arc or plasma welding. This protects the cutter tool from wear and extends its service life.

Design?principle?of shield tunneling cutter matters

Cutter performance is not only related to the manufacturing process but also closely linked to the tool design. If the design is excellent, the performance of each part of the cutting tool can be utilized to its fullest potential. The tool may encounter a problem where the steel billet was bent and fractured. This problem was solved by changing the structure, which eliminated the issues of tool fracture and bending.

Reduce tunneling advancement methods’ impact

1 Attention should be paid to controlling the excavation parameters during shield tunneling. For example, when there is a geological mutation, the speed of the cutterhead and the advancement of the shield should be cut to reduce the impact of the geological mutation on the tools and avoid tool damage.

2 During the tunneling process, foam should be added in a timely manner and slurry soil should be improved to enhance its performance and reduce wear. The temperature of the slurry soil should be monitored to prevent excessive temperature that can aggravate tool wear.

3 Strengthen tool management, regularly inspect and replace tools to prevent one tool from failing and causing increased loads on adjacent tools, thus accelerating their damage rate. After replacing the tool, a low thrust and low-speed test push should be conducted to give the tool a buffering time.

結(jié)論

1 In order to improve the service life of cutting tools, it is difficult to achieve significant results by improving them in only one aspect. Only by improving cutting tools from multiple aspects such as design, manufacturing, and application can their performance be maximized.

2 After abnormal failure of cutting tools, starting from the mechanism of damage and finding the root cause of the failure can achieve good results by adopting targeted measures.