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HEAT PROTECTION TECHNOLOGY ON TUNGSTEN CARBIDE BEAD WELD

Abstract: This paper summarizes the application of the tungsten carbide composite alloys wear - resistance bead welding in the industrial fields and the physical chemistry reaction of the tungsten carbide during high temperature process. Feasible technolo2 gy methods are introduced to the selection , fabrication , heat protection result assessment of insulator according to the heat protec2 tion of the tungsten carbide bead welding in resent years. At the same time , the related research result of modern science and technology , and the new theories and methods are briefly stated.

Keywords: tungsten carbide, bead weld, heat protection technology, insulator

With the development of industrial technology, many mechanical parts due to poor operating conditions, often caused by surface damage and failure. Increasing working parameters, the performance of the surface of the parts made a higher demand, and this requirement is often contradictory with the requirements of the matrix material. In order to solve these contradictions, a variety of surface technology came into being, in which surfacing is a more mature welding technology, especially for serious wear and tear under the surface of the parts to strengthen or repair the surface of the parts have been worn. There are many types of alloys or other wear-resistant materials used for surfacing, and the welding technology is also different. This paper mainly discusses the application status of wear-resistant surfacing of tungsten carbide composite alloy in various industrial sectors and the thermal protection technology in the related tungsten carbide surfacing. From the aspects of the selection, preparation and heat-proof effect evaluation of tungsten carbide adiabatic agent Put forward the technical ideas for reference.

Application Status of Tungsten Carbide Composite Surfacing Alloy: Surfacing is one of the most widely used surface technologies in recent years.

Its working principle is the use of welding methods in the surface of a part of the material with a specific performance of the material process. With the wear-resistant materials in the industrial application continues to expand, surfacing has become a solution to metal parts due to wear and tear of the important technical measures. According to statistics, in the industrial field of various types of wear and tear caused by economic losses, the proportion of abrasive wear nearly half. Therefore, anti-abrasive wear is one of the most important applications of wear-resistant layer surfacing. Anti-abrasive wear surfacing alloy has various types, such as high chromium alloy cast iron, martensitic alloy cast iron, austenitic high manganese steel surfacing alloy. For some special conditions, the use of the workpiece, such as blast furnace gas, hopper, oil drilling drill, drilling centralizer, mining machinery bucket teeth, geological core drill, concrete mixer blades, bulldozers high-speed sandbox and other components , Domestic and foreign commonly used tungsten carbide composite alloy surfacing, in order to obtain a strong abrasive wear or gas - grain wear, and by a certain impact on the working surface.

Tungsten carbide composite alloy with high hardness, high wear resistance, high strength and high positive elastic modulus, in all surfacing alloys, the highest abrasive wear resistance. Document 1 points out that the wear resistance of cast alloy composed of cast tungsten carbide and manganese white copper is 5 to 7 times that of Solmayite alloy, which is more than twice that of high manganese steel. Therefore, the tungsten carbide composite alloy becomes an irreplaceable composite surfacing alloy working under the condition of strong abrasive wear and gas-particle wear. In the oil drilling, metallurgical mining and coal mining, civil construction, building materials, sugar, power generation, agricultural and other industries have been more and more widely used.

Technical Features of Tungsten Carbide Composite Surfacing Material and Surfacing Technology: There are three types of tungsten carbide for surfacing: one is cast tungsten carbide, which is composed of WC and W2C eutectic; the other is cobalt-based sintered tungsten carbide; in addition to a special coarse grain tungsten carbide , Is a special process of tungsten and carbon black mixed in high-temperature carbonization furnace made of carbonization. There are many types of tungsten carbide composite surfacing materials, commonly used tube-like casting ceramic tungsten carbide electrode, can be used for oxygen acetylene flame surfacing. With this electrode for the welding core, coated with alkaline low hydrogen type can be made of welding electrode for arc welding. In addition can also be made of tungsten carbide flux cored wire or welding tape for automatic arc welding. Tungsten carbide composite alloy welding commonly used oxygen acetylene flame surfacing, plasma arc surfacing, argon arc welding, infiltration surfacing (furnace brazing), induction welding,Electroslag welding and other methods. Which in the welding, furnace brazing, electroslag welding and other methods in the 20th century, 70 ~ 80 years have been used for large workpieces such as blast furnace bell, hopper and the pressure of the joints and easy to wear parts of the surfacing [ 2, 7], get better use results. In recent years, foreign countries have tungsten carbide flux cored wire, welding application development reports. The high abrasion resistance of tungsten carbide composite surfacing alloys is highly dependent on the tungsten carbide hard phase in the alloy material, and in the above-mentioned different surfacing processes, the final performance of the tungsten carbide hard phase Big difference. Experiments show that [9], with the same volume of casting tungsten carbide as a hard phase, NiCrBSi alloy as a binder phase, respectively, with induction welding, oxygen acetylene flame welding and arc welding surfacing alloy wear resistance So that the welding temperature of different surfacing methods is one of the important reasons for the difference of hard phase performance.

Arc welding tungsten carbide thermal protection technology: hand arc surfacing and flux cored wire, welding surfacing is widely used in equipment wear surface repair technology, since the 1970s domestic and foreign manufacturers have been used for carbonation Tungsten material surfacing process. Due to the high temperature effect of the arc and the physical and chemical reaction between the tungsten carbide and the bonding phase, the performance of the hard phase in the surfacing layer will inevitably change. Therefore, how to keep the carbonization in the surfacing alloy as much as possible during the arc welding process Tungsten and other particles of the original physical and chemical properties, that is, arc welding tungsten carbide thermal protection technology, welding metallurgists has been concerned about the issue. Specific technical measures introduced at home and abroad can be divided into two categories. One of the technical solutions is from the welding process point of view, by improving the form of tungsten carbide particles added to the hard particles in the process of surfacing away from high temperature heat source to prevent excessive dissolution and melting. This method has been achieved in the tungsten carbide plasma arc surfacing, melting argon arc welding and electroslag welding process has another effect of another technical solution is from the perspective of the material according to the idea of ​​internal thermal protection [16], in the surfacing material To add a powdered adiabatic agent to prevent the tungsten carbide particles in the arc under high temperature conditions change the nature. The internal thermal protection method is the use of certain substances in the physical and chemical reactions at high temperatures to absorb away a lot of heat and cut off the protected objects and the environment of material and energy exchange, resulting in a thermal barrier to prevent the purpose of protecting objects overheating The This is a reference to the aerospace industry, aircraft industry, ablation and heat prevention research results As the welding process conditions and the operating environment of the aircraft is very different, so the study of tungsten carbide adiabatic two key technical problems, one adiabatic type The choice and preparation, another problem is the way to join the adiabatic agent. The literature on this is extremely limited. The research results, research methods and new theories in the field of modern science and technology are applied to the research of adiabatic agents, and the combination with surfacing material manufacturing process and surfacing process will help solve these problems.

3. 1 choice of adiabatic material Insulation agent in the arc surfacing should meet the following requirements: with low thermal conductivity, and in the instant contact with 4 500 ℃ above the arc high temperature and 1 800 ℃ above the weld pool area can occur sublimation or decomposition and absorption of welding hard The heat around the phase, while not producing toxic dust. There are two types of substances with the above properties: organic polymers and inorganic compounds. Good thermal insulation and low linear ablation rate (AR) characteristics of organic polymers Some transition metal oxides or hydrides at high temperature when the state changes or heat decomposition will absorb a lot of heat, such as TiH2 decomposition reaction enthalpy Becomes 125. 4 kJ / mol [19]. There is also a class of chemical reactions that can absorb large The amount of heat, such as metal and silicon dioxide at 1 000 ℃ or more from the endothermic reaction. In addition, the organic polymer in addition to low temperature gas in addition to produce a large number of carbonaceous material will produce a large number of carbonaceous residues, some of which with silica and so on in more than 1 500K chemical reaction to absorb a lot of heat, the gas generated from the heat In addition, the material at high temperatures in the physical and chemical changes can absorb a lot of heat is the prerequisite for the selection of adiabatic agents, thermodynamic calculations for the choice of insulation materials laid the foundation.

3. Application of 2-particle-wrapped surface technology in preparation of tungsten carbide adiabatic agent Screening of adiabatic materials should be combined with their preparation methods and addition methods. In the past, the preparation of surfacing material is a mixture of various ingredients mechanical bonding with adhesive bonding, pressure coated into a solid electrode or into the tubular electrode. Whether this kind of joining method can effectively play the role of adiabatic agent still need to verify. The use of particle coating method, the adiabatic agent is pre-coated on the surface of tungsten carbide particles, is a viable technical ideas. In recent years, the surface modification of inorganic granular materials has become a hot research topic in materials science and interface and surface science [20]. Some technical achievements in the ceramic industry, dye industry, thermal spraying and powder metallurgy and other fields of production are widely used. For the surface coating of tungsten carbide, most of the problems involved in the packaging of metal materials for the production of spray or powder metallurgy products [21]. Therefore, how to form an adiabatic film on the surface of tungsten carbide particles is a new technology. For different types of adiabatic agents, different coating methods should be used. Coated with organic heaters, generally can be used dip coating method, the key technology is the dispersion of coated particles. There are many methods of coating the inorganic heat insulator, such as homogeneous precipitation method, alkoxide hydrolysis method, sol-gel method, non-uniform coagulation method and nonuniform nucleation method [22]. High-pressure hydrogen reduction is a mature method for the production of metals and low-cost oxide powders in hydrometallurgy, which has been used to prepare metal-coated coating powders [21, 23]. When the inorganic heat insulating agent coating such as oxide is prepared by this method, the firmness of the encapsulated particles and the coating and the control of the coating thickness will become a technical difficulty. Work in this area has not been reported in the literature. In summary, in order to obtain a good anti-heat effect of tungsten carbide adiabatic composite coating, in addition to choice of insulation materials, the coating process technology should be able to ensure that the coating thickness requirements, uniformity and wrapped tungsten carbide particles and coating Layer firmness. Low coating cost is an important factor in the formation of industrial production.

3. Evaluation of the thermal protection effect of tungsten carbide adiabatic agent The parameters of the heat-resisting properties are as follows: ablation rate AR (mm / s), mass ablation rate MAR (g / cm2 · s), adiabatic index IT (s), ablation performance index A PI, Ablation heat AH (kJ / kg) [24]. Among the above indicators, AR and A PI are important evaluation parameters. They reflect the anti-ablation performance and thermal insulation properties of the material. The smaller the AR, the larger the A PI, the higher the ablation resistance of the material and the good heat insulation. AR and A PI parameters of tungsten carbide adiabatic choice of a certain reference value. However, the requirements for surfacing on the adiabatic agent are different from those required for aerospace ablation. For surfacing, adiabatic effect of heat insulation is directly reflected in the tungsten carbide particles by the role of high temperature after the arc changes. For this change there are quantitative indicators to be assessed, and in recent years the rapid development of fractal theory may be described as tungsten carbide in the surfacing layer in the form of effective means. Fractal theory was founded in the mid-1970s, and its research object is a disorderly (or irregular) and self-similar system, which has become a new geometric language that quantifies the seemingly irregular phenomena The It divides the fractal dimension of the system with its behavior and its changing laws and causes, and achieves remarkable results in the fields of metalwork, fracture mechanics and inorganic materials science.

4 concluding remarks At present, the number of carbide surfacing materials used for wear-resistant surfacing is small, but in the case of severe abrasive wear parts and tool surfacing, carbide surfacing occupies an important position, while tungsten carbide surfacing alloys are used in other types of carbides (Such as titanium, molybdenum, tantalum, vanadium and chromium carbide) composite materials still remain dominant in the welding. As the tungsten carbide hard particles in the surfacing high temperature conditions, physical and chemical changes, resulting in wear resistance of the surfacing layer, so that tungsten carbide and other wear-resistant surfacing materials in the price performance ratio is not dominant, which also affected the The expansion of the application of tungsten carbide surfacing materials and processes in different industrial fields. Therefore, the research and development of tungsten carbide surfacing thermal protection technology for the full use of China's resource advantages, the rational use of wear-resistant materials, reduce production costs and use costs, and further expand the scope of tungsten carbide surfacing materials in the machinery manufacturing industry, are There's important meaning.

Characteristics and Application of Tungsten Carbide Carbide

Carbide is a high hardness and melting point of the carbide, with Co, Mo, Ni as a binder sintered powder metallurgy products. Its room temperature hardness of up to 78 ~ 82 HRC, 850 ~ 1000 ℃ high temperature, cutting speed comparable to high-speed steel 4 to 10 times higher. But the impact toughness and bending strength than the high-speed steel is poor, so rarely made the overall tool.

Tungsten carbide is a hard alloy family of raw materials, pure tungsten carbide is not commonly used for black hexagonal crystals, metallic luster, hardness and diamond similar to the electric, heat good conductor. Melting point 2870 ℃, the boiling point of 6000 ℃, the relative density of 15.63 (18 ℃). Tungsten carbide is not soluble in water, hydrochloric acid and sulfuric acid, soluble in nitric acid - hydrofluoric acid mixed acid. Pure tungsten carbide fragile, if mixed with a small amount of titanium, cobalt and other metals, can reduce the brittleness. Tungsten carbide used as a steel cutting tool, often added titanium carbide, tantalum carbide or their mixture, in order to improve the anti-explosive ability. The chemical properties of tungsten carbide are stable.

In tungsten carbide, carbon atoms embedded in the gap between the tungsten metal lattice, does not destroy the original metal lattice, the formation of gap solid solution, it is also known as interstitial (or insert) compounds. Tungsten carbide can be prepared by heating the mixture of tungsten and carbon at high temperature, and the presence of hydrogen or hydrocarbons accelerates the reaction. If the preparation of tungsten with oxygen compounds, the product must be vacuum at 1500 ℃ to remove carbon oxides. Tungsten carbide suitable for high temperature machining, can produce cutting tools, furnace structure materials, jet engines, gas turbines, nozzles and so on.

Design of the Screw Steel Roller Crescent Groove Special CNC Milling Machine

With the rapid development of China's national economy, especially the South-to-North Water Diversion, West-East Gas Pipeline, the Three Gorges Project, the western development, the 2008 Olympic venues, the 2010 World Expo venues and the nationwide urban construction and other large-scale real estate projects have started, Water, electricity, road and bridge industry has brought unprecedented opportunities for development, a variety of rebar (also known as hot-rolled ribbed steel bar, the brand by the HRB and grades of the minimum point of submission) applications are growing, according to national Statistics in 2008 the amount of construction with rebar to 100 million tons. To meet the demand, the steel rolling mill project have been launched.

The rebar roll is an important part of the rolling mill and rolls the different sizes of rebar profiles with the pressure generated by rolling the rolls of one or a group of rebar rolls. The difference between the grooved rebar and the round bar is that it has longitudinal and transverse tendons on its surface, usually two longitudinal bars and a plurality of transverse tendons distributed evenly along the length. In the process of hot-rolled forming rebar, the service life of the rebar roll is mainly affected by the dynamic and static loads and the harsh working environment caused by high temperature, high pressure and high speed. It is easy to wear, pitting and even breakage The In order to further improve the service life of the roll, tungsten carbide as a matrix, with cobalt or cobalt and nickel as a bonding material made of cemented carbide roll, because of high hardness, wear resistance, is widely used in high-speed wire Finishing mill. Sintered tungsten carbide roll has a very good wear resistance, it is easy to meet the requirements, but the toughness is not easy to achieve, it is generally used split structure, that is made of sintered tungsten carbide roll ring.

The main function of the machine to be designed is to realize the processing of the crescent groove in the annular groove of the tungsten carbide roll, taking into account the requirements for the marking, writing and patterning of the rebar rolled by the roll.

Alveolar processing program to determine: At present, both domestic and foreign, screw steel roll crescent groove CNC machine tools as the preferred machine for processing rebar roll has been accepted by all rebar roll manufacturing enterprises. But for a long time, due to the continuous improvement of the use of manufacturers and machine tool manufacturers and technical development level and practical experience of the restrictions, so that the existing models of various CNC thread.

The difference between the properties of silicon carbide and tungsten carbide mechanical seal material

Silicon carbide density 3.2g/cm3, black silicon carbide and green silicon carbide two commonly used basic varieties, are α-SiC. ① black silicon carbide containing about 98.5% SiC, its toughness is higher than the green silicon carbide, mostly used for processing tensile strength of low materials, such as glass, ceramics, stone, refractories, cast iron and nonferrous metals. ② green silicon carbide with SiC99% or more, self-sharpening, and most used for processing carbide, titanium and optical glass, but also for honing cylinder liner and grinding high-speed steel cutting tools. In addition to cubic silicon carbide, which is made in a special process of yellow-green crystals, for the production of abrasive for the super-finishing of the bearing, the surface roughness can be raised from Ra32 ~ 0.16 microns to Ra0.04 ~ 0.02 microns.

Silicon carbide due to chemical stability, high thermal conductivity, thermal expansion coefficient is small, good wear resistance, in addition to abrasive, there are many other uses, such as: a special process to apply silicon carbide powder turbine or cylinder The inner wall, can improve its wear resistance and extend the service life of 1 to 2 times; used to make high-grade refractory, heat shock, small size, light weight and high strength, energy saving effect. Low grade silicon carbide (containing about 85% SiC) is an excellent deoxidizer that accelerates steelmaking and facilitates the control of chemical composition and improves the quality of the steel. In addition, silicon carbide is also used in large quantities for the production of silicon carbide rods. Silicon carbide hardness is great, with excellent thermal conductivity, is a semiconductor, high temperature can be antioxidant.

Silicon nitride has a diamond-like three-dimensional lattice structure, so it has high temperature thermal stability, thermal shock resistance, chemical stability and good electrical insulation and hard. Silicon nitride melting point 1900 ℃, the relative density of 3.2 to 3.4, hardness 1500 ~ 1900Hv, bending strength of 600 ~ 1000MPa, elastic modulus of 310GPa. In the air heated to 1450 ~ 1550 ℃ is still stable. Soluble in hydrofluoric acid, insoluble in cold, hot water and dilute acid, for concentrated sulfuric acid and concentrated sodium hydroxide solution is also very slow.

Tungsten carbide is a hard alloy family of raw materials, pure tungsten carbide is not commonly used for black hexagonal crystals, metallic luster, hardness and diamond similar to the electric, heat good conductor. Melting point 2870 ℃, the boiling point of 6000 ℃, the relative density of 15.63 (18 ℃). Tungsten carbide is not soluble in water, hydrochloric acid and sulfuric acid, soluble in nitric acid - hydrofluoric acid mixed acid. Pure tungsten carbide fragile, if mixed with a small amount of titanium, cobalt and other metals, can reduce the brittleness. Tungsten carbide used as a steel cutting tool, often added titanium carbide, tantalum carbide or their mixture, in order to improve the anti-explosive ability. The chemical properties of tungsten carbide are stable.

In tungsten carbide, carbon atoms embedded in the gap between the tungsten metal lattice, does not destroy the original metal lattice, the formation of gap solid solution, it is also known as interstitial (or insert) compounds. Tungsten carbide can be prepared by heating the mixture of tungsten and carbon at high temperature, and the presence of hydrogen or hydrocarbons accelerates the reaction. If the preparation of tungsten with oxygen compounds, the product must be vacuum at 1500 ℃ to remove carbon oxides. Tungsten carbide suitable for high temperature machining, can produce cutting tools, furnace structure materials, jet engines, gas turbines, nozzles and so on.