Depending on the requirements for the final product, heat treatment is carried out by various methods.
Drying processes used in the production of final intermediates in the form of granules, briquettes, as well as for dehydration of solutions, sludges and suspensions; by subsequent drying, firing or sintering of the granulated or shaped material, the final product is obtained. In these cases, the patterns of heat and mass transfer are the same as during the main technological processes drying in chemical industry and in the production of building materials.
IN sintering process agglomerates and molded blanks, powder particles are combined into a monolithic polycrystalline solid body with properties close to those of a compact material. The heat treatment process consists of two stages.
The first stage - the removal of the technological binder - occurs at the temperatures of evaporation and melting of the binder and ends at the temperature at which powder particles begin to sinter. The second stage - sintering - begins at a temperature corresponding to the mutual adhesion of the particles to each other, and continues until the temperature for obtaining a monolithic body, which is approximately 0.8 of the melting temperature of the ceramic material. The firing mode is selected based on the chemical and particle size distribution of the charge from the waste, the method of molding or pressing, as well as the size and type of product.
During sintering, the initial charge (molded or pressed) is a thermodynamically unstable disperse system with a large reserve of free energy.
The sintering process can be conditionally divided into three stages.
At the first stage, the driving force is the excess free surface energy of fine particles, which tends to compress the workpiece due to the pressure that arises and reduce its free surface. The particles slide along the grain boundaries, which leads to compaction of the workpiece and its shrinkage.
At the second stage, the particles are baked at the points of contacts created at the first stage. During firing, the contacts between the particles expand, and the shape and size of the pores continuously change. The kinetics of this process is determined by the speed of the viscous flow of the medium in which the pores are located. At this stage, the viscous flow of the medium is determined by the mechanism of surface diffusion of atoms over the surfaces of sintering particles to the region of the contact isthmus.
At the third stage, only closed isolated pores remain in the sintering body, and further compaction is possible only by reducing their number and volume (healing process). The final stage of sintering is the longest.
pyrolysis process finds application in the processing of waste wood, plastics, rubber products, MSW and oil refining sludge and is a process of decomposition of waste wood, other plant materials when they are heated to a temperature of 450-1050 ° C without air access. In this case, gaseous and liquid products are formed, as well as solid coal.
native remainder ( charcoal in the processing of wood, carbon black in the recycling of tires).
Depending on the heating temperature, pyrolysis plants are divided into low-temperature (450-500 ° C), characterized by a minimum gas output, a maximum amount of resins, oils and solid residues; medium temperature (up to 800 °C) with an increased yield of pyrolysis gas and a reduced yield of resins and oils; high-temperature (over 800 °C) with a maximum yield of gases and a minimum of resinous products.
High temperature intensifies waste disposal. The rate of reactions increases exponentially with increasing temperature, while heat losses increase linearly. In this case, a more complete yield of volatile products occurs and the volume of the resulting solid residue is reduced. During pyrolysis, the temperature range of 1050-1400 °C is undesirable, since it leads to the formation of slags, especially in MSW.
The pyrolysis process is carried out in furnaces of periodic or continuous operation of various designs (chamber, tunnel, shaft, with moving layers) with external and internal heating. At the initial stage, with an increase in temperature, endothermic processes occur. When wood or other plant waste is heated to 150 ° C, moisture is removed, and at temperatures of 170-270 ° C, CO and CO 2 gases and small amounts of methyl alcohol and acetic acid are formed. At 270-280 °C, exothermic transformations begin. The output of non-condensable gases, such as CO and CO 2, decreases and at the same time the output of other gaseous and vaporous substances (CH 4, C 2 H 4 , H 2), as well as methyl alcohol and acetic acid, increases. The speed of the process is affected by the size of the pieces of processed waste, their humidity and temperature.
The gases leaving the furnace cool and release valuable components from them. The resulting charcoal is used in the production of active carbons, black powders and other processes.
High tech and difficult process, which requires special equipment and special tools. This is due to the fact that such alloys have high elasticity and strength, and therefore strongly resist cutting, drilling, grinding and other machining. At the same time, the quality of the corresponding process largely depends on the characteristics of the metal and the correct selection of the cutting tool.
Carbide Features
Hard-to-cut metals include heat-resistant and stainless steels and alloys. These materials are a solid solution of the austenitic class, so they have such qualities as high resistance to corrosion, the ability to work in a stressed state for a long time, and resistance to chemical destruction. In addition, some types of these metals have a highly dispersed structure. Due to this, the sliding process practically does not occur.
Processing is also complicated for the following reasons:
- when cutting, the material is hardened;
- alloys of this nature have low thermal conductivity, and therefore the contact part of the workpiece and tool begin to seize;
- original strength is retained even at very high temperature;
- high abrasion ability of alloys leads to the formation of inclusions that adversely affect the tool;
- the vibration resistance of metals is determined by the uneven flow of the cutting process, which means that it will not work to obtain the desired quality of processing.
Tool selection
In order to avoid all the problems described above and to carry out high-quality processing of hard alloys, it is necessary first of all to choose the right tool. It must be made of a metal that has higher cutting properties than the workpiece. At the same time, experts recommend using carbide cutters for pre-treatment, and high-speed cutters for finishing. The latter include steel grades R14F4, R10K5F5, R9F5, R9K9.
For the manufacture of tools from carbide metals, three types of alloys are used:
- T30K4, T15K6, VKZ - wear-resistant;
- T5K7, T5K10 - are distinguished by high viscosity;
- VK6A, VK8 - are insensitive to shocks, have the least resistance to wear.
To harden the tools and improve their performance, the second layer of carbide metal, cyanidation, chromium plating, and cladding are additionally applied.
coolant
The correct selection of coolants and the method of their application is no less important process if it is necessary to machine hard alloys. For drilling, experts recommend using mineral-based materials. They especially increase productivity when working with titanium, which is very difficult to work with. For turning alloyed steels, semi-synthetic coolants are suitable, for honing and grinding cast iron - a fluid without mineral oils. There are also universal materials that are very beneficial to use if the nature of metal processing is constantly changing.
The most optimal way of supplying coolant when working with hard metals is considered to be high-pressure, in which the liquid is supplied in a thin stream to the back wall of the tool. Equally effective are liquid atomization and carbon dioxide cooling. All this allows to increase tool life and improve the quality of processing.
equipment requirements
Equipment for working hard metals is very different from standard machine tools. These models are different:
- increased rigidity of all mechanisms;
- vibration resistance;
- high power;
- the presence of channels for chip removal;
- special landing places for fixing a short tool.
Hard metals and alloys are wear-resistant materials that can maintain their characteristics at elevated temperatures (900-1100 degrees). They have been known to man for over a hundred years.
general characteristics
Hard alloys are made mainly on the basis of chromium, tantalum, titanium, tungsten with the addition of various amounts of nickel or cobalt. In the production, durable carbides are used that are not subject to decomposition and dissolution at high temperatures. Hardmetal can be cast or sintered. Carbides are brittle. In this regard, to form a solid material, their grains are bound with suitable metals. The latter are iron, cobalt, nickel.
Cast connections
The carbide tool obtained by this method is characterized by high resistance to abrasion by the material of the workpiece and descending chips. They do not lose their characteristics at a heating temperature of 750 to 1100 degrees. It has been established that products made by melting or casting with the addition of a kilogram of tungsten can process five times more material than high-speed steel objects with the same W content. One of the disadvantages of such compounds is their brittleness. With a decrease in the proportion of cobalt in the composition, it increases. The speed that carbide cutters have is 3-4 times higher than that of steel.
Sintered materials
They include a metal-like joint bonded by an alloy or metal. As a basis, as a rule, titanium or tungsten carbide (including complex) is used, as well as tantalum, titanium carbonide. Less often, borides are used in the manufacture. The matrix for holding the grains of the material is a binder - an alloy or metal. As a rule, it is cobalt. It is a carbon neutral element. Cobalt does not form its own carbides and does not destroy others. Less often, nickel and its combination with molybdenum are used in a bundle.
Comparative characteristics
Sintered materials are obtained by powder method. Processing of hard alloys of this type is carried out only by grinding or by physical and chemical methods (laser, etching in acids, ultrasound, etc.). Cast products are subjected to hardening, annealing, aging and so on. They are designed for hardfacing on tools. Powdered materials are attached by soldering or mechanically.
Classification
It depends on the content of cobalt, tantalum, tungsten and titanium carbides. In this regard, the materials under consideration are divided into three groups. When designating brands of compounds, letters are used:
- Tungsten carbide - "B".
- Cobalt - "K".
- Titanium carbide - the first "T".
- Tantalum carbide is the second "T".
The numbers after the letters indicate the approximate percentage of components. The rest in the compound (up to 100%) is tungsten carbide. The letters indicated at the end indicate the granularity of the structure: "B" - large, "M" - small, "OM" - extra fine. The industry produces hard alloys grades VK (tungsten), TTK (titanium-tantalum-tungsten) and TK (titanium-tungsten).
Features
The main properties of hard alloys are their high strength and wear resistance. At the same time, the considered materials are characterized by lower viscosity and thermal conductivity in comparison with steel. This must be taken into account when using the products. When choosing a hard alloy, you must adhere to a number of recommendations:
- Tungsten products, in comparison with titanium-tungsten products, are distinguished by a lower temperature of weldability with steel. In this regard, they are used to work with cast iron, non-ferrous metals and non-metallic materials.
- For steel, it is advisable to use compounds of the TK group.
- Carbide grade TTK has increased toughness and accuracy. It is used to work with steel forgings, castings in adverse conditions.
- Fine and fine turning with a small chip section is provided by carbide burrs with a fine-grained structure and a lower cobalt content.
- Under adverse conditions and rough work with materials with impact loading, it is advisable to use compounds with a high content of cobalt. At the same time, they should have a coarse-grained structure.
- Finishing and roughing in the continuous cutting process are carried out mainly with compounds with an average percentage of cobalt.
Powder materials
They are represented by two groups: containing and not containing tungsten. In the first case, the hard alloy is presented in the form of a mixture of technical powdered W and ferrotungsten with carburizing components. It was made in the USSR. This hard alloy is called "vokar". The manufacturing process of the material is as follows:
- High percentage ferrotungsten and technical powdered W are mixed with ground coke, carbon black and other similar components.
- The resulting mass is kneaded on sugar syrup or resin into a thick paste.
- The mixture is pressed into briquettes, which are lightly fired. This is necessary to remove volatile compounds.
- After firing, the briquettes are ground and sieved.
The finished material thus has the appearance of brittle black grains. Their size is 1-3 mm. Distinctive feature of such materials is their large bulk density.
stalinite
This hard alloy does not contain tungsten, which makes it a low cost material. It was also invented in Soviet years and is widely used in industry. As practice has shown, despite the fact that this hard alloy does not contain tungsten, it has high mechanical characteristics, in most cases satisfying technical requirements. Stalinite has significant advantages over tungsten materials. First of all, it is a low (1300-1350 degrees) melting point. Tungsten materials only change from 2700 degrees. The melting temperature of 1300-1350 degrees greatly facilitates surfacing, increases its productivity. A mixture of cheap powdered ferroalloys, ferromanganese and ferrochrome is used as the basis of stalinite. The production of this material is similar to the production process for tungsten compounds. Stalinite contains 16-20% chromium, 13-17% manganese.
Application
In modern industry, hard alloys are widely used. At the same time, materials are constantly being improved. The development of this production sector is carried out in two directions. First of all, the compositions of alloys are improved, the technology of their manufacture is being improved. In addition, innovative methods of applying compounds to products are being introduced. Carbide tools contribute to a significant increase in labor productivity. This is ensured by high wear resistance and heat resistance of products. Such characteristics allow you to work at speeds 3-5 times higher than for steel. Such advantages, for example, have modern burrs. Carbide materials manufactured using advanced technologies (electrochemical and electrophysical methods), including the use of diamond blanks, are today one of the most demanded in the industry.
Developments
Today, various studies are being carried out in the domestic industry, including a deep analysis of the possibility of improving the characteristics of hard alloys. They mainly concern the granulometric and chemical composition materials.
As a fairly successful example over the past few years, compounds of the TSN group can be cited. Such alloys are specially designed for friction units operating in an aggressive acid environment. This group continues to develop new compounds in the VN group proposed by the All-Russian NIITS.
During the research, it was found that with a decrease in the grain size of the carbide phase, such characteristics as the strength and hardness of the alloys significantly increase. The use of technologies for regulation and plasma reduction of particle size distribution today makes it possible to produce materials with a fraction size of less than a micron. Alloys of the TSN brand are widely used today in the production of oil and gas and chemical pump assemblies.
Russian industry
One of the leading enterprises engaged in the field of production and scientific development is the Kirovograd Hard Alloy Plant. KZTS has an extensive own experience on the introduction of innovative technologies in production. This allows him to take first positions in the industrial market of Russia. The company specializes in the production of sintered hard-alloy tools and products, metal powders. The issue has been established since January 1942. In the late 1990s, the company was modernized. Over the past few years, the Kirovograd Hard Alloy Plant has focused its activities on the production of improved multifaceted wear-resistant multi-layered wear-resistant indexable inserts. The company is also developing new tungsten-free compositions.
Conclusion
Many positive experiences industrial enterprises suggests that in the near future tungsten-free alloys will not only become even more popular, but will also be able to replace other materials used for the production of stamping and cutting products, machine elements that work in difficult conditions, fixtures and tooling. Today, a whole group of compounds based on carbonitride and titanium carbide has already been created. They are used in many industrial areas. Widespread, in particular, hard alloys TV4, LCK20, KTN16, TN50, TN20. New developments include materials of the tantalum TaC, niobium NbC, hafnium HfC, titanium TiC groups. The release of tools using these alloys makes it possible to replace tungsten with relatively cheap additives, thus expanding the range of raw materials used. This, in turn, ensures the production of products with specific properties and higher performance.
